CONTINUATION OF EASTERN CALIFORNIA SHEAR ZONE INTO SOUTHWEST ARIZONA AND NORTHWEST SONORA, MEXICO

We present a reconstruction of Late Cenozoic extension and dextral shear within a region of southwest Arizona and northwest Sonora, Mexico, roughly on strike with the Eastern California Shear Zone. Our analysis utilizes mapped geologic tie points across generally buried faults between Yuma-Puerto Penasco and Ajo-Sonoyta-Quitovac. Displaced rocks include Late Cretaceous plutons of the Gunnery Range batholith, intruded into Proterozoic basement and overlapped by Pliocene-Quaternary basalts of the Pinacate volcanic field. Strain is distributed across five northwest-trending structures with right-lateral displacements as great as 30 km. Noteworthy piercing relationships are: (1) offset of an Eocene conglomerate at Sierra Los Alacranes (Sonora) from distinct bedrock sources in Sierra Choclo Duro, (2) separation of a prominent Proterozoic-Cretaceous contact in Sierra Pinta (Arizona) from the Drift Hills, and (3) restoration of oblique extension across the Mohawk detachment by matching range boundaries and re-entrants farther south near Sierra Pinta and Sonoyta, Sonora. Additional large displacement is inferred for a structure near Yuma by restoring Proterozoic gneiss of Pilot Knob to the southern Gila Mountains. Our model yields aggregate dextral displacement of 71 km, coupled with 34 km east-west extension. Additional extension (and dextral shear) is implied by several other alluvial valleys across which geologic ties are not constrained.

Kinematic connection to the Eastern California Shear zone is feasible if one employs the northeast-trending Gila River lineament as a clockwise-rotating sinistral transfer structure akin to the Garlock fault and the Chiriaco-Pinto Mtn fault system of the eastern Transverse Ranges. Projection of the Sonoran faults farther southeast is permissible given the wide gaps between ranges. Our model alleviates the “missing slip” problem recognized by Atwater and Stock (1998) and Oskin and Stock (2003), in which approximately 75 km of dextral shear between 12 to 6 Ma is needed across coastal Sonora to attain the 375 km separation of the Eocene Poway conglomerate from its Sonora bedrock source (Abbot, 1989). Faults identified in our reconstruction are strategically located to resolve that issue. Additional work is required to constrain their timing but disrupted of Middle to Late Miocene basalts in the region offer potential control.