THE WALKER LANE AND GULF OF CALIFORNIA: RELATED EXPRESSIONS OF PACIFIC-NORTH AMERICAN PLATE BOUNDARY DEVELOPMENT

The Walker Lane (WL) and Gulf of California (GC) have undergone similar and, according to us, related evolution but are currently in different stages of development of the Pacific–North American plate boundary. Nevertheless, the GC illustrates the future of the WL. In both areas, a major episode of extension began ~12-13 Ma (proto-Gulf extension, GC) in what had been a continental volcanic arc. ENE proto-Gulf extension, which represented the orthogonal component of oblique Pacific–North American plate motion along the transform boundary west of Baja California (Stock and Hodges, 1989, Tectonics), was mostly sandwiched between the Cretaceous Peninsular Ranges batholith and the mostly 32-20 Ma Sierra Madre Occidental caldera province (SMO). In the WL, extension may have accommodated minor clockwise rotation of the Sierra Nevada block relative to the central Great Basin as western North America expanded toward the transform boundary. This interpretation implies that the amount of extension pre-dating strike-slip faulting should decrease northward along the WL. Extension in the WL was mostly sandwiched between the Sierra Nevada batholith and ~35-10 Ma calderas of the ignimbrite flare-up in central Nevada.

In both areas, extension avoided the mid-Cenozoic caldera belt. Calderas are underlain by large, crustal magma chambers, so a nearly continuous batholith probably underlies both belts. The core SMO remains unextended, but the central Nevada caldera belt has undergone ±10% WNW extension, probably related to WL dextral shear. Also in both areas, extension, and subsequently strike-slip faulting, cut through the Cretaceous batholiths because arc magmatism had heated and weakened the crust.

Strike-slip faulting and the plate boundary either totally (GC) or partly (WL) stepped into the area that had undergone arc magmatism followed by extension. Development of the ridge-transform system in the GC ~6 Ma placed the WL in a favorable position to accommodate part of the transform motion. We predict that WL strike-slip faulting will propagate northwestward, following the migrating Mendocino triple junction and arc, and that the WL – GC will eventually accommodate all plate boundary motion. Whether the WL part will be a single, major strike-slip system (e.g., San Andreas) or a “leaky transform” (e.g., GC) is unknown.