Geology of the Purcell Trench rift valley and Sandpoint Conglomerate: Eocene en echelon normal faulting and synrift sedimentation along the eastern flank of the Priest River metamorphic complex, northern Idaho
The Priest River metamorphic complex is separated from less metamorphosed rocks to the east by the Purcell Trench, a north-trending, glaciated valley. Between Sandpoint and Bonners Ferry, Idaho, exposures of variably dipping Sandpoint Conglomerate are juxtaposed with mid-crustal metamorphic rocks that extend across the valley. The presence and location of faulting in the Purcell Trench have been controversial, and the age and tectonic significance of the Sandpoint Conglomerate have been enigmatic. New geologic mapping, metamorphic thermobarometry, and 40Ar/39Ar thermochronology verify the existence and location of the Eocene (45–55 Ma) east-dipping Purcell Trench normal fault zone, which consists of en echelon, right-stepping northern and southern fault segments. An unfaulted east-dipping homocline occupies the gap between them. It formed in the footwall of the west-dipping East Newport normal fault, which roots on the opposite side of the metamorphic culmination. “Chrontours” of K-Ar and 40Ar/39Ar cooling dates record progressive and nearly synchronous Eocene quenching of the metamorphic infrastructure as it was being exhumed by both the East Newport and the Purcell Trench normal faults. The Sandpoint Conglomerate is an Eocene basin-fill deposit that accumulated above the East Newport fault. Displacement on the east-dipping southern Purcell Trench fault eventually propagated across the footwall of the East Newport fault, transferring the Sandpoint Conglomerate to the hanging wall of the southern Purcell Trench fault and stranding it in the Purcell Trench. These relationships, which provide new insights on the process of extensional exhumation, should be helpful in tectonic interpretations of the Cordillera and other orogenic belts.