InSAR Measurements and Theoretical Modeling of Deformation due to Large Mid-crustal Magma Bodies: Investigation of the Dynamics and Timescales of Crustal Anatexis

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Fialko EAR-0208165 The proposed models regarding characteristic spatiotemporal scales of formation of parental silicic magma bodies in the continental crust range from partial melting of the tectonically thickened fluid-rich lower crust to magmatic underplating, whereby the heat required to produce crustal melting is advected by intrusions of mafic sills (presumably, from the mantle source). These models predict very different time scales for the production of granitic magma. The pertologically-based model of slow equilibrium melting of the lower crust implies that granites are generated on "orogenic'' time scales of the order of millions of years. In contrast, theoretical models of magmatic underplating predict that anatectic melts are produced on quite short timescales of the order of the crystallization time of typical mafic underplates (e.g., 100-1000 years for sill intrusions that are a few tens to a few hundred meters thick). In principle, the intrusion of mafic underplates, the volume changes associated with in situ melting, and the subsequent evacuation of the resulting granitoid magmas can each generate geodetically observable deformation. Geodetic measurements in areas of contemporaneous large active magma bodies may therefore provide critical constraints on the timescales and dynamics of crustal anatexis. This project uses Interferometric Synthetic Aperture Radar (InSAR) observations in regions of the ongoing crustal magmatism to constrain typical rates of the large-scale melt generation and/or migration, and to test the proposed models of the granitic melt production. The primary targets include large mid-crustal magma bodies imaged by seismic studies, in particular, the Socorro (New Mexico, USA), the Altiplano-Puna (South America), and the Southern Tibet (Asia) magma bodies. The observed spatial patterns and average rates of magma-induced deformation are combined with theoretical modeling that explicitly includes thermodynamics of melting/freezing, realistic variations in the mechanical properties of the host rocks, and inelastic deformation, to develop a detailed quantitative understanding of the dynamics of crustal anatexis. Independent observables that are brought to bear on the model results include constraints on the lateral extent and thickness of the mid-crustal magma bodies from seismic imaging, inferences about the duration and total amplitude of the magma-induced uplift from geomorphologic studies, and structural and geochemical signatures of the exhumed ancient magma bodies. Results of this work are relevant to interpretations of seismic observations of "bright spots'' in the mid-to-lower continental crust, field and theoretical studies of the crustal magmatism, and origin and evolution of the continental crust.