Late Cretaceous intra‐arc thrusting in southern California

Abstract

Thick zones of mylonitic rock are exposed in or between a number of the suspect crystalline terranes in the Transverse Ranges and eastern Peninsular Ranges of southern California. After palinspastic reconstruction of strike‐slip displacements along Neogene faults in the region the mylonite zones appear in a semicontinuous belt several hundred kilometers long. Mylonitic deformation in these areas is of a similar style and age. Lineation trends in the mylonitic rocks are closely aligned when palinspastic reconstruction takes into account the block rotations in the region, and the sense of shear is consistently west directed. The mylonite zones appear to have formed concurrently in a major midcrustal thrust system that juxtaposed the basement terranes during the Late Cretaceous. A kinematic model is proposed in which a west directed ductile thrust zone along the eastern side of the Peninsular Ranges is offset by a sinistral tear fault or lateral thrust ramp along the southern margin of the San Gabriel Mountains from a ductile thrust underlying crystalline basement terranes to the north. This model requires the suspect basement rocks of the Peninsular and Transverse Ranges to be linked tectonically by the latest Cretaceous. This Late Cretaceous thrust system also appears to continue directly into autochthonous basement rocks in the San Bernardino Mountains. The Late Cretaceous westward displacement of basement rocks in the Transverse Ranges relative to the northern end of the Peninsular Ranges batholith is mirrored by similar tectonic movement around the southern end of the Sierra Nevada batholith. It is argued here that a formerly continuous batholithic belt was breached by ductile west directed thrusting during the Late Cretaceous between the two still recognizable batholiths. The Salinia terrane represents the westernmost portion of this westward escaping arc segment and would have been established as a crustal salient by the latest Cretaceous. This model implies that the basement terranes involved in this thrusting episode are parautochthonous fragments of a Cretaceous continental arc and refutes suggestions that they have been accreted in the Cenozoic.