Melt evolution and residence in extending crust: Thermal modeling of the crust and crustal magmas

Abstract

Available online xxxx Editor: T. Elliott Keywords: thermal model tectonic extension crustal melting fractionation crystal mush magma longevityTectonic extension and magmatism often act in concert to modify the thermal, mechanical, and chemical structure of the crust. Quantifying the effects of extension and magma flux on melting relationships in the crust is fundamental to determining the rate of crustal melting versus fractionation, magma residence time, and the growth of continental crust in rift environments. In order to understand the coupled control of tectonic extension and magma emplacement on crustal thermal evolution, we develop a numerical model that accounts for extension and thermal-petrographic processes in diverse extensional settings. We show that magma flux exerts the primary control on melt generation and tectonic extension amplifies the volume of melt residing in the crustal column. Diking into an extending crust produces hybrid magmas composed of 1) residual melt remaining after partial crystallization of basalt (mantle-derived melt) and 2) melt from partial melting of the crust (crustal melt). In an extending crust, mantle-derived melts are more prevalent than crustal melts across a range of magma fluxes, tectonic extension rates, and magmatic water contents. In most of the conditions, crustal temperatures do not reach their solidus temperatures to initiate partial melting of these igneous lithologies. Energy balance calculations show that the total enthalpy transported by dikes is primarily used for increasing the sensible heat of the cold surrounding crust with little energy contributing to latent heat of melting the crust (maximum crustal melting efficiency is 6%). In the lower crust, an extensive mush region develops for most of the conditions. Upper crustal crystalline mush is produced by continuous emplacement of magma with geologically reasonable flux and extension rates on timescales of 10 6 yr. Addition of tectonic effects and non-linear melt fraction relationships demonstrates that the magma flux required to sustain partially molten regions in the upper crust is within the range of estimates of magmatic flux in many rifting regions (∼10 −4 to 10 −3 km 3 /yr) and at least an order of magnitude lower than previous modeling estimates. Our results demonstrate the importance of tectonics in augmenting melt production, composition, and crustal evolution in active magmatic systems.

High‐Resolution Numerical Modeling of Heat and Volatile Transfer from Basalt to Wall Rock: Application to the Crustal Column beneath Long Valley Caldera, CA

“…Among the parameters explored in this study, three exert the strongest control: (1) random versus fixed emplacement of sills, (2) the initial depth range of sill emplacement, and (3) the time interval over which a thickness of basalt is emplaced. These results are broadly consistent with previous work in the literature (e.g., Annen et al, ; Colón et al, ; Dufek & Bergantz, ; Karakas & Dufek, ; Leeman et al, ). An important factor additionally explored in this study is the development of a mixed crustal lithology as the crust is gradually heated through the periodic and random emplacement of basaltic sills.…”

Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)

“…These conditions were attained during the evolution of the Sesia Magmatic System, where the crustal section was thickened by the Variscan orogeny and likely pre-heated by the emplacement of the huge lower Mafic Complex and by sporadic mafic pulses at higher crustal levels (Klötzli et al, 2014;Sinigoi et al, 2011). The transtensional tectonic of Permian Europe (Wilson et al, 2004) is consistent with extension having been moderate enough to avoid efficient diking, which would promote a different scenario, with minimal assimilation, as observed in continental flood basalts and predicted by thermal models for extending crust (Karakas and Dufek, 2015). The proposed model may be effective if and when crustal rocks above underplated basalt exceed their solidus in a hot layer whose thickness depends on the established geotherm, a condition which may trigger the process of density-driven stoping.…”

“…Karakas and Dufek, 2015) and thus call for fractionation as the primary means for producing silicic magmas (e.g. Jagoutz, 2010;Lee and Bachmann, 2014;Rioux et al, 2010;Walker et al, 2015).…”

Are granites and granulites consanguineous?

“…Karakas and Dufek, 2015). This process allowed the preservation of refractory granulite with remnants of crustal-sourced granite in the southwest, but to the northeast produced granites that require the involvement of an increasingly higher proportion of mantle melt in their genesis.…”