Analysis of fault populations in western Spitsbergen: Implications for deformation partitioning along transform margins
Mesoscale faults, expressed in the Paleogene Central basin and Cretaceous basement along the West Spitsbergen orogenic belt, are used to define the history of continental deformation associated with the development of the transform system that accompanied the opening of the Eurasian basin and the Norwegian-Greenland Sea. A fundamental change in the orientation of faults from dominantly north-south in the basal part of the basin to northeast striking in the upper units broadly correlates with a major shift in plate motion during late Paleocene–early Eocene time. The two main populations of faults are interpreted as extension fractures that were variably reactivated as shear fractures and formed in response to north-south compression followed by northeast-southwest compression. Paleostress stratigraphy, or the analysis of striated faults measured in successive stratigraphic units of the Central basin, corroborates this interpretation and allows a time sequence of paleostress tensors to be defined. Results show that a Late Cretaceous-early Paleocene north-south compression, consistent with plate motion, was followed by three relatively short-lived and regionally limited faulting events leading up to a change in plate motion during late Paleocene time (post-Chron 25). Major transpressive tectonism followed, producing northeast-trending fractures that were reactivated as dextral and sinistral faults defining a prolate-shape paleostress ellipsoid with a horizontal σ 1 . This regionally reproducible tensor, consistent with the prediction of dextral transpression along the paleotransform, was probably initiated at approximately the Paleocene-Eocene boundary. Finally, a post–early Eocene deformation, pervasively recorded in the study area by an oblate shape paleostress ellipsoid with horizontal σ 3 , is tentatively correlated with the dextral transtension that characterized the post–Chron 13 opening of the Norwegian-Greenland Sea and Eurasian basin. This study offers documentation of deformation partitioning during the evolution of a transform margin. The Late Cretaceousearliest Paleocene compression associated with the prototransform did not involve substantial partitioning, and a north-south maximum principal stress pervaded the continental crust away from the transform. In contrast, the late Paleocene–Eocene transpression reflects total partitioning of deformation along the evolved, mature transform margin. Oblique plate motion was presumably partitioned into a strikeslip zone(s) offshore, whereas the onshore Central basin recorded only the orthogonal component of oblique convergence and divergence, a pattern similar to modern orogenic belts developed in zones of oblique plate motion.