Late Cenozoic strike-slip faulting in the Mojave Desert, California

Recent tectonic models for southern California treat the entire Mojave Desert Block as the site of distributed simple shear during late Cenozoic time. These models consider that much of the region is composed of a series of narrow blocks, bounded by active NW striking, right-slip faults that have facilitated the distortion and rotation of the region about vertical axes during translations. As much as 100 km of cumulative right slip is predicted for these faults by some of these models. These kinematic models require that the faults of the Mojave Desert Block merge with the Garlock fault, which is viewed as the intact northern boundary that served to accommodate the distortion of the Mojave Desert Block by simple shear. Map-scale structural relations are used to test explicit and implicit features of kinematic models proposed for the region. These relationships indicate that late Cenozoic NW striking, right-slip faults of the Mojave Desert Block possess the following characteristics: (1) the faults are discontinuous, with only the Calico-Blackwater fault spanning the entire Mojave Desert; (2) the faults terminate before reaching the Garlock fault; (3) faults south of an irregular line extending from near Barstow eastward to Ludlow and to Soda Lake are continuous and well developed and have a cumulative net slip of >40 km, whereas faults to the north are discontinuous and display <12 km of right slip; and (4) there is a northwestward decrease in net slip along most of the faults. A new kinematic model is proposed to reconcile these new observations with existing data. We assert that integrated strain within the province since middle Miocene time is not regionally homogeneous as predicted by simple shear models but is instead partitioned into six major domains. The domains probably have deformed and rotated about vertical axes independently of each other and are separated by zones of shortening or extension or by strike-slip faults. Strike-slip faults and folding have likely accommodated internal deformation and rotation of some of the domains. The model predicts that the Mojave Desert has been the site of ∼65 km of right shear since middle Miocene time. The broad network of faults of the Mojave Desert Block along with similar strike-slip faults of the Death Valley region constitute a regional zone of right shear, named here, the Eastern California shear zone. Because of its probable physical connection to the San Andreas fault system, the Eastern California shear zone may have accommodated a significant portion of Pacific-North American transform motion. The Eastern California shear zone accounts for 9-14% of the total shear, predicted from plate tectonic reconstructions, along the Pacific-North American transform boundary since ∼10.6 Ma. The kinematic connection of the normal faults of the Death Valley region, with the San Andreas fault system via the faults of the Mojave Desert accords with the deduction of Atwater (1970) that late Cenozoic extension in portions of the Basin and Range province is related to Pacific-North American transform shear. Finally, the present arcuate trace of the Garlock fault is ascribed to oroclinal folding within the broad zone of distributed shear of the Eastern California shear zone.