The Role of Partial Melting and Extensional Strain Rates in the Development of Metamorphic Core Complexes

Tectonophysics 477 (2009) 135–144 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto The role of partial melting and extensional strain rates in the development of metamorphic core complexes P.F. Rey a,⁎, C. Teyssier b, D.L. Whitney b a Earthbyte Research Group, School of Geosciences, University of Sydney, NSW2006 Sydney, Australia b Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA article info abstract Article history: Orogenic collapse involves extension and thinning of thick and hot (partially molten) crust, leading to the Received 11 September 2008 formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. Two- Accepted 12 March 2009 dimensional thermo-mechanical Ellipsis models evaluate the parameters that likely control the formation and Available online 20 March 2009 evolution of MCC: the nature and geometry of the heterogeneity that localizes MCC, the presence/absence of a partially molten layer in the lower crust, and the rate of extension. When the localizing heterogeneity is a Keywords: normal fault in the upper crust, the migmatite core remains in the footwall of the fault, resulting in an Continental extension Migmatite asymmetric MCC; if the localizing heterogeneity is point like region within the upper crust, the MCC remains Metamorphic core complex symmetric throughout its development. Therefore, asymmetrically located migmatite domes likely reflect the Gneiss dome dip of the original normal fault system that generated the MCC. Modeling of a severe viscosity drop owing to the Partial melting presence of a partially molten layer, compared to a crust with no melt, demonstrates that the presence of melt Modeling slightly enhances upward advection of material and heat. Our experiments show that, when associated with boundary-driven extension, far-field horizontal extension provides space for the domes. Therefore, the buoyancy of migmatite cores contributes little to the outer envelope of metamorphic core complexes, although it may play a significant role in the internal dynamics of the partially molten layer. The presence of melt also favors heterogeneous bulk pure shear of the dome as opposed to the bulk simple shear, which dominates in melt-absent experiments. Melt presence affects the shape of P-T-t paths only slightly for material located near the top of the low-viscosity layer but leads to more complex flow paths for material inside the layer. The effect of extension rate is significant: at high extension rate (cm yr−1 in the core complex region), partially molten crust crystallizes and cools along a high geothermal gradient (35 to 65 °C km−1); material remains partially molten in the dome during ascent. At low strain rate (mm yr−1 in the core complex region), the partially molten crust crystallizes at high pressure; this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20 to 35 °C km−1) during ascent. Therefore, the models predict distinct crystallization versus exhumation histories of migmatite cores as a function of extensional strain rates. The Shuswap metamorphic core complex (British Columbia, Canada) exemplifies a metamorphic core complex in which an asymmetric, detachment-controlled migmatite dome records rapid exhumation and cooling likely related to faster rates of extension. In contrast the Ruby Mountain-East Humboldt Ranges (Nevada, U.S.A.) exhibits characteristics associated with slower metamorphic core complexes. © 2009 Elsevier B.V. All rights reserved. 1. Introduction angle normal faults (detachments) to form the core of high-grade structural domes. McMCC offer an opportunity to study deep crustal Metamorphic core complexes (MCC) are common crustal-scale processes; they contribute to the thermal and mechanical re- features that develop when thermally mature orogens undergo equilibration and differentiation of orogenic crust, and, as we show horizontal extension. This extension can be driven either by volume here, record the kinematic boundary conditions prevailing during the forces (active extension) or surface forces (passive extension) final stages of orogeny. The structural and thermal characteristics of following a change in plate kinematics. Migmatite-cored MCC develop McMCC result from the interplay of plate boundary forces, viscous when partially molten continental crust is exhumed beneath low- forces (strength of the lithosphere) and body forces (induced by lateral variation in densities), and from syn-deformational processes that affect the rheology and/or buoyancy of the system, such as partial ⁎ Corresponding author. Tel: +61 2 9351 2067; fax: +61 2 9351 0184. melting and strain localization. Physical and numerical experiments E-mail address: [email protected] (P.F. Rey). have shown that strain localization is essential to generate 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.03.010 136 P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 metamorphic core complexes and is easily achieved around rheolo- with δ the Kronecker delta. The conservation equation for momentum, gical and/or density anomalies such as a pre-existing detachment fault in which f is the body force, is: in the brittle upper crust (Buck, 1993; Lavier et al., 1999; Koyi and τ − ð Þ Skelton, 2001; Dyksterhuis et al., 2007; Gessner et al., 2007), a ij;j pi = fi 2 rheological and/or density anomaly in the lower crust (Brun et al., υ υ 1994; Brun, 1999; Tirel et al., 2004, 2008), or a severe density jump subject to the incompressible continuity equation: i,j =0, in which along the brittle-ductile transition (Wijns et al., 2005). More recently, is the velocity. Acceleration is neglected as material deforms via slow fl Regenauer-Lieb et al. (2006) demonstrated that localization takes viscous ow. Temperature derives from solving the energy equation: place by natural physical processes even in the absence of any DT AT particular weakness zone. The shape and orientation of the strain u m Á Δ κj2 ð Þ A + T = T + H 3 localization agent may have important consequences for the asym- Dt t metry of McMCC. In particular, when strain localization is achieved via D κ with Dt the material time derivative, T the temperature, the thermal a weak layer beneath the brittle/ductile transition, normal fault diffusivity and H the rate of energy production (in our models this bounded symmetric graben accommodate extension in the upper amounts to the radiogenic heat in the continental crust and the fl crust (Tirel et al., 2004, 2008). In contrast, asymmetric ow unfolds constant heat flow at the base of the models). Ellipsis has been used to when a detachment-like rheological anomaly occurs in the upper model crustal deformation in extension (Wijns et al., 2005; Gessner et crust (Lavier et al., 1999). This detachment fault rotates into a sub- al., 2007) and compression (Mühlhaus et al., 2002). We have horizontal décollement (a rolling-hinge detachment) with ongoing implemented a melt function to account for the thermal and shear strain localized along the hanging wall block (Brun et al., 1994; mechanical effects of partial melting (cf. O'Neill et al., 2006 for Lavier et al., 1999). Upon extension, localized thinning of the upper details). This function however doesn't account for melt extraction fl crust is isostatically compensated by the ow of lower crust into the processes (i.e. compaction or diking) so the code is restricted to MCC (Block and Royden, 1990; Wdowinski and Axen, 1992). A weak models involving partially melted regions in which the melt remains lower crust is therefore important for metamorphic domes to grow in its source. For the melt fraction F, Ellipsis uses McKenzie and Bickle (Brun et al., 1994; Wijns et al., 2005; Tirel et al., 2008). The viscosity of (1988) parameterization, which expresses F as a function of the partially molten rocks decreases dramatically with increasing melt supersolidus temperature Tss, the solidus temperature Tsol and the fraction (Richet and Bottinga, 1995; Scaillet et al., 1996; Baker, 1998, liquidus temperature Tliq: Arbaret et al., 2007) and, in the common case of fluid-absent partial melting, density also decreases (Clemens and Droop, 1998). Because T ðÞz + T ðÞz T − sol liq most fluid-absent melting reactions have positive dP/dT slopes, a T = 2 ð4Þ ss T ðÞz − T ðÞz positive feedback between extension-induced decompression and liq sol melting could be important in the structural and thermal development of McMCC (Teyssier and Whitney, 2002; Whitney et al., 2004). The melt fraction is then: The degree of coupling between the brittle upper crust and the ductile ðÞ : 2 − : Á ðÞð: : Á Þ lower crust is also known to impact the structural evolution of MCC. FTss =05+Tss + Tss 0 25 0 4256 + 2 988 Tss 5 Physical experiments show that a strongly coupled continental crust leads to distributed surface extension, whereas a mechanical The density of partially melted regions changes with both T and F: decoupling between a very weak lower crust and a much stronger upper crust leads to localized surface extension (Brun et al., 1994; ρðÞ; ρ Á ðÞ− α Á Δ − β Á ðÞ T F = 0 1 T F 6 Brun, 1999). Partial melting produces a strong decoupling because it reduces drastically the viscosity of the lower continental crust. Very where α is the coefficient of thermal expansion and β is the coefficient of few studies have investigated the role of partial melting in the expansion related to phase change. The starting model (Fig. 1)is270km development of McMCC (Tirel et al., 2004, 2008) and none of them long and 90 km thick, and includes 15 km of air-like material (low take into account temperature-dependent melt fraction and/or melt- density, low viscosity) and a 60 km thick crust above 15 km of upper dependent viscosity and density.

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