Towards a Coupled Model for Deformation and Heat Transfer Around a Composite Felsic Intrusion

Part of the deformation that accompanies the emplacement of any pluton is caused by the thermal stresses and rheological changes that occur in its host rocks in response to the heat it releases. Although some aspects of the thermal evolution of intrusions can be understood by studying bodies with simple emplacement geometries (e.g., Yoshinobu et al., J Struct Geol, 20:1208-1218; Hirt, GSA Bull, 119:1185-1200) such models do not capture the feedbacks that occur when deformation and heat transfer occur simultaneously around an intrusion growing in a medium with temperature-dependent physical properties. As a step towards developing such a coupled model we have used 2-D finite element simulations to study the thermal evolution of a composite intrusion emplaced during a series of discrete deformation and heat transfer steps. During each deformation step elastic host rock is displaced in response to stresses created by: (1) an elliptical, sheet-like intrusion with a constant internal pressure and final volume, and (2) differential thermal expansion due to the temperature gradient across the modeled region. During the succeeding heat transfer step latent and sensible heat from the crystallizing intrusion are conducted through the deformed region for a specified interval. This sequence of steps was repeated to build an intrusion similar in size and emplacement history to the Mount Whitney Intrusive Suite of California’s Sierra Nevada. The model indicates that vertical displacements of the surrounding crust accommodate most of the intrusion’s growth, and that downward movement of the warmer underlying crust is greater than upward movement of the cooler overlying crust due to greater thermal expansion. Construction of the intrusion by emplacing progressively shorter sheets over time, consistent with the suite’s outcrop geometry, leads to thickening of its central part. This development of a thicker central region may be linked to the more negative gravity anomaly observed over the center of the suite and to the more extensive internal differentiation of its youngest member.