HOW DOES THE EASTERN CALIFORNIA SHEAR ZONE PROJECT INTO SONORA, MEXICO?

Much discussion on reconciling the Late Cenozoic North American-Pacific relative plate motion budget has addressed the Walker Lane and Eastern California Shear Zone. An important piece of the story resides along the SE projection of these zones in Mexico. One broad constraint from latitudes 31^o-33^o N involves 370±50 km dextral offset of the Eocene Poway conglomerate from its northern Sonora bedrock source (Abbot and Smith, 1989). Only ~250 km of this displacement can be accommodated by the San Andreas-San Gabriel-Gulf of California fault system, implying that significant missing slip has accumulated on faults between the Gulf and central Sonora since Eocene time. Several candidate structures are identified in the Arizona-Sonora border region SE of Yuma. Across a 50 km–wide belt between Cerro Pinto, Sonora and Sierra Pinta, Arizona, distinctive geologic contacts are dextrally offset along five hypothetical NW-trending faults. Cumulative distributed displacement across this shear zone is 50±10 km. One structure records ~21km of dextral displacement of an Eocene(?) river channel and overlying Mid-Miocene volcanic strata from bedrock sources composed of distinctive Proterozoic and Cretaceous units. Other offset markers include various Late Cretaceous-Proterozoic intrusive contacts. This dextral shear zone projects SE into the Quaternary Pinacate volcanic field and does not appear to disrupt it. Farther SE near Caborca, Sonora, cryptic evidence for additional pre-Quaternary, post-Mid Miocene dextral faulting and associated vertical-axis block rotation is preserved. Jurassic arc strata of the Juarez-San Francisco mining district are broken into diamond-shaped blocks by NW dextral faults and NE sinistral faults that disrupt brittle structures related to Mid Miocene detachment faulting. Easterly mylonitic lineation of the 15 Ma Cerro Carnero core complex are misaligned from the regional N60E trend, suggesting block rotation between two NW-trending dextral faults transecting the Caborca-Altar valley. Such field relationships and the relative seismic quiescence of NW Sonora support existence of a significant dextral shear zone operative during the important Late Miocene time period within a region where 100+ km of right slip is needed to balance the relative plate motion budget.