WHAT CAN THE ALAMO MOUNTAIN AND PIRU CREEK SHEAR ZONES TELL US ABOUT CRETACEOUS TECTONICS IN SOUTHERN CALIFORNIA?

The beginning of the Laramide Orogeny approximately 90-80 Ma marks the transition from thin-skinned, Sevier style deformation to thick-skinned, basement involved thrusting and uplift, and has been attributed to the subduction of two relatively buoyant oceanic rises. The timing of this transition remains uncertain, and it is unclear what the tectonic response of the overriding crust was at the frontal arc during this transition. The Alamo Mountain (AM) and Piru Creek (PC) shear zones, located within the Transverse Ranges in Southern California, are believed to be part of a regionally extensive shear system that developed along the western edge of North America during the Late Cretaceous. These shear zones occur within Precambrian porphyritic gneiss and Mesozoic plutons (Lockwood Creek and Sewart Mountain plutons), none of which have been dated. High-strain rocks consist of quartzofeldspathic mylonites, phyllite and schists up to 300 m thick. Pressure temperature estimates indicate deformation conditions at 500-600°C and 4 kbar. Both shear zones record sinistral oblique motion with top to the NW sense of shear.

We report U-Pb laser-ablation-sector field-inductively coupled plasma mass spectrometry zircon dates from deformed and undeformed host rocks and dikes within the shear zone to bracket the timing of deformation. We find that two weakly deformed and mylonitic porphyritic granodiorites give similar ages of 1658 ± 21 and 1656 ± 10 Ma, respectively, and two banded quartzofeldpsathic gneisses give ages of 1700 ± 10 Ma and 1706 ± 12.0 Ma. Undeformed granodiorites from the Lockwood Creek Pluton at the margin of the shear zone gave two ages at 150.7 ± 0.9 Ma and 150.4 ± 1.2 Ma, and a mylonitic leucogranite dike, interpreted to have been derived from the Lockwood Creek Pluton, was dated at 146.7 ± 2.9 Ma. Undeformed biotite granites from the Sewart Mountain Pluton yielded ages at 65.5 ± 0.9 Ma and 73.3 ± 0.8 Ma, and field observations suggest that the margin of this pluton is deformed as well. These data suggest that deformation may be as young as 65 Ma; however, ongoing dating of deformed and undeformed dikes at the margin of the Sewart Mountain Pluton will help clarify the timing of deformation.