Links between episodic Mesozoic crustal thickening events, angular unconformities, and magmatic surges in the Sierra Nevada, CA

Our new field mapping and geochronologic analyses have further constrained the significance of three major Mesozoic crustal thickening and erosional events resulting in three angular unconformities in the eastern Sierra pendants of the central Sierra Nevada (CSN). The oldest unconformity is the angular unconformity separating the Paleozoic allochthonous or autochthonous North America passive margin sequence from the Mesozoic arc volcanic sequences. Typically 100-150 m.y. of strata are missing across this contact. Timing of this unconformity is bracketed between 260 and 300 Ma: it is subsequently folded during a 219-174 Ma event that is constrained in the Twin Lakes area. The second unconformity separates the Late Triassic-Early Jurassic volcanics (Koip sequence) from Early Jurassic marine sediments (Sawmill Canyon sequence) across which 10-50 Myr strata are missing. The timing of this unconformity is bracketed between 195 and 185 Ma in Virginia Canyon area and 164 and 203 Ma in Ritter Range pendant. This timing is compatible with the event that folded the Pz/Mz unconformity. This unconformity is faulted by Late Cretaceous faults. The third unconformity separates 96-115 Ma Cretaceous volcanic-sedimentary rocks from tilted Jura-Cretaceous rocks and is intruded by Late Cretaceous plutons. The timing of this unconformity is bracketed between 115 and 132 Ma in eastern Sierran pendants and 110 and 148 Ma in Oak Creek pendant. These three unconformities, separated by crustal shortening and vertical thickening events suggest the deformation and erosion within the CSN arc are episodic and took place between periods of sedimentation and little crustal thickening. We propose that much of the deformation is driven by thrusts and ductile shortening. The Mesozoic magmatic flare-ups in the Sierra initiate at ~250, 180, 120 Ma; thus crustal thickening and erosion occurred before or during surge initiation indicating a connection between crustal thickening and increased volcanism and plutonism within the arc.