TIME SERIES PERSPECTIVE OF EVOLVING JURASSIC ARC BEHAVIOR IN THE CENTRAL SIERRA NEVADA ARC

In the central Sierra Nevada (CSN), the ~200 km wide Jurassic arc consists of 175-145 Ma volcanic rocks (peak at 168 Ma; shoulder at 150 Ma) and 168 to 145 Ma plutons (peak at 150 Ma) with the latter replacing ~16% of host rocks. Marine sediments deposited throughout the Jurassic show continuous Jurassic detrital zircon ages (peak at 165 Ma; shoulder at 150 Ma) with recycled zircons as old as 2.8 Ga forming a larger percentage of ages in early Jurassic samples.

Similar spatial and temporal geochemical patterns occur in volcanic, hypabyssal, and plutonic units with typical continental arc signatures of calcic (outboard) to calc-alkaline (inboard), magnesian, and metaluminous to (rarely) peraluminous compositions with a weak trend towards more mafic (Zr vs. SiO₂) and primitive (Sr_i, _eNd, Hf_zirc) through time. Sr_i, _eNd and Pb isotopes broadly become more enriched from west to east. Sr/Y ratios hint at Moho depths from ~15 km (west) to 30 km (east) with no measurable change through time.

Deformation in the western (WMB), central (CSN pendants) and eastern (White-Inyos) arc preserve a ~175 to 140 Ma, arc-scale, doubly-vergent, fan structure marked by a SW-vergent fold and thrust belt (20-40% shortening strains; WMB), steeply rotated and strained beds in CSN pendants (40-60% shortening strains) and the NE-vergent East Sierra thrust system (30% shortening strains; White-Inyos), the latter potentially linked to steep thrusts in the Ritter and Saddlebag pendants. Sinistral transpressive shear zones and Independence dikes recording local extension (13%) formed between 160-140 Ma in scattered localities within the fan. Evidence for regional extension is lacking, including during the Early Jurassic when other evidence is permissive of extension.

Thus in spite of significant arc-perpendicular shortening, with extension only prior to 180 Ma or locally afterwards, the Jurassic arc may have remained at or below sea-level, with a moderate to thin crust, due to its width, low volume of plutons, a dense root(?) and arc parallel extension.