The role of hot crust in mountain building: Testing the alpha-beta quartz transition as a crustal geothermometer

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Of all physical parameters that control the behavior of the Earth's crust, temperature is arguably the most difficult to determine accurately. At the same time, temperature controls important processes such as melting (generation of magma), the depth at which deformation processes change from brittle (usually accompanied by earthquakes) to ductile (without earthquakes), and phase transitions between minerals that change the density of the crust, making it more likely to lift high terrain (less dense) or sink into the mantle (denser). The ability to determine crustal temperature is therefore important for the prediction of a wide range of crustal behaviors. Quartz makes up a large portion of the Earth's crust and undergoes a change in mineral structure at a known temperature. This change, and therefore the temperature, can be measured by detecting a characteristic pattern in the velocity of seismic waves traversing the material. We use a systematic combination of seismic techniques to pinpoint the quartz transition and determine crustal temperature. The α- to β-phase transition of quartz occurs in a narrow temperature range that lies between 580 to 800°C for pressures seen from the Earth?s surface to ~40 km depth. The phase transition generates a sharp compressional seismic velocity (Vp) increase with no accompanying shear velocity (Vs) contrast, unlike other mechanisms such as melting and compositional boundaries, which create contrasts in both Vp and Vs. The hypothesis tested in this study is: In orogens with a hot felsic middle crust, an interface can be detected that generates P reflections but no P to S conversions, and the inferred temperature and pressure are consistent with the presence of the α-β quartz transition and the absence of melting. This study uses a systematic set of teleseismic observation techniques (tests for the occurrence of P-S conversions and P-P reflections/conversions from the same discontinuity) in combination with petrophysical modeling to investigate crustal temperature and its geodynamic consequences in the Himalaya-Tibet and Taiwan orogens; if successful, the method can be widely applied to constrain the temperature of orogens worldwide.