Abstract

Structural relief resulting from middle Tertiary extensional deformation in the Chemehuevi Mountains of California exposes a unique cross section through an extensive (>280 km2) calc-alkalic, compositionally zoned, sill-like granitic intrusion of Late Cretaceous age. Minimum estimates for emplacement pressure, 4 to 6 kbar, imply that the Chemehuevi Mountains plutonic suite was initially intruded at mid-crustal depths and has undergone 10° to 15° of postemplacement tilting, tectonic denudation, and erosion. Reconstruction of the pre-Tertiary (pre-tilt) configuration suggests that this metaluminous to peraluminous granitic suite exhibits crude normal, vertical, and temporal zonation from granodiorite to granite. The zonation involves a decrease in age and an increase in silica away from the walls and roof, the youngest and most evolved members being concentrated toward the center and floor of the intrusion. The lower part of the intrusion had a flat floor, which was penetrated by at least three feeder dikes providing magma to the chamber. The chamber grew laterally in stages, each pulse being more evolved than the preceding one. Structural reconstruction indicates that the roof is less than 1 km above the exposed top of the intrusion, suggesting a tabular geometry with an aspect ratio of about 5:1. The magma apparently ponded along the contact between undeformed Proterozoic basement above and subhorizontally foliated mylonitic gneisses below, and was therefore structurally controlled. This reconstruction provides information on the geometry of mesozonal intermediate to silicic magma chambers. In addition, the restoration provides a rare opportunity to observe crosscutting relations between different types of mid-crustal structures (thick mylonitic shear zones, granitic intrusions, and temporally unrelated detachment faults), the geometry of which emphasizes the need for careful evaluation of seismic reflection profiles across complexly deformed and intruded continental crust.

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