TESTING TECTONIC MODELS FOR MESOZOIC SHORTENING, INTRUSION, AND EXTENSION IN SOUTHEASTERN CALIFORNIA USING DETAILED CRUSTAL THICKNESS DATA

Methods for estimating crustal thickness using geochemical proxies for igneous rocks have been applied to large regions and over long time spans to test models of many kinds. The methods use geochemical proxies sensitive to proportions of higher- and lower-pressure phases during melting. We applied these approaches in detail with a dense data set of 320 geochemical analyses across the eastern extents of batholiths ranging from ~180 Ma to ~69 Ma, in southeastern California. This 100 x 80 km area is one of frontal thrust belt activity, Late Cretaceous extensional tectonics, and multiple plutonic episodes; it stretches from the Bristol Mountains on the SW to the New York Mountains on the NE. We find the Sr/Y metric for crustal thickness to be unreliable for many Jurassic plutons, perhaps due to the shoshonitic affinity of the magmas and widespread albitization. Crustal thickness from the La/Yb method increases monotonically through time from ~48 km to ~60 km. Spatial patterns are difficult to discern in the noisy data, with all areas having approximately the same crustal thickness at a given time. However, isolated small 153-150 Ma plutons (n=3) and one 69 Ma pluton that average 40 km thickness lie on the east side of batholiths and may record thinner crust lacking the batholithic additions. These plutons are in different thrust plates, however, and they may simply represent statistical variability in a homogeneous population. These data can be interpreted as crustal thickening driven by thrust faulting and plutonism. Our data yield similar values to crustal thickness estimates using La/Yb ratios from Laramide aged plutons in southern Arizona, suggesting the crust was thickened prior to the Laramide orogeny. These data do not show purported excision from the base of the crust suggested to accompany ~80 Ma Laramide shallow subduction, although crustal thickening due to accretion of trench-derived schists may have balanced subduction erosion. In addition, signals of two widespread ~75-65 Ma mylonitic extensional systems are not seen, indicating that they were either too minor to be recorded by the methods employed or most extension occurred after emplacement of the ~69 Ma plutons and dikes used in our analyses. Using dense data at sub-regional scales places new limits on tectonic models and promises to be powerful.