Flexure and isostatic residual gravity of the Sierra Nevada

Abstract

The Sierra Nevada of California displays an isostatic gravity anomaly of +35 mGal over the western foothills and −45 mGal at the crest of the range. A flexural isostatic model designed to explain the rapid Neogene uplift of the Sierra Nevada is consistent with these observations and can explain a portion of both gravity anomalies by contributions from Moho depths. The present‐day crustal root causing regional compensation in this model differs from the simple local Airy compensation assumed for the Sierra in calculating the isostatic residual gravity. Two‐dimensional gravity modeling indicates that this difference results in a residual gravity high of +9 mGals over the western foothills, which are buoyed up by the excess root to the east, and a low of −16 mGals over the crest of the Sierra. The remainder of the isostatic gravity anomaly is attributed to upper crustal density contrasts because high‐density rocks underlie the foothills, and a low‐density batholith forms the crest. Any model for the isostatic residual gravity must assume, implicitly or explicitly, a flexural response of the lithosphere. With the stiff Sierran lithosphere, the isostatic deflection of the Moho due to the upper crustal bodies will be small. If a similar gravity model were to assign no stiffness to the lithosphere (local Airy compensation), the mass anomalies of the upper crustal bodies would necessitate a change in depth of the Moho to achieve compensation. This deflection of the Moho would significantly reduce the amplitude of the anomaly produced by the upper crustal block. For the unbroken, pre‐uplift case of the Sierra Nevada of 10 Ma, our model predicts an isostatic gravity low over the range crest of approximately −80 mGals and a flanking high of +55 mGals. These values are larger than would be predicted for a more typical eroded magmatic arc that does not have abnormally low heat flow and consequent high flexural rigidity.