PLATE BOUNDARY INFLUENCES ON THE COMPLEX MIOCENE STRAIN FIELD OF THE LAKE MEAD REGION AND NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR, SOUTHERN NEVADA AND NORTHWESTERN ARIZONA: FROM PROPAGATING RIFTS TO INDENTER TECTONICS

Southern NV-northwest AZ record a complex Miocene strain field whereby large-magnitude E-W extension in the northern Colorado extensional corridor (NCREC) occurred proximal to major strike-slip faulting and N-S shortening in the Lake Mead region (LMR). Ernie Anderson’s foundational work documented the tantalizing complexities of both regions, and his legacy lives on in generations of subsequent research. Placing these disparate domains in the context of plate boundary events yields insights on their origin. Major ~E-W extension propagated north in the NCREC, culminating 19-16 Ma near the southern tip of NV, 16-13 Ma N-ward to the LMR, and 13-9 Ma in the western LMR. From 13-6 Ma, 60 km of dextral shear on the NW-striking Las Vegas Valley shear zone (LVSZ), tens of km’s of sinistral slip on the NE-striking Lake Mead fault system, and ~55 km of N-S shortening occurred in the LMR while extension to the south waned abruptly ~13 Ma.

The Walker Lane (WL) currently accommodates ~20% of the Pacific-North American dextral motion. The LVSZ is an early manifestation of the WL, as it developed ~13 Ma parallel to relative plate motion and inboard of where the San Andreas (SAF) initially developed following microplate elimination offshore southern CA, south steps in the Rivera triple junction, and increase in plate motions. N-S shortening and NE sinistral faults in the LMR directly east of LVSZ may have accommodated collision of more mobile crust against stable North America, resulting in localized indenter tectonics with nearby E-W extension partly induced by tectonic escape. However, WL initiation effectively terminated the N-ward propagating CREC. In the late Miocene, the south part of the transform shifted to the Gulf of California area (~13-6 Ma), the Big Bend developed, and plate motions changed from ~N60W to N37W (11-6 Ma). Coincidentally (~11-6 Ma), WL dextral shear largely shifted west from LVSZ to a NNW belt in the western Great Basin where the WL remains focused today. Dextral shear was favored there as it paralleled the new plate motion, aligned with the Gulf, and avoided the Big Bend. This west shift in the WL reduced rates of strike-slip faulting and N-S shortening in the LMR. These relations suggest that plate boundary events had a profound effect on the distribution of inboard dextral shear and deformation style in the Basin and Range.