53236834
Thermochronological insights into reactivation of a continental shear zone in response to Equatorial Atlantic rifting (northern Ghana)
West Africa was subjected to deformation and exhumation in response to Gondwana break-up. The timing and extent of these events are recorded in the thermal history of the margin. This study reports new apatite fission track (AFT) data from Palaeoproterozoic basement along the primary NE-SW structural trend of the Bole-Nangodi shear zone in northwestern Ghana. The results display bimodality in AFT age (populations of ~210-180 Ma and ~115-105 Ma) and length distributions (populations of 12.2 ± 1.6 and 13.1 ± 1.4 µm), supported by differences in apatite chemistry (U concentrations). The bimodal AFT results and associated QTQt thermal history models provide evidence for multiple cooling phases. Late Triassic – Early Jurassic cooling is interpreted to be related with thermal relaxation after the emplacement of the Central Atlantic Magmatic Province (CAMP). Early to middle Cretaceous cooling is thought to be associated with exhumation during the Cretaceous onset of rifting between West Africa and Brazil. Late Cretaceous – Cenozoic cooling can be related with exhumation of the Ivory Coast – Ghana margin and NNW-SSE shortening through western Africa. Furthermore, our data record differential exhumation of the crust with respect to the Bole-Nangodi shear zone, preserving older (CAMP) cooling ages to the south and younger (rifting) cooling ages to the north of the shear zone, respectively. This suggests that the Palaeoproterozoic BN shear zone was reactivated during the Cretaceous as a result of deformation in the Equatorial Atlantic region of Africa.
128663231
A 3.5 Ga granite–gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton
Abstract A granite–gneiss formation (Guelemata Orthogneiss) was mapped on the northern and western slopes of the Mount Nimba hill range in Guinea. The original rocks were high-Al, low-Yb, medium to high-K granites similar to most Archean TTG. Analyses of U and Pb isotopes in zircons, using an ion-microprobe (SHRIMP), gave ages of 3542 and 3535 Ma respectively for the granite–gneiss and a neighbouring granulitic metagabbro. These are the oldest ages so far reported from the Archean Kenema-Man domain of West Africa. Partial melting of an unfractionated basalt under eclogite facies conditions appears to be a suitable model for the origin of the granite–gneiss.
129089946
Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating
Abstract The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. UPb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 ± 2 Ma; Sefwi belt, 2179 ± 2 Ma) are ∼ 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 ± 1 Ma). Belt granitoid are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.
54921096
Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse
We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25–2.00 Ga) West African Craton in northern Ghana, based on the interpretation of field observations and airborne geophysical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and low-grade volcano–sedimentary belts, separated by shear zones and assembled during the Paleoproterozoic Eburnean orogeny (2.15–2.07 Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite–granulite facies transition, during a (D1) deformation event, consistent with N–S horizontal shortening, which generated reverse shear zones. High and low metamorphic grade rocks were brought in contact along extensional shear zones formed during N–S extension (D2). These structures were overprinted by contractional deformation associated with E–W shortening and N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolution (D4–D7) is characterised by strain localisation in successive generations of narrow shear zones, and the re-activation of inherited structures in a dominantly transcurrent regime. U–Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110 Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the collision of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics.
128831103
Transform tectonics and thermal rejuvenation on the Côte d’Ivoire-Ghana margin, west Africa
Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of >120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting c. 1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.
29706677
Flexural unloading and uplift along the Côte d'Ivoire‐Ghana Transform Margin, equatorial Atlantic
Recent Ocean Drilling Program sampling of the Cote d'Ivoire-Ghana margin of West Africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent-ocean transition after the end of intracontinental wrenching. We model its evolution using a two dimensional flexural backstripping technique. The model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores. Following rifting at 120 Ma (Aptian), intracontinental wrenching continued until ∼105 Ma (mid Albian), when South America and Africa separated along this transform. While the possible presence of thicker crust under the Marginal Ridge compared to the Deep Ivorian Basin may explain some of the ridge's topography, the entire uplift can be readily modeled as a flexural response to unloading along a shallow-dipping (∼25°) fault at the time of continental separation. Forward subsidence modeling of the Marginal Ridge suggests that an effective elastic thickness of 2.5 km at 105 Ma is most appropriate to match the observed structure. Conduction of heat from the oceanic plate across the continent-ocean transition drove temporary uplift of at least 1200 m peaking just before 89 Ma, when sedimentary data show deposition within the photic zone (0–50 m). This study shows that significant, transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant.
128400126
Fission track study: heating and denudation of marginal ridge of the Ivory Coast–Ghana transform margin
Abstract Six sandstone blocks sampled during dives along the southern slope of the Ivory Coast–Ghana continental margin have been studied using fission tracks in apatite and zircon. Measurements demonstrate that the rocks were heated above 120°C but below 390°C and cooled quickly. The ages of cooling recorded by the apatite crystals are 90 Ma in the western part of the margin, and 80–70 Ma in the central and eastern part. Heating is interpreted by the heat liberation due to the friction along the active transform fault and by the vicinity of an oceanic spreading center, which slipped along the margin. Cooling is interpreted by two stages of denudation due to minor faults and landslides produced by the increasing of the bathymetric step between the continental margin and the oceanic crust.
129429973
Early Cretaceous rifts of Western and Central Africa: an overview
Abstract The structure and evolution of Early Cretaceous rift basins in Western and Central Africa are described. Two stages of rift development and fracturing have been identified: (1) from Neocomian to Early Aptian roughly E-W and NW trending troughs (Upper Benue, N Cameroon, S Chad, Sudan etc.) opened in response to a submeridian extensional regime in Central Africa while in Western Africa the N-S trending transsaharian fault zone acted as a sinistral wrench; (2) from Middle Aptian to Late Albian large northwest trending troughs (E Niger, Sudan, Sirte, etc.) opened in response to a northeast extensional regime while the Central African fault zone (from Benue to Sudan) exhibited strike-slip movements, generating pull-apart basins. These rift and fracture systems delimit three large blocks within the African plate: a Western block, an Arabian-Nubian block and an Austral block. The Arabian-Nubian block tends to separate from the two other blocks, migrating towards the north during the first stage of basin development and then towards the NE during the second stage. The opening of the Atlantic Ocean was the dominant driving force for the Western and Austral blocks while the Arabian-Nubian block probably moved in response to the opening of the Indian Ocean and to the evolution of the Tethyan margin.
73638920
The geophysical signatures of the West African Craton
Abstract This paper examines existing and newly compiled geophysical representations of the West African Craton (WAC) in terms of its large-scale tectonic architecture. In order to build an interpretation with a significant depth extent we draw upon a range of geophysical data, principally seismic tomographic inversions, receiver functions, gravity and magnetics. We present these results as a series of layers providing a series of depth slices though the cratonic lithosphere. The different geophysical methods suggest partitioning of the WAC into two tectonic elements at the largest scale which is observed in both seismic tomographic images, lithosphere–asthenosphere boundary (LAB) models and long wavelength gravity signals. The different models of the Moho, or crust-mantle boundary, based on these gravity or seismic datasets show little or no correlation, either for short or long-wavelength features, and show little correlation with new receiver function inferred crustal thickness estimates. Manual interpretation of low-wavelength gravity and magnetic data suggest a possible continuation of the WAC across the western margin of the modern boundary, and also highlight distinct domains interpreted to be of Birimian age.
129065439
Basin modelling of a transform margin setting: structural, thermal and hydrocarbon evolution of the Tano Basin, Ghana
ABSTRACT This study explores the structural and thermal evolution of the Ghana transform margin. The main objective is to explore how the opening of the Atlantic Ocean and subsequent interaction with the Mid-Atlantic Ridge (MAR) has affected the margin's structural and thermal evolution. Two representative evolution scenarios are described: a reference case that neglects the influence of continental breakup and a second scenario that accounts for a possible heat influx during the passage of the MAR as well as magmatic underplating. These two scenarios have further been analysed for the implications for the hydrocarbon potential of the region. The scenario analysis builds on a suite of 2D realizations performed with TECMOD2D, modelling software for automated basin reconstructions. As the observed stratigraphy is input, the structural and thermal evolution of the basin is automatically reconstructed. This is achieved through the coupling of a lithosphere scale forward model with an inverse algorithm for model parameter optimization. We find that lateral heat transport from the passing MAR in combination with flexure of the lithosphere can explain the observed uplift of the margin. These results were obtained for a broken plate elasticity solution with a relative large value for the effective elastic thickness (Te=15) and necking level (15 km). Lateral heat flow from oceanic lithosphere is clearly visible in elevated basement heat flow values up to 50 km away from the ocean–continent transition (OCT). This influx of heat does not seem to have affected the maturation history along the margin significantly. Only the deepest sediments close to the OCT show slightly elevated vitrinite reflectance in simulations that account for the passage of the MAR. In conclusion, it appears that that lateral heat transport from the oceanic lithosphere is instrumental in shaping the Ghana transform margin but seems to have only limited control on the maturation history.
129019177
Constraints on provenance, stratigraphic correlation and structural context of the Volta basin, Ghana, from detrital zircon geochronology: An Amazonian connection?
Abstract We have dated 1060 detrital zircon grains from the Neoproterozoic to lower Palaeozoic Volta basin, Ghana, and from sandstones in the adjoining thrust sheets belonging to the Pan-African Dahomeyide belt. All dated zircons in the lower Voltaian Bombouaka Group are older than ~ 1000 Ma, while samples from the middle Voltaian Oti Group and the upper Voltaian Obosum Group contain numerous zircons of 600–1000 Ma. The samples we have studied from the Dahomeyide thrust sheets (Buem and Togo structural units) have zircon age spectra similar to those from the Bombouaka Group, confirming a correlation of the investigated sandstones with lower Voltaian strata. The Bombouaka Group was deposited between ~ 1000 and 600 Ma, perhaps shortly after 1000 Ma, as suggested by earlier Rb–Sr data on clay minerals. Deposition of the Oti and Obosum Groups took place shortly after 600 Ma, perhaps continuing into the lower Palaeozoic. Most samples contain Palaeoproterozoic zircons with ages of 2000–2200 Ma that probably have been derived from the surrounding crystalline (Birimian) basement. Archaean zircons, present in smaller proportions, may have come from Archaean rocks in the West-African craton. Most zircons of 1000–1900 Ma were probably derived from sources outside the West-African craton, the Amazonian craton being a plausible source region. Zircons with ages around 1200 Ma are believed to have been derived from Grenvillian orogenic belts, perhaps those that fringe the Amazonian craton. If this is so, no major seaways could have been present between these belts and the West-African craton during the early Neoproterozoic. Zircons younger than 1000 Ma were probably derived from an eastern continental block that collided with the West-African craton during the Pan-African orogeny ~ 600 Ma ago.
130054160
A review of Wilson Cycle plate margins: A role for mantle plumes in continental break-up along sutures?
Abstract The Wilson Cycle theory that oceans close and reopen along the former suture is a fundamental concept in plate tectonics. It was named after J. Tuzo Wilson who recognised that dissimilar marine palaeo-faunas on both sides of the present-day Atlantic Ocean were best explained by an earlier proto-Atlantic ocean. The Wilson Cycle theory implies that collision zones may localise extensional deformation hundreds of millions of years after collision has waned. We review the passive margins of the Atlantic and Indian Oceans with the aim to evaluate the extent in which oceanic openings used former sutures and analyse the potential role of mantle plumes in continental break-up. We summarise the time of collision, onset of rifting, break-up, and main phase of flood basalt emplacement (if applicable) for eighteen conjugate margins. We find that conjugate margins open along former sutures with the exception of the Madagascar–Seychelles–India system. There is no relationship between suture age and break-up age. Continental break-up occurred on relatively young sutures, such as Morocco–Nova Scotia, and on very old sutures, such as the Greenland–Labrador and East Antarctica–Australia systems. We identified two cases where a suture was reactivated as a transform fault: the Charlie Gibbs Fracture Zone follows the Iapetus suture and the Agulhas–Falkland Fracture Zone possibly follows a Late Palaeozoic–Early Mesozoic suture between Patagonia and Western Gondwana. Continental extension and break-up is not always associated with large amounts of volcanism, as illustrated by the magma-poor margins of Iberia–Newfoundland, the Equatorial Atlantic Ocean, and East Antarctica–Australia. But twelve of the conjugate margins in our review are linked to large igneous provinces, such as, the North Atlantic margins (NAIP), Northwest Africa–Florida (CAMP), Arabia–Northeast Africa (Afar), and South Africa–East Antarctica (Karoo). For these margins we find that break-up occurs concurrent with emplacement of the associated large igneous province. In many margins, rifting began before the main phase of volcanism. This suggests that rifting was initiated by tectonic forces and that plume material flowed to the thinned rifted lithosphere to help trigger final continental break-up.
130922005
The role of mantle plumes in the evolution of the African segment of Pangea and the formation of the Atlantic Ocean
In this paper, we discuss a broad range of issues related to the formation of large igneous provinces in the African segment of Pangea on the basis of modern seismic tomography data. The formation of older igneous provinces (Central American and Karoo) is attributed to a prolonged phase of upwelling of hot mantle material or fluids in separate jets within a much larger area than the supposed plume head. Owing to its huge size and the thick, dense continental crust, Pangea acted as a shield promoting the accumulation and lateral channeling of heat energy beneath the lithosphere. The changes in global Earth dynamics and the generation of extensional stresses alone may have led to the breakup of Pangea, triggering the eruption of large volumes of magma over short period of time. The same factors led to the opening of the Atlantic Ocean. We provide arguments that the African superplume represents a Cenozoic structure not associated with the emplacement of the Karoo province. At the same time, the hot material brought under the lithosphere by this superplume synchronously with the start of magmatism in east Africa then spread out to the northwest to form local melting areas in Central and Northwestern Africa. We suggest that magmatic activity within the same region may have lasted, with interruptions, over tens of millions of years. Because of plate motion, these lowvelocity zones acting as heat sources appear to have lost their deep-seated roots, so that mantle reservoirs surviving at the base of the lithosphere may have fed magmatism and drifted together with the lithosphere.
134196427
Geochronology and thermochronology of the gneisses of the Brasiliano/Pan-African Araguaia Belt: Records of exhumation of West Gondwana and Pangea break up
Abstract The 1200 km long Araguaia Belt, central-north Brazil, was formed as a result of the Neoproterozoic (Ediacaran) to Early Cambrian collage of West Gondwana and has several dome structures in whose nuclei crop out basement inliers of Archean and Paleoproterozoic orthogneisses. Thus, the orthogneisses from Lontra, Cocalândia, Colmeia and Cantao dome structures were selected for fission-track thermochronology and U-Pb double dating. Three Archean magmatic episodes ranging from 2830.9 ± 6.9 Ma (Colmeia dome) to 2905 ± 5.1 Ma (Lontra dome) and one Paleoproterozoic magmatic event at 1835 ± 31 Ma (Cantao dome) are recognized by U-Pb LA-MC-ICP-MS zircon dating. Fission-track thermochronology in zircon permitted to identify three well-defined age populations, which correspond to major regional geological events. The older population (489 ± 15 Ma to 498±8Ma) records the exhumation process and the orogenetic collapse of the Araguaia Belt. The intermediate population (331 ± 8 Ma to 345 ± 13 Ma) is related to the Gondwanides orogeny which affected the western margin of Gondwana and therefore the interior of South American. Finally, the younger fission track ages (197 ± 3 Ma, 197 ± 4 Ma, 198 ± 2 Ma and 208 ± 10 Ma) reflect crustal heating around 200 Ma, which is associated with the Lower Jurassic Mosquito tholeiitic magmatism and extended the influence of the Central Atlantic Magmatic Province to the Araguaia Belt. This mafic magmatism has been recognized in different regions of the South American and was an important source of heat for hydrocarbon generation in some Paleozoic intracratonic basins.
128965645
Tectonothermal evolution of the northeastern margin of Iberia since the break-up of Pangea to present, revealed by low-temperature fission-track and (U–Th)/He thermochronology: A case history of the Catalan Coastal Ranges
Abstract The Catalan Coastal Ranges (CCR) form part of the northeastern Iberian Mediterranean margin. This margin typifies the transition zone between the thickened crust of the Pyrenees and Ebro Basin (32 to 40 km), and the extremely thinned continental crust of the offshore Valencia Trough (8 km). Despite intensive geological research in recent decades, the issue of the unsolved thermochronological history has hardly been addressed for the Catalan Margin. This study provides for the first time a continuous low-temperature thermochronological dataset covering the period from Late Paleozoic to present-day. More than 100 analyses have been performed in an area less than 200 km long by 50 km wide. Zircon and apatite fission-track (ZFT and AFT) results of the Hercynian basement yield cooling ages between 254 ± 41–104 ± 16 Ma and 223 ± 27–16 ± 3 Ma, respectively. Apatite (U–Th)/He ages range between 58 ± 3 and 2.0 ± 0.2 Ma. These thermochronological data provide evidence for a very active geothermal past, revealing above all an important thermal period coeval with the Triassic–Early Jurassic break-up of Pangea. Hereby we discuss and suggest various mechanisms responsible for the elevated crustal temperatures detected. This new dataset also provides new constraints on the allocation of areas where exhumation/uplift occurred synchronous with some tectonic phases, the widespread presence of hydrothermal mineralizations and the present-day overmaturity state of the Mesozoic sediments, with regard to oil potential, in the CCR. Despite the local scale of the Catalan Margin, the very detailed study here presented sheds some light on the geological implications of results from other more regional thermochronological studies on the Iberian Microplate. Studies that yield similar Mesozoic thermochronological record for areas that also experienced extensional fracturing and subsidence during the break-up of Pangea.
140575779
The farthest record of the Central Atlantic Magmatic Province into West Africa craton: Precise 40Ar/39Ar dating and geochemistry of Taoudenni basin intrusives (northern Mali)
The Taoudenni basin dike swarm (northern Mali) is one of the densest dike complexes of the Central Atlantic Magmatic Province (CAMP). A combined geometrical (dike directions), geochemical (major and trace elements, Sr and Nd isotopes), and geochronological (40Ar/39Ar dating) study is presented for the first time on the dikes and sills in this area. The CAMP-related magmatic event is confirmed, revealing a main peak in activity at 198.1 Ma. Dike orientations are clearly correlated with geochemical signatures: the E–W trending dikes have intermediate-Ti concentrations with La/Yb > 8; the N–S or NW–SE trending dikes (and the sills) have low-Ti contents, with La/Yb < 7. Although almost all 40Ar/39Ar plateau ages of both dike groups overlap, N–S and intermediate trending dikes could be slightly younger (peak at 197.2 Ma) than the E–W trending dikes (peak at 198.1 Ma). Timing and geochemical evolution on the dikes supported by comparison with contemporaneous lava flows in Morocco suggest that a single sub-continental lithospheric mantle source underwent increasing extents of partial melting (4% during initial stages and 7–15% during later ones) with more pronounced magmatic evolution and crustal contamination in the final stages. Finally, the dike directions are not in agreement with the radial pattern that would be predicted by a plume model for the CAMP. Alternatively, the evolution of the Taoudenni dike pattern may have been controlled by reactivation of older crustal structures and by later massive magma storage at depth in response to thermal incubation beneath the Pangea supercontinent.
129235315
Mesozoic igneous activity in the Maranha˜o province, northern Brazil: 40Ar/ 39Ar evidence for separate episodes of basaltic magmatism
Low- and high-Ti basaltic rocks of the Maranha˜o province in northern Brazil appear to be related to the opening of the Atlantic Ocean. 40Ar/39Ar dating was carried out on powdered whole-rock basalts, both washed and unwashed in dilute nitric acid, to document this relationship. Acid treatment does not alter the argon retention properties of retentive phases, but is effective in removing alteration phases (high Ca/K) degassed at low laboratory temperatures. Our results identify that at least two periods of volcanic activity formed the Maranha˜o province. Low-Ti basalts from the western sector are ∼ 200-190 Ma and are related to the opening of the North Atlantic. High-Ti basalts from the eastern sector are ∼ 129-124 Ma, slightly younger than compositionally similar rocks in the Serra Geral flood basalt province of southern Brazil, and are related to the opening of the South Atlantic. Other flood basalts, such as the Columbia River, Deccan, Rajmahal, Ferrar and Siberian provinces, were each predominantly formed in 1–2 Ma. The Maranha˜o basalts, remnants of separate igneous events spanning ∼ 70 Ma, should not be classed as a single flood basalt province. The temporal complexities noted in the Maranha˜o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the Atlantic Ocean. 
 
Two sets of Mesozoic basaltic rocks occur in southern Africa, India and South America, resulting from events leading to the fragmentation and dispersal of sections of Gondwanaland; in each (sub)continent the two provinces were formed by events separated by ∼ 50–70 Ma. Of these pairs, only the Maranha˜o and Serra Geral show geochemical similarities, indicating compositionally distinct upper-mantle source regions under sections of Brazil separated by ∼ 3000 km.
140558542
40Ar/39Ar ages and Sr–Nd–Pb–Os geochemistry of CAMP tholeiites from Western Maranhão basin (NE Brazil)
The Central Atlantic Magmatic Province (CAMP), emplaced at the Triassic-Jurassic (T-J) boundary (~ 200 Ma), is among the largest igneous provinces on Earth. The Maranhao basin in NE Brazil is located around 700 km inland and 2000 km from the site of the earliest Pangea disruption. The CAMP tholeiites occur only in the western part of the basin and have been described as low and high-Ti. Here we document the occurrence of two sub-groups among the high-Ti tholeiites in the Western Maranhao basin. The major and trace elements and the Sr-Nd-Pb isotopic ratios define three chemical groups corresponding to the low-Ti (TiO2 3 wt.%) western Maranhao basin tholeiites (WMBT). The new 40Ar/39Ar plateau ages obtained on plagioclase separates for high-Ti (199.7 ± 2.4 Ma) and evolved high-Ti WMBT (197.2 ± 0.5 Ma and 198.2 ± 0.6 Ma) are indistinguishable and identical to those of previously analyzed low-Ti WMBT (198.5 ± 0.8 Ma) and to the mean 40Ar/39Ar age of the CAMP (199 ± 2.4 Ma). We also present the first Re-Os isotopic data for CAMP basalts. The low and high-Ti samples display mantle-like initial (187Os/188Os)i ranging from 0.1267 to 0.1299, while the evolved high-Ti samples are more radiogenic ((187Os/188Os)i up to 0.184) We propose that the high-Ti WMBT were derived from the sub-lithospheric asthenosphere, and contaminated during ascent by interaction with the subcontinental lithospheric mantle (SCLM). The evolved high-Ti WMBT were derived from the same asthenospheric source but experienced crustal contamination. The chemical characteristics of the low-Ti group can be explained by partial melting of the most fertile portions of the SCLM metasomatized during paleo-subduction. Alternatively, the low-Ti WMBT could be derived from the sub-lithospheric asthenosphere but the resulting melts may have undergone contamination by the SCLM. The occurrences of high-Ti basalts are apparently not restricted to the area of initial continental disruption which may bring into question previous interpretations such as those relating high-Ti CAMP magmatism to the initiation of Atlantic ridge spreading or as the expression of a deep mantle plume. We propose that the CAMP magmatism in the Maranhao basin may be attributed to local hotter mantle conditions due to the combined effects of edge-driven convection and large-scale mantle warming under the Pangea supercontinent. The involvement of a mantle-plume with asthenosphere-like isotopic characteristics cannot be ruled out either as one of the main source components of the WMBT or as a heat supplier.
128604689
The Cretaceous opening of the South Atlantic Ocean
Abstract The separation of South America from Africa during the Cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted between ∼121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Africa during that period. The kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (
129357615
A geological field trip to the Côte d'Ivoire-Ghana transform margin
Abstract During the Equanaute survey (June 1992), fourteen submersible dives were performed between 4950 and 2250 m water depths across the southern slope of the Cote d'Ivoire-Ghana Marginal Ridge (CIGMR), in the eastern Equatorial Atlantic. The CIGMR, a high-standing topographic marginal ridge along the Cote d'Ivoire-Ghana transform margin, is believed to result from a complex structural evolution due to the specific wrench-related rifting between Western Equatorial Africa and Northeastern Brazil, in Early Cretaceous times. In this paper we report and discuss geological observations made during dives, and sample analyses to resolve the lithology, paleoenvironmental conditions, age and origin of the CIGMR. The data help in better characterizing the different sedimentary and tectonic regimes which successively prevailed during the CIGMR formation and assessing the thermal regime operative during the fabrication and subsequent evolution of the margin. The thick sedimentary pile exposed along the southern CIGMR slope is made up of a repetitive clastic sequence indicative of a deltaic-to-prodeltaic environment. This sedimentary pile, of Early Cretaceous age, has recorded different stages of the transform margin structural evolution. 1. (1)|Syn- to post-lithification deformations first record extensional deformations related to the rifting of the adjacent northern divergent margin segment (the Deep Ivorian Basin). 2. (2)|Wrench tectonics has, at a later stage, produced intense fracturing and participated in local folding, chiefly detected upslope. The integrated studies of geological samples and in situ observations obtained during the Equanaute survey support models for transform margin evolution proposed mainly from geophysical data.
129902424
The Ivory Coast-Ghana transform margin: A marginal ridge structure deduced from seismic data
Abstract The Ivory Coast-Ghana (ICG) marginal ridge is a prominent feature of the ICG transform margin and includes a fossil ridge partially buried by a thick, undeformed sedimentary cover. The fossil ICG ridge is 130 km long and 25 km wide, and towers over the adjacent rifted basin (deep Ivorian basin, DIB) and the oceanic crust by 1.3 km and more than 4 km, respectively. It formed in three successive stages. 1. (1) During the rifting of the DIB, both vertical and horizontal motions between the DIB and the South American plate varied along the plate boundary. This relative motion occurred in an accommodation zone that tilted the northern slope of the ICG ridge along en-echelon, mainly strike-slip, faults. 2. (2) After the rifting of the DIB, the relative motion remained constant along the transform plate boundary. At this time strike-slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone. 3. (3) Finally, the transform motion occurred between the DIB and an occanic plate. Thermal exchanges between the two adjacent plates induced thermal uplift of the ICG ridge that amplified previous tectonic tilting.
128633497
Rift–shear architecture and tectonic development of the Ghana margin deduced from multichannel seismic reflection and potential field data
Abstract The Ghana margin displays one of the best-known transform margins. Studies of the margin have provided the framework for a number of conceptual models aimed at understanding transform margin development worldwide. However, the deep structure of the margin is poorly known as knowledge is based only on wide-angle refraction measurements obtained from two separate localities on the margin. Consequently, complexities in the rift–shear margin architecture have been overlooked by current interpretations of margin development. Based on combined analysis of a detailed grid of ∼2710 km multichannel (MCS) lines and potential field data, we provide new insights into the structural architecture and tectonic development of the Ghana margin. In particular, we outline the deep structure of the entire margin using a series of 2D gravity modelled transects constrained by MCS and published wide-angle data. Our study reveals more complex rift–shear margin architecture than previously envisaged. We demonstrate that the main transform boundary representing the continental extension of the Romanche Fracture Zone, is actually composed of two distinct margin segments, i.e., the ENE–WSW trending sheared margin segment of the Cote d’Ivoire-Ghana Ridge and the NE–SW trending rift-influenced sheared margin segment of the Ghana Platform. These segments evolved under varying stress regimes, and during different time intervals. West of the transform margin, divergent rifting during the Early Cretaceous initiated the development of the Deep Ivorian Basin, essentially, as a single major pull-apart structure. However, east of the shear zone, oblique rifting resulted in the development of the Eastern Ghana Slope Basin as a composite of at least two coalescing pull-apart basins displaced along strike-slip faults. Our structural interpretation of the transform boundary geometry shows that the ridge and platform margin segments were each subjected to separate thermal influences from two different migrating spreading centres. Tectonic uplift of the ridge began through transpression during mid-Albian time following a change in relative direction of plate motion from NE–SW to ENE–WSW. However, the ridge uplift was amplified by thermal heating from a previously undocumented spreading centre whose progressive westward migration along the ridge followed closely after the Albian transpressional phase. The structural architecture of the Ghana margin resulted from a combination of factors, notably, pre-existing basement structure, plate boundary geometry, the relative direction of plate motion and thermal heating.
128887606
The role that plate tectonics, inferred stress changes and stratigraphic unconformities have on the evolution of the West and Central African Rift System and the Atlantic continental margins
The Muglad rift basin of Sudan, is a good example of polyphase rifting, with at least three major phases of basin development. Each phase has resulted in the generation of source rock, reservoir and seal geology with structural traps often closely linked to basement highs. In this paper we investigate on a regional scale the tectonic processes that have contributed to rift basin development. 
 
On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to relative movements of African sub-plates and to global plate tectonic processes and plate interactions. Changes in plate interactions are observed in the oceanic crust as azimuth changes of fracture zone geometries and by inference have caused significant modifications to both the orientation and magnitude of the motions of the African sub-plates. Such plate motion processes have controlled the polyphase development of the West and Central African Rift System. On the basinal scale, changes of sub-plate motions have resulted in changes in the stress field which have had a clear impact on the deformation and fault geometries of rift basins and on the resulting stratigraphy. The construction of the first unified stratigraphic chart for the West and Central African Rift System shows a close correlation in the timing of the major unconformities with the timing of changes in relative plate motion as observed in the changes of the azimuthal geometry of the oceanic fracture zones in the Central Atlantic. Since similarly timed unconformities exist along the continental margins of Africa and South America, we propose that the causative mechanism is change in relative plate motion which leads to an increase or decrease in the tension on the plate and thus controls the strength or effective elastic thickness, Te, of the crust/plate beneath the margins. This results in a focused change in isostatic response of the margin during short-period changes in relative plate motion; i.e. more tension will mean that loads are not compensated locally resulting in local uplift of the margin.
129678114
Field study of the positive flower structures of the Gombe inlier, upper Benue trough, northeastern Nigeria system
The Gombe inlier can be considered as a geological microcosm of the Gongola basin as it vividly displays general stratigraphic and structural styles within a relatively small, easily accessible area. The Cretaceous lithostratigraphic sequence of the Gombe inlier has been affected by sinistral strike-slip faults; the Gombe and Wuro Ladde-Wurin Dole faults with a Benue and Gongola trend respectively.Detailed study of these two faults shows that the Gombe fault is older and synsedimentary fault movement started in at least the mid-Santonian. Precambrian basement upthrust along the Gombe Fault is likely to have initiated during the first of the two compressive events that affected the upper Benue trough; the mid-Santonian N — S to N 174° E compression. The Wuro Ladde — Wurin Dole fault is relatively younger because (1) it sinistrally truncates the Liji hill anticlines that occur at the northern end of Gombe hill, resulting in a NEE — SWW trend, and (2) the orientation of transpressional structures along this fault suggest they developed during the end-Cretaceous N 140° E to N 150° E compression.Well-preserved transpressional structures along these faults provide the structural geologist a rare field examples to study their 3-dimensional (3D) geometry and kinematic evolution; and also as an analog for interpreting seismic reflection data that images complex strike-slip structures in the sub-surface.
128715622
Thermal annealing of fission tracks in apatite: 1. A qualitative description
Confined fission-track lengths have been used to study the degree of annealing of induced fission tracks in samples of a single fluorapatite crystal (Durango apatite), heated for various times (between 20 min. and 500 days) at temperatures between 95° and 400°C.

In all annealed samples the mean confined track length is always less than that in unannealed control samples. As annealing progresses, the mean length is reduced and the length distribution broadens, slowly at first, and then more rapidly below a length reduction (ll0) of ∼ 0.65. In addition, the variation of track length with angle to the crystallographic c-axis becomes progressively more anisotropic. As the mean track length approaches zero, the only tracks left are aligned parallel to the c-acis.

In heavily annealed samples (ll0 < 0.65) observation of individual tracks after sequential etching steps shows the presence of unetchable “gaps” in a small proportion of tracks. The existence of these gaps is borne out by annealing studies of heavy-ion tracks in apatite slices. These studies also show the anisotropy characteristic of annealing in apatite.

The annealing of fission tracks in apatite appears to be characterised by two processes. For small degrees of annealing the dominant process brings about a progressive shrinking of the track from each end, with tracks perpendicular to the c-axis shortening more rapidly than those parallel to it. As annealing becomes more severe (ll0 < 0.65) the tracks begin to break up into discontinuous portions. Track length distributions in annealed apatites reflect the interplay of these processes.

The effect of apatite composition on annealing has been studied using spontaneous tracks in apatites from a sample in which considerable geological annealing has occurred. Electron microprobe studies show that apatite grains rich in Cl are more resistant to annealing, while fluorapatite is more readily annealed.
140642473
Evidence of the contrasted fission-track clock behavior of the apatites from In Ouzzal carbonatites (northwest Hoggar): The low-temperature thermal history of an Archean basement
This fission-track study is performed on the apatites from the In Ouzzal (northwest Hoggar, Algeria) carbonatite complex and permits us to present a complete thermal history of this Archean core. Two types of apatites, the pink ones (early carbonatites) and the green ones (late carbonatites), have been dated. The temperature during the Pan-African orogeny would have reached 100-120 °C in the In Ouzzal core, as both types of apatites from the carbonatites have recorded this event (500-630 m.y.). A mild Permian thermal event reset only the fission-track clock of the green, late apatites. This Permian thermal event is of quite a large extent in the Hoggar shield; it has also been detected in the Pharusian belt and in central Hoggar. This event is related to the opening of the Atlantic Ocean. The very high U-Th content of some of these apatites, and consequently, the amount of radiation damage in the mineral lattice, has been shown to possibly influence the fission-track clock behavior of these minerals. The F-Cl content of the apatites does not show any correlation with the degree of age reductions.
134940497
Thermal and exhumation history of Sakhalin Island (Russia) constrained by apatite U-Pb and fission track thermochronology
Abstract Sakhalin Island represents a key locality to study the tectonic evolution of the western Pacific. The island is located at the Amur-Okhotsk plate margin and records a complex thermotectonic history. Apatite double dating (U-Pb and fission track) and thermal history modelling were applied to three late Eocene granitoid massifs within central and southern Sakhalin: the Aniva, Okhotsk and Langeri complexes. Apatite U-Pb results yield consistent late Eocene (∼40–37 Ma) ages, suggesting rapid post-magmatic cooling. Apatite fission track results reveal bimodal age distributions with late Eocene – early Oligocene (∼38–33 Ma) and early Miocene (∼20–17 Ma) age populations that can be correlated with variations in Uranium and Chlorine concentrations. Thermal history modelling translates the AFT age bimodality into two-phase cooling histories. The timing of the early cooling phase (∼38–33 Ma) corresponds with the apatite U-Pb ages, indicating rapid cooling to at least ∼100 °C during the late Oligocene. The second cooling phase at ∼20–17 Ma cooled the samples to near-surface temperatures. Both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate, suggesting that cooling was a response to rapid exhumation. In addition, our data suggests that the studied terranes record differential exhumation with respect to the structural architecture. The Miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the Kuril Basin.
128416263
Apatite fission track and (U-Th)/He data from Fennoscandia: An example of underestimation of fission track annealing in apatite
Abstract Apatite fission track (AFT) data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation, often in the absence of any remnant sedimentary rocks or other supporting geological evidence. Reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic. Also, (U-Th)/He ages from cratonic igneous rocks that are older than paired AFT data are commonly discarded as ‘too old’ even when the former are repeatable at the 2σ confidence level. For example, AFT data from central Fennoscandia have been interpreted to reflect, first, burial by a Caledonian foreland basin several kilometres in thickness and, subsequently, its denudation. However, throughout Fennoscandia there is no direct evidence that such a deep, extensive foreland basin ever existed and indeed some geological evidence points to the contrary. Additionally, a significant number of (U-Th)/He ages from Fennoscandia are older than their AFT pairs but have been interpreted by their authors as ‘too old’ relative to the AFT ages and not geologically meaningful despite repeatable results. Here we propose a new interpretation of AFT data from Fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history. Discarding the hypothesis of a thick Caledonian foreland basin, we suggest that 1) AFT ages in Finland are ‘too young’ rather than that the (U-Th)/He ages are ‘too old’; 2) repeatable sequences of (U-Th)/He ages are more likely to record the true denudation history of cratons; 3) the importance of radiation-enhanced annealing at low temperatures has been overlooked; and 4) Fennoscandian AFT data may be divided into an eastern domain strongly affected by radiation-enhanced annealing (where current AFT annealing models are invalid) and a western domain much less affected by radiation-enhanced annealing (where current modelling methods may appropriately be used to extract thermal histories). In principle, radiation-enhanced annealing is also pertinent to the interpretation of young AFT ages from apatites rich in α-emitter actinides. The construction of a properly calibrated annealing model that fully incorporates the effects of radiation-enhanced annealing is fundamentally important.
205617732
In situ TEM observation of alpha-particle induced annealing of radiation damage in Durango apatite
A major issue in thermochronology and U-Th-Pb dating is the effect of radiation damage, created by α-recoils from α-decay events, on the diffusion of radiogenic elements (e.g., He and Pb) in host mineral. Up until now, thermal events have been considered as the only source of energy for the recovery of radiation-damage. However, irradiation, such as from the α-particle of the α-decay event, can itself induce damage recovery. Quantification of radiation-induced recovery caused by α-particles during α-decay events has not been possible, as the recovery process at the atomic-scale has been difficult to observe. Here we present details of the dynamics of the amorphous-to-crystalline transition process during α-particle irradiations using in situ transmission electron microscopy (TEM) and consecutive ion-irradiations: 1 MeV Kr2+ (simulating α-recoil damage), followed by 400 keV He+ (simulating α-particle annealing). Upon the He+ irradiation, partial recrystallization of the original, fully-amorphous Durango apatite was clearly evident and quantified based on the gradual appearance of new crystalline domains in TEM images and new diffraction maxima in selected area electron diffraction patterns. Thus, α-particle induced annealing occurs and must be considered in models of α-decay event damage and its effect on the diffusion of radiogenic elements in geochronology and thermochronology.
129893179
Lithospheric mantle evolution monitored by overlapping large igneous provinces: Case study in southern Africa
Abstract Most of the studies on the large igneous provinces (LIPs) focus on Phanerozoic times, and in particular, those related to the disruption of Pangea (e.g. CAMP, Karoo, Parana–Etendeka) while Precambrian LIPs (e.g. Ventersdorpf, Fortescue) remain less studied. Although the investigation of Precambrian LIPs is difficult because they are relatively poorly preserved, assessment of their geochemical characteristics in parallel with younger overlapping LIP is fundamental for monitoring the evolution of the mantle composition through time. Recent 40Ar/39Ar dating of the Okavango giant dyke swarm (and related sills) in southern Africa showed that ~ 90% of the dykes were emplaced at 179 ± 1 Ma and belong to the Karoo large igneous province whereas ~ 10% of dykes yielded Proterozoic ages (~ 1–1.1 Ga). Here, we provide new major, trace and rare earth elements analyses of the low-Ti Proterozoic Okavango dyke swarm (PODS) that suggest, combined with age data, a cognate origin with the 1.1 Ga Umkondo large igneous province (UIP), southern Africa. The geochemical characteristics of the PODS and UIP basalts are comparable to those of overlapping low-Ti Karoo basalts, and suggest that both LIPs were derived from similar enriched mantle sources. A mantle plume origin for these LIPs is not easily reconciled with the geochemical dataset and the coincidence of two compositionally similar mantle plumes acting 900 Myr apart is unlikely. Instead, we propose that the Umkondo and Karoo large igneous provinces monitored the slight evolution of a shallow enriched lithospheric mantle from Proterozoic to Jurassic.
129708615
Denudation history and landscape evolution of the northern East-Brazilian continental margin from apatite fission-track thermochronology
Abstract We reconstruct the history of denudation and landscape evolution of the northern East- Brazilian continental margin using apatite fission-track thermochronology and thermal history modeling. This part of the Brazilian Atlantic margin is morphologically characterized by inland and coastal plateaus surrounding a wide low-lying inland region, the Sertaneja Depression. The apatite fission track ages and mean track lengths vary from 39 ± 4 to 350 ± 57 Ma and from 10.0 ± 0.3 to 14.2 ± 0.2 μm, respectively, implying a protracted history of spatially variable denudation since the Permian at relatively low rates (  60 Myrs. In contrast, the coastal regions record up to 2.5 km of Late Jurassic-Early Cretaceous (150–120 Ma) denudation, coeval with rifting of the margin. The samples from elevated coastal regions, the Borborema Plateau and the Mantiqueira Range, record cooling from temperatures above 120 °C since the Late Cretaceous extending to the Cenozoic. We interpret this denudation as related to post-rift uplift of these parts of the margin, possibly resulting from compressional stresses transmitted from the Andes and/or magmatism at that time. Several samples from these areas also record accelerated Neogene (
53119015
Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean
Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ∼20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.
130632167
Transition from passive to active rifting: Relative importance of asthenospheric doming and passive extension of the lithosphere
We present quantitative modeling results of the dynamic interplay of passive extension and active convective thinning of the mantle lithosphere beneath intracontinental rift zones investigating the relative importance of thermal buoyancy forces associated with asthenospheric doming and far-field intraplate stresses on the style of rifting. To this aim we employ a two-dimensional numerical code based on a finite element method formulation for nonlinear temperature dependent viscoelastoplastic rheology. Brittle behavior is modeled using Mohr-Coulomb plasticity. The models support a scenario in which passive stretching leads to an unstable lithospheric configuration. Thermal buoyancy related to this asthenospheric doming subsequently drives active upwelling in a lithosphere scale convection cell. In the late synrift to early postrift the lithospheric horizontal stresses caused by the active asthenospheric upwelling may start to compete with the far-field intraplate stresses. At this stage the domal forces may dominate and even drive the system causing a change from passive to active rifting mode. If this transition occurs, the model predicts (1) drastic increase of subcrustal thinning beneath the rift zone, (2) lower crustal flow towards the rift flanks, (3) middle crustal flow towards the rift center, (4) the coeval occurrence of tensional stresses within and compressive stresses around the upwelling region, and (5) possible surface uplift. Late postrift thermal cooling removes the thermal buoyancy forces. At this stage the far-field forces dominate the stress state again and the lithosphere becomes more sensitive to small changes in the intraplate stresses. The model results may explain several key observations that are characteristic of a large number of intracontinental rift basins. These features include differential thinning of extending lithosphere, the discrepancy between fault-related extension and crustal thinning, late (end of synrift to early postrift) mantle related volcanism, surface domal uplift succeeding rifting, and rift flanks uplift associated with extension of a weak lithosphere.
128639328
Neotectonic reactivation of shear zones and implications for faulting style and geometry in the continental margin of NE Brazil
Abstract The eastern continental margin of South America comprises a series of rift basins developed during the breakup of Pangea in the Jurassic–Cretaceous. We integrated high resolution aeromagnetic, structural and stratigraphic data in order to evaluate the role of reactivation of ductile, Neoproterozoic shear zones in the deposition and deformation of post-rift sedimentary deposits in one of these basins, the Paraiba Basin in northeastern Brazil. This basin corresponds to the last part of the South American continent to be separated from Africa during the Pangea breakup. Sediment deposition in this basin occurred in the Albian–Maastrichtian, Eocene–Miocene, and in the late Quaternary. However, our investigation concentrates on the Miocene–Quaternary, which we consider the neotectonic period because it encompasses the last stress field. This consisted of an E–W-oriented compression and a N–S-oriented extension. The basement of the basin forms a slightly seaward-tilted ramp capped by a late Cretaceous to Quaternary sedimentary cover ~ 100–400 m thick. Aeromagnetic lineaments mark the major steeply-dipping, ductile E–W- to NE-striking shear zones in this basement. The ductile shear zones mainly reactivated as strike-slip, normal and oblique-slip faults, resulting in a series of Miocene–Quaternary depocenters controlled by NE-, E–W-, and a few NW-striking faults. Faulting produced subsidence and uplift that are largely responsible for the present-day morphology of the valleys and tablelands in this margin. We conclude that Precambrian shear zone reactivation controlled geometry and orientation, as well as deformation of sedimentary deposits, until the Neogene–Quaternary.
53614348
Tectonic history of the Kyrgyz South Tien Shan (Atbashi‐Inylchek) suture zone: The role of inherited structures during deformation‐propagation
[1] Multimethod chronology was applied on intrusives bordering the Kyrgyz South Tien Shan suture (STSs) to decipher the timing of (1) formation and amalgamation of the suturing units and (2) intracontinental deformation that built the bordering mountain ranges. Zircon U/Pb data indicate similarities between the Tien Shan and Tarim Precambrian crust. Caledonian (∼440–410 Ma) and Hercynian (∼310–280 Ma) zircon U/Pb ages were found at the edge of the STSs, related to subduction and closure of the Turkestan Ocean and the formation of the suture itself. Permian-Triassic (∼280–210 Ma) titanite fission track and zircon (U-Th)/He data record the first signs of exhumation when the STSs evolved into a shear zone and the adjacent Tarim basin started to subside. Low-temperature thermochronological (apatite fission track, zircon and apatite (U-Th)/He) analyses reveal three distinct cooling phases, becoming younger toward the STSs center: (1) Jurassic-Cretaceous cooling ages provide evidence that a Mesozoic South Tien Shan orogen formed as a response to the Cimmerian orogeny; (2) Early Paleogene (∼60–45 Ma) data indicate a renewed pulse of STSs reactivation during the Early Cenozoic; (3) Neogene ages constrain the onset of the modern Tien Shan mountain building to the Late Oligocene (∼30–25 Ma), which intensified during the Miocene (∼10–8 Ma) and Pliocene (∼3–2 Ma). The Cenozoic signals may reflect renewed responses to collisions at the southern Eurasian border (i.e., the Kohistan-Dras and India-Eurasia collisions). This progressive rejuvenation of the STSs demonstrates that deformation has not migrated steadily into the forelands, but was focused on pre-existing basement structures.
129127676
Confined fission track lengths in apatite: a diagnostic tool for thermal history analysis
Fission-track ages in apatite are generally accepted as giving a measure of the time over which a sample has been exposed to temperatures below approximately 100° C. A compilation of the lengths of confined fission tracks in a wide variety of apatites from different geological environments has shown that the distribution of confined track lengths can provide unique thermal history information in the temperature range below about 150° C over times of the order of 106 to 109 years. The distribution of confined lengths of freshly produced induced tracks is characterised by a narrow, symmetrical distribution with a mean length of around 16.3 μm and a standard deviation of the distribution of approximately 0.9 μm. In volcanic and related rocks which have cooled very rapidly, and never been reheated above about 50° C, the distribution is also narrow and symmetric, but with a shorter mean of 14.5 to 15 μm, and a standard deviation of the distribution of approximately 1.0 μm. In granitic basement terrains which are thought never to have been significantly disturbed thermally, since their original post-emplacement cooling, the distribution becomes negatively skewed, with a mean around 12 or 13 μm and a standard deviation between 1.2 and 2 μm.This distribution is thought to characterise slow continuous cooling from temperatures in excess of 120° C, to ambient surface temperatures. More complex thermal histories produce correspondingly complex distributions of confined tracks. The continuous production of tracks through time, coupled with the fact that the length of each track shrinks to a value characteristic of the maximum temperature it has experienced, gives a final length distribution which directly reflects the nature of the variation of temperature with time. Most distinctive of the myriad possible forms of the final distribution are the bimodal distributions, which give clear evidence of a two-stage history, including high and low temperature phases. The study of confined length distributions therefore offers invaluable evidence on the meaning of any fission-track age, and bears the potential of providing rigorous constraints on thermal history in the temperature regime below about 150° C. The results of this study strongly suggest that any apatite fission-track age determination should be supported by a confined track length distribution.
133909636
Meso-Cenozoic tectonic evolution of the Talas-Fergana region of the Kyrgyz Tien Shan revealed by low-temperature basement and detrital thermochronology
Abstract This study provides new low-temperature thermochronometric data, mainly apatite fission track data on the basement rocks in and adjacent to the Talas-Fergana Fault, in the Kyrgyz Tien Shan in the first place. In the second place, we also present new detrital apatite fission track data on the Meso-Cenozoic sediments from fault related basins and surrounding intramontane basins. Our results confirm multi-staged Meso-Cenozoic tectonic activity, possibly induced by the accretion of the so-called Cimmerian blocks to the Eurasian margin. New evidence for this multi-staged thermo-tectonic activity is found in the data of both basement and Meso-Cenozoic sediment samples in or close to the Talas-Fergana Fault. Zircon (U–Th)/He and apatite fission track data constrain rapid Late Triassic–Early Jurassic and Late Jurassic–Early Cretaceous basement cooling in the Kyrgyz Tien Shan around 200 Ma and 130–100 Ma respectively. Detrital apatite fission track results indicate a different burial history on both sides of the Talas-Fergana Fault. The apatite fission track system of the Jurassic sediments in the Middle Tien Shan unit east of the Talas-Fergana Fault is not reset, while the Jurassic sediments in the Fergana Basin and Yarkand-Fergana Basin, west of the fault zone, are partially and in some cases even totally reset. The totally reset samples exhibit Oligocene and Miocene ages and evidence the Cenozoic reactivation of the western Kyrgyz Tien Shan as a consequence of the India-Eurasia convergence.
131397981
U-Pb LA-ICPMS dating using accessory mineral standards with variable common Pb
Precise and accurate U-Pb LA-ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA-ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U-Pb LA-ICP-MS dating using a modified version of the VizualAge U-Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U-Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the Pb-204, Pb-207 or Pb-208((no Th)) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments. VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide "model" U-Pb fractionation curves to the time-resolved U-Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. Pb-204- and Pb-208((no) (Th)) corrected concordia diagrams and Pb-204-, Pb-207- and Pb-208((no Th))-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline. The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U-Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q-ICP-MS) coupled to a Photon Machines Analyte Excite 193 nm ArF Excimer laser. Madagascar apatite, OLT1 titanite and R10 rutile were used as primary standards and were corrected for variable common Pb using the new VizualAge_UcomPbine DRS. The secondary Durango (31.44 +/- 0.18 Ma) apatite standard yielded a U-Pb TW concordia intercept age of 31.97 +/- 0.59 Ma (MSWD = 1.09; primary standard corrected by the Pb-207-method) and a U-Pb concordia age of 31.82 +/- 0.40 Ma (MSWD = 1.4; primary standard corrected by the Pb-204-method). McClure Mountain (523.51 +/- 1.47 Ma) yielded a U-Pb TW concordia intercept age of 524.5 +/- 3.7 Ma (MSWD = 0.72) while the Fish Canyon Tuff (28.201 +/- 0.046 Ma) and Khan (522.2 +/- 2.2 Ma) titanite standards yielded U-Pb TW concordia intercept ages of 28.78 +/- 0.41 Ma (MSWD = 1.4) and 520.9 +/- 3.9 Ma (MSWD = 4.2) respectively. The suitability of the Pb-208((no Th))-correction is demonstrated by the agreement between a U-Pb TW concordia intercept age of 452.6 +/- 4.7 Ma (MSWD = 0.89) and a Pb-208((no Th))-corrected TW concordia age of 448.6 +/- 4.5 Ma (MSWD = 1.4) on a c. 450 Ma rutile which exhibits variable incorporation of common Pb. A range of LA-ICPMS U-Pb dating applications are presented and include U-Pb dating of apatite from >3.8 Ga gneisses from Akilia, SW Greenland. These apatites host C-13-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U-Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core-rim age differences of up to 300 Ma. These data explain the scatter and poor precision of earlier U-Pb apatite age determinations on Akilia apatite. Other LA-ICPMS dating applications include U-Pb apatite dating as a rapid method for determining the age of mafic intrusions, U-Pb titanite and apatite dating of ash fall tuffs, determining temperature-time histories using multiple U-Pb thermochronometers and improving concordance in LA-ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage. (C) 2013 Elsevier B.V. All rights reserved.
54765379
Iolite: Freeware for the visualisation and processing of mass spectrometric data
Iolite is a non-commercial software package developed to aid in the processing of inorganic mass spectrometric data, with a strong emphasis on visualisation versus time of acquisition. The goal of the software is to provide a powerful framework for data processing and interpretation, while giving users the ability to implement their own data reduction protocols. It is intended to be highly interactive, providing the user with a complete overview of the data at all stages of processing, and allowing the freedom to change parameters and reprocess data at any point. The program presents a variety of windows for the selection and viewing of data versus time, as well as features for the generation of X-Y plots, summary reports and export of data. In addition, it is capable of generating X-Y images from laser ablation rasters, and combining information from up to four separate elemental concentrations (intensities of red, green and blue, and the z-axis) in a false-colour three-dimensional image. By virtue of its underlying computing environment—Igor Pro—Iolite is capable of processing very large datasets (i.e., millions of timeslices) rapidly, and is thus ideal for the interrogation of multi-hour sessions of laser ablation data that can not be easily manipulated in conventional spreadsheet applications, for example. It is also well suited to multi-day sessions of solution-mode inductively-coupled plasma mass spectrometer (ICPMS) or thermal ionisation mass spectrometer (TIMS) data. A strong emphasis is placed on the interpolation of parameters that vary with time by a variety of user selectable methods including smoothed cubic splines. Data are processed on a timeslice-by-timeslice basis, allowing outlier rejection and calculation of statistics to be employed directly on calculated results. This approach can reduce the risk of processing biases associated with the manipulation of integrated datasets, while also allowing the implementation of more complex data reduction methods.
123982709
Statistics for Fission Track Analysis
Statistical analyses of the numbers, lengths and orientations of etched fission tracks in minerals yield dating and thermal history information in geoscience applications, particularly in oil exploration. Fission tracks can be represented mathematically by a stochastic process of line segments, randomly oriented in three dimensional space. This model provides understanding of observed data and a rigorous foundation for quantitative modelling and analysis. This book deals both with the formal mathematics and with its application to fission track analysis. Methods are also derived for analysing and interpreting fission track data, with application to designing laboratory studies andf to understanding field data. This book is a volume in the Chapman and Hall/CRC Press Interdiscplinary Statistics series.
53493394
Transdimensional inverse thermal history modeling for quantitative thermochronology
[1] A new approach for inverse thermal history modeling is presented. The method uses Bayesian transdimensional Markov Chain Monte Carlo and allows us to specify a wide range of possible thermal history models to be considered as general prior information on time, temperature (and temperature offset for multiple samples in a vertical profile). We can also incorporate more focused geological constraints in terms of more specific priors. The Bayesian approach naturally prefers simpler thermal history models (which provide an adequate fit to the observations), and so reduces the problems associated with over interpretation of inferred thermal histories. The output of the method is a collection or ensemble of thermal histories, which quantifies the range of accepted models in terms a (posterior) probability distribution. Individual models, such as the best data fitting (maximum likelihood) model or the expected model (effectively the weighted mean from the posterior distribution) can be examined. Different data types (e.g., fission track, U-Th/He, 40Ar/39Ar) can be combined, requiring just a data-specific predictive forward model and data fit (likelihood) function. To demonstrate the main features and implementation of the approach, examples are presented using both synthetic and real data.
131509995
A reporting protocol for thermochronologic modeling illustrated with data from the Grand Canyon
Apatite (U–Th)/He and fission-track dates, as well as ^4He/^3He and fission-track length data, provide rich thermal history information. However, numerous choices and assumptions are required on the long road from raw data and observations to potentially complex geologic interpretations. This paper outlines a conceptual framework for this path, with the aim of promoting a broader understanding of how thermochronologic conclusions are derived. The tiered structure consists of thermal history model inputs at Level 1, thermal history model outputs at Level 2, and geologic interpretations at Level 3. Because inverse thermal history modeling is at the heart of converting thermochronologic data to interpretation, for others to evaluate and reproduce conclusions derived from thermochronologic results it is necessary to publish all data required for modeling, report all model inputs, and clearly and completely depict model outputs. Here we suggest a generalized template for a model input table with which to arrange, report and explain the choice of inputs to thermal history models. Model inputs include the thermochronologic data, additional geologic information, and system- and model-specific parameters. As an example we show how the origin of discrepant thermochronologic interpretations in the Grand Canyon can be better understood by using this disciplined approach.