The α–β Quartz Transition in Some Common Continental Crust Rocks: Implications for Crustal Seismology

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Abstract
Laboratory measurements show that quartz‐bearing rocks undergo a decrease in P‐wave velocity when approaching the α–β quartz transition, followed by an abrupt increase afterwards. S‐wave velocity changes only slightly. This change can have a signature in crustal geophysical data. Here, we use the AbersHackerMacro to explore how the modal abundance of quartz affects the Vp/Vs ratio and reflectivity across the α–β transition in a number of high‐grade metamorphic rocks. The modelling shows that even with a low modal abundance of quartz, the α–β transition is recognisable in the Vp/Vs ratios of these rocks. α quartz has a strong influence on these Vp/Vs ratios, whereas β quartz does not. There is an increase in the reflection coefficient across the α–β transition with an increase in modal abundance of quartz. Reflection coefficients between different lithologies with less than about 25% quartz can increase or decrease. With greater than 25% quartz, reflection coefficients decrease.

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