GULF OF CALIFORNIA AND EASTERN CALIFORNIA SHEAR ZONE – WALKER LANE BELTS: TWO MAJOR TRANSTENSIONAL BELTS IN THE GREATER PACIFIC – NORTH AMERICA PLATE BOUNDARY

The Gulf of California (GC) and Eastern California shear zone – Walker Lane belt (ECWLb) are two major transtensional belts in the Pacific – North America plate boundary; the major difference is that the GC has complete continental rupture, while the ECWLb is a lesser, but significant component of the plate boundary. The GC formed along a former Miocene volcanic arc and the eastern edge of the Cretaceous batholith; most of the ECWLb formed along the edge of the same batholith. GC rifting began at 16 Ma in the north and 12 Ma in the south. From 16/12 to 8/6 Ma, the margins of the Gulf were dominated by ENE – WSW to E-W extension. There was strain partitioning both across the plate boundary and within the proto Gulf. During proto GC, there was major strike-slip faulting west of Baja California, while minor? strike-slip and major normal faulting was in the GC. About 280 km of the 300 – 450 km of NW translation across the GC is known after 6.3 Ma in the north. The first marine incursion was at ~8 to 6.5 Ma from S to N. There was a major increase in strike-slip faulting and a localization of faulting toward the center of the Gulf after 6 Ma. The ECWLb and GC have many of the same traits. (i) Both belts developed from ~15/12 to 0 Ma. (ii) The GC localized along a former volcanic arc and both belts formed along a Late Cretaceous suture that guided later strain. (iii) Strain partitioning across the plate boundary in both regions has the more transtensional belt to the east and the more translational belt (Tocso – Abreojos fault, San Andreas fault) to the west. (iv) Strain partitioning within the belts has normal faulting along the western margin and more strike-slip faulting in the center. (v) Strain localized over time from the margins to the center of the GC, and this process is beginning in the ECWLb. (vi) The transform – ridge plate boundary in the GC formed from en echelon strike-slip – normal fault systems, while en echelon faults dominate the ECWLb. (vii) The en echelon fault systems commonly have transtensional basins at the margins. (viii) Along the E Sierra Nevada, there is a pattern of major normal faults that step left and north; this pattern is similar to parts of the E edge of Baja California along the GC plate boundary. Other workers have suggested that the ECWLb is evolving into a GC-like belt as the main plate boundary with a connection to the Juan de Fuca plate in SW Oregon.