Late Cretaceous ductile deformation, metamorphism and plutonism in the Piute Mountains, eastern Mojave Desert

Abstract

Late Cretaceous deformation in the Piute Mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones. The shear zones often separate Proterozoic lithologies with strong competency contrasts, suggesting that Proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network. Shear zones now divide the crust into wedge and lozenge‐shaped blocks. The Piute Mountains can be separated into two kinematic domains, each characterized by a dominant sense of shear. Thrusting in the southwest directed kinematic domain ended at 85±7 Ma, the age of the late synkinematic East Piute pluton. Thrusting in the northeast directed kinematic domain could have lasted until 74±3 Ma, the age of the Lazy Daisy pluton. Postthrusting northwest‐southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage. F2 strain increases toward the south and is concentrated around the margins of the Lazy Daisy pluton. This deformation was synchronous with pluton emplacement. Late Cretaceous peak metamorphism outlasted all ductile deformation. Peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the Lazy Daisy pluton is approached. Temperature and pressure of approximately 450°C and 2.5–4.0 kbar were achieved during ductile thrusting. Temperatures of 500°‐540°C existed at the onset of upright open folding, and peak temperatures as high as 620°C near the Lazy Daisy pluton occurred after F2 upright folding. All penetrative deformation ended before 72–71 Ma, by which time the area had cooled to below 300°C (Foster et al., 1989).