Footwall rocks to the Mid‐Tertiary Chemehuevi Detachment Fault: A window into the middle crust in the Southern Cordillera

Abstract

Crystalline rocks unroofed from the middle crust during Tertiary crustal extension by the Chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently SW‐plunging stretching lineation, which are intruded by members of the voluminous laccolith‐shaped Chemehuevi Mountains plutonic suite. Proterozoic and Mesozoic gneisses, migmatites and early members of the Chemehuevi Mountains plutonic suite in the eastern Chemehuevi Mountains are deformed into a gently southwest‐dipping zone of protomylonite, mylonite and locally, ultramylonite in excess of 1.5 km thick. A belt of steeply foliated mylonitic gneiss up to 5 km across, traceable for 12 km along strike (NE‐SW) crops out in the northern part of the range. This belt passes abruptly downward into the subhorizontal shear zone exposed in the eastern Chemehuevi Mountains. A preliminary shear sense study indicates NE‐directed shear within the sub‐horizontal mylonitic rocks, and sinistral shear parallel to the steeply foliated mylonites. Deformation in these two zones took place under upper greenschist to middle amphibolite facies conditions. Mylonitic deformation associated with the two shear zones was spanned by intrusion of the Chemehuevi Mountains plutonic suite. Cross‐cutting contacts within the suite indicate that ductile deformation occurred after intrusion of hornblende‐biotite granodiorite. Four zircon fractions from this unit are discordant, but define a chord with a lower intercept of around 90 Ma and an upper intercept of ∼1900 Ma. Thirteen zircon fractions from two separate undeformed members of the suite (biotite granodiorite and porphyritic biotite granodiorite) are variably discordant, but fractions for each unit define chords that yield latest Cretaceous crystallization ages with upper intercepts of ∼1600 Ma. Together these data constrain the timing of plutonism and ductile deformation, with associated thick zones of fabric reorientation and mylonitic transposition, to the Late Cretaceous. Superimposed on this fabric and involving all rocks types in the footwall, are the localized brittle and ductile effects of the mid‐Tertiary extensional deformation. Movement direction and the timing of mylonitic deformation in the Chemehuevi Mountains suggest that the effects of the Cordilleran fold and thrust belt extends into the Chemehuevi Mountains region, where it in part overlapped with, and was heated by, plutonism along the eastern margin of the Mesozoic magmatic arc during late Cretaceous time.