Thermal Evolution of Crustal Magmatic Systems in Extensional Settings

Extensional tectonic environments can occur in a variety of magmatic settings including rifted arcs, continental rift zones and oceanic rift zones, and can encourage successive magma intrusions and lithospheric thinning. Both extension and intrusion modify the thermal, mechanical, and chemical structures of the crust. In this work, we monitor the crustal evolution in diverse extensional settings through a two-dimensional thermal model. The model simulates repetitive basaltic intrusions from the mantle-crust boundary and covers a wide range of tectonic extension (2 cm/yr to 2 mm/yr) and magma intrusion rates (10^-2 to 10^-3 m^3/m^2/yr) with varying magmatic water contents (wet: 5.6 wt.%, dry: 0.5 wt.%, and anhydrous: 0.0 wt.% H2O). The model tracks the amount of mantle-derived and crustal melts, the efficiency of crustal melting, and volume, composition, and fraction of melt in the magmatic system. In particular, we give constraints on the melt intruded and crystallizing from the mantle as well as the amount of crustal melt that is generated. We find that extensional tectonic rates and basalt flux have a coupled control on melt amount in the crust and the influence of basalt flux on melt generation typically dominates over extension. The total amount of melt residing in the crustal column typically increases with increasing tectonic extension rates and basalt flux, and for hydrous conditions is dominated by the residual mantle-derived melt, while more crustal melting can occur with the same crustal lithologies and anhydrous mantle melts. The efficiency of crustal melting in hydrous environments with high flux conditions reaches its maximum value of 5%. We report on the optimal crustal melting conditions as well, occurring in regions of extreme extension. Millions of years of coupled tectonic extension and repeated basaltic injections generate a thermally evolved crust and create an extensive partially melted zone in the lower crust. The zone is mainly dominated by low melt fraction (<0.3) after 2x10^6 years of high basalt flux. We also find that there exist a number of transient partially melted regions in the crust and we track their size and longevity. The results allow us to evaluate the role tectonics plays in augmenting melt production and crustal evolution in active magmatic systems.