We calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free‐energy minimization routine Perple_X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust‐forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53 wt % SiO2) and (ii) andesitic (64 wt % SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection.

Arc lower crust plays a critical role in processing mantle-derived basaltic melts into the intermediate continental crust, yet can only be studied indirectly or in exposed arc sections. Compared with the relatively well-studied oceanic arc sections (e.g., Kohistan and Talkeetna), the composition and formation mechanisms of continental arc lower crust remain less clear. Here we present a geochronological and geochemical study on the Lilong Complex and the Wolong granitoids from the Gangdese arc deep crustal section in southern Tibet. The Lilong Complex is composed of the early (85-95 Ma) mafic-intermediate sequence and late (85-86 Ma) ultramafic sequence. The Lilong crustal section exposed crustal depth extending from ~42-17 km based on the geobarometry. The maficintermediate sequence is a damp (low H2O) igneous differentiation sequence characterized by the subsequent appearance of pyroxene → plagioclase → amphibole → biotite. The ultramafic sequence represents a wet igneous differentiation sequence composed of olivine → pyroxene → amphibole → plagioclase. The 74-84 Ma Wolong granitoids were formed by fractional crystallization of wet magma and intra-crustal assimilation. Calculated seismic properties of the Gangdese deep arc crust are comparable to the average continental crust at similar depth.The average composition of the Gangdese arc lower crust is basaltic andesite with SiO2 of ~54 wt%. The highly incompatible elements in the Gangdese arc lower crust are systematically higher than those of the oceanic arc and are comparable with the estimates of lower continental crust, suggesting continental arc magmatism significantly contributes to the formation of continental crust.

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