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

Tectonophysics 477 (2009) 135–144

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Tectonophysics

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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 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 − : ðÞð: : 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 ðÞ− α Δ − β 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. We have therefore investigated the mantle. A weak prismatic region dipping 45° simulates a detachment thermal and mechanical evolution of McMCC via a series of 2D fault in the upper crust. The geotherm is based on a constant heat flow numerical experiments that address the impact of partial melting on imposed at the base of the model (0.022 W m− 2), a constant temperature, density, and viscosity. In this paper, we compare temperature imposed at the top (20 °C), a depth independent radiogenic simulation results for cases involving a partially molten (low viscosity) heat production in the crust (7.67·10−7 Wm−3)andzeroheatflow vs solid-state lower crust for different extension rates and different across the lateral sides of the model. For all rocks, we assume a heat types of strain localization agents. Strain, flow paths, and P-T-t paths capacity of 1000 J kg−1 K−1 and thermal diffusivity of 9·10−7 m2 s−1. generated from the models allow us to better interpret first-order The crustal thermal conductivity is therefore 2.45 W m−1 K−1.The characteristics of McMCC. temperature field is allowed to equilibrate until the Moho of the 60 km thick crust reaches 975 °C. 2. Numerical code and model setup Because the composition of the continental crust changes with depth, an increase in density is expected toward the Moho. However, we assume We use Ellipsis, a Lagrangian integration point finite-element code, that this density increase is balanced by the density decrease related to to solve the governing equations of momentum, mass and energy in thermal expansion, and therefore we choose a depth and temperature −3 incompressible flow (Moresi et al., 2001, 2002, 2003). The equations independent density for the continental crust of 2720 kg m . At room −3 fi of motion are solved using a multigrid solver coupled to a continuity conditions, the density of the mantle is 3370 kg m and has a coef cient −5 −1 equation via a multigrid Uzawa iteration. The explicit timestepping of of thermal expansion of 2.8·10 K . a Petro-Galerkin upwind method solves the energy equation (Moresi The crust and the mantle have a visco-plastic rheology with a and Solomatov, 1995, 1998). In Ellipsis, the stress tensor is decom- temperature and stress dependent viscosity for stresses below the posed into pressure p and deviatoric stress τ: yield stress, and a depth dependent plastic branch above it. We use power law relationships between strain rate and stress to describe dislocation creep. The viscosity varieshi with the temperature and stress σ τ − δ ð Þ − = ðÞ− = ij = ij p ij 1 η 1 1 n Q e 1 n n according to: = 2 A exp nRT . For the continental P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 137

Fig. 1. Model geometry, parameters and boundary conditions, showing the solidus and liquidus, temperature, melt fraction, strength as a function of depth, and the locations of Lagrangian markers used to track temperature and pressure history, ﬂow paths and ﬁnite strain in the central part of the model. The density in crust decreases with the melt fraction (cf. text) but is otherwise temperature independent. The models are 270 km long and 90 km deep. They include 15 km of air-like material, a 60 km thick continental crust and 15 km of mantle. A weak detachment-like rheological anomaly is introduced in the upper brittle crust to localize deformation. The thermal parameters are such that a 40 km thick crust would have a steady-state Moho temperature at 540 °C. Following thickening, thermal relaxation was let to proceed until the Moho temperature reached 975 °C. Then, extension is driven at both ends of the model by imposing a constant velocity condition.

Fig. 2. Comparison of experiments in which strain localization is initiated and proceeds from a point-like rheological anomaly located at the brittle/ductile transition (left column), and the same experiment (right column) with a detachment-like rheological anomaly dipping 45° to the right and cutting through the entire brittle crust and slightly extending into the ductile crust. In both experiments, strain rate is 2 10− 15 s− 1. 138 P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 crust, A, n and Q are those of quartz-rich rock (pre-exponent 250 kJ kg− 1 K− 1 is embedded into the energy equation. The density of − 6 − nc − 1 5 − 1 Ac =5·10 MPa s , nc =3, Q c =1.9·10 Jmol , Goetze, the partially melted region decreases linearly by 13% between the 1978). The pre-exponent factor for the fault is 10 · Ac which makes solidus and the liquidus (Clemens and Droop, 1998). In nature, the the fault ten times less viscous than the crust in which it is embedded. viscosity of partially melted regions likely drops over many orders of

The rheology of the mantle is that of dry olivine (pre-exponent Am = magnitude when the melt forms a connected network (Arzi,1978; Van 4 − nc − 1 5 − 1 7·10 MPa s , nm =3,Q m =5.2·10 J mol , Brace and Kohlstedt, der Molen and Paterson, 1979; Scaillet et al., 1996). In our 1980). experiments, the viscosity decreases linearly by 3 orders of magnitude In the crust and the mantle, frictional sliding is modeled via a Mohr when the melt fraction increases from 15 to 30%. When the melt

Coulomb criterion with a cohesion (C0) of 15 MPa and a coefficient of fraction is b15%, the viscosity of the melted crust is that of the non- friction (μ) of 0.44. The cohesion and coefficient of internal friction in the melted surrounding; when the melt fraction is N30%, its viscosity is a detachment fault are C0/10 and μ/10, respectively. In all material, the thousand times lower than in surrounding material. Rosenberg and yield stress linearly drops to a maximum of 20% of its initial value when Handy (2005) showed that significant weakening occurs at 7% melt the accumulated strain reaches 0.5 (cf. Wijns et al., 2005 for details). For fraction. In our experiments, the melt fraction increases up to 30% over differential stresses reaching the yield stress, the material fails and a few kilometres. Consequently, the results are not affected by shifting deformation is modeled by an effective viscosity: ηyield =τyield/(2 · E)in this critical melt fraction down to 2–12%. In the models, there is no which E is the second invariant of the strain rate tensor. For semi-brittle segregation of the melt from its source. Although this is a limitation of effects, we impose a maximum yield stress of 250 MPa for the crust, the code, it is a reasonable approximation for many McMCC in which 400 MPa for the mantle and 10 MPa for the fault. melt and solid fractions move en masse (Teyssier and Whitney, 2002; The crust has a solidus and liquidus with positive dP/dT that is Whitney et al., 2004). representative of fluid-absent partial melting reactions (Clemens and Moving boundary conditions are imposed at both lateral ends of Droop, 1998). We did not attempt to use the solidus/liquidus of any the model. We have tested a range of initial strain rates in between specific felsic lithology. The solidus and liquidus, which are defined as 6·10− 15 s− 1 and 2·10− 16 s− 1 (i.e., 25.5 mm/y to 0.85 mm/y at both polynomial functions of temperature, are adjusted to obtain a peak sides). These strain rates are only a guide, as they are achieved for the melt fraction of 35% at the Moho (Fig. 1). A latent heat of fusion of first increment of deformation and decrease as extension proceeds.

Fig. 3. All models have recorded 25% extension following 40 m.y. of extension at a strain rate of 2 10− 16 s− 1 (a, c), and 4 m.y. of extension at a strain rate of 2 10− 15 s− 1 (b, d). (a, b) Models with melt function turned off, contoured for temperature. (c–d) Models with melt function turned on contoured for temperature (c1–d1) and melt fraction (c2–d2). Flow paths of the deeper grid nodes are show as thick white lines. P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 139

Since deformation is strongly localized around the detachment fault, dome is most likely due to the initial dip direction of the planar zone of much higher strain rates are achieved around the MCC. A free slip weakness. In nature, the position of migmatite cores relative to MCC condition is imposed on both the upper and lower horizontal detachments could be used to infer the dip direction of the original boundaries of the models, and no vertical motion is possible along detachment fault. the base of the models. The latter condition could limit somehow the Models deformed at relatively low (2·10− 16 s− 1)andhigh capacity for the Moho to rise during extension. However, the hot (2·10− 15 s− 1) extension rates are compared for the same surface geotherm, which makes the sub-continental mantle very weak, extension (25%) reached after 40 m.y. (lower) and 4 m.y. (higher) enhances the ability of the mantle to flow horizontally and vertically extension rates (Fig. 3). In melt-absent experiments (Fig. 3aandb), above the base of the model, hence allowing the Moho to move higher extension rate promotes significant heat advection (relief of vertically. isotherms, Fig. 3b) and internal deformation of material in the upwelling region, resulting in a slightly wider dome. This result 3. MCC geometry, temperature and partial melt distribution seems at odds with results from physical experiments, which suggest that fast extension leads to distributed surface extension The distribution and nature of extensional features at the surface (Brun et al., 1994; Brun, 1999). This discrepancy is likely due to the are strongly affected by the distribution and shape of rheological more realistic visco-plastic stratification in our numerical experi- anomalies that existed in the crust before the onset of extension (e.g. ments compared to the shear stress discontinuity at the sand/ Dyksterhuis et al., 2007). A point-like anomaly results in symmetric silicone contact in physical experiments. extension accommodated by a graben bounded by two normal faults With the melt function turned on (Fig. 3c1–2 and d1–2), the core (Brun et al., 1999 and references therein) (Fig. 2). As extension complex broadens slightly relative to the no-melt cases. Slow proceeds (2·10−15 s− 1 up to 25% surface extension), both normal extension with melt (Fig. 3c1–2) enhances upward motion of deep faults remain active. A migmatite core, which crystallizes during its crust compared to the no-melt case (Fig. 3a), resulting in larger exhumation, is exhumed at the center of the graben, which remains exhumation of rocks that were once partially molten. Fig. 3c2 shows symmetric. In contrast, a normal fault anomaly cutting through the an antiform of crystallized partial melt (in blue), akin to a 3D upper crust leads to asymmetric extension accommodated by a migmatite dome that represents preferential exhumation of deep rolling-hinge detachment (Brun, 1994; Lavier et al., 1999). As rocks in the detachment footwall. The solidus at 40 m.y. (Fig. 3c2) is extension proceeds, strain localizes along the margin of the hanging relatively flat and located at about 27 km depth. In contrast, fast wall, and the migmatite core is asymmetrically exhumed beneath the extension (Fig. 3d1–d2) produces greater heat advection that results active segment of the detachment fault. Despite the symmetry of in upward motion of the partial melt layer. After 4 m.y., the solidus is kinematic boundary conditions, the crystallized migmatite core is located at ca. 7 km depth and the partial melt antiform is cored by a systematically shifted toward the hanging wall. Burg et al. (2004) have region of N30% melt at shallow levels (Fig. 3d1). The exhumation also noted an asymmetry in the metamorphic zoning of detachment- magnitude of the solidus, and the crystallization versus exhumation/ controled metamorphic domes, with higher grades lying against the deformation history, are the most fundamental differences between hanging wall. This shift does not exist when a point-like rheological models extending at fast and slow strain rates. Initially at a depth of anomaly is the strain localization agent. Therefore, the shift of the 37 km, fast extension brings the solidus to about 7 km to the surface in

Fig. 4. P-T-t paths in McMCC for slow and fast strain rates in (a, b) no-melt models and (c, d) melt-present models. The color of the P-T-t paths refers to the markers shown in the inset in (a); their original depths are 20, 30 and 40 km. 140 P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 a few m.y. In this case, cooling and crystallization of partially molten contrasting crystallization versus exhumation/deformation history crust occurs after the bulk of deformation accommodating the also suggests that low-pressure migmatites (Pb400 MPa, i.e b ca. exhumation of the dome, possibly even after extension has stopped, 15 km) require fast extensional strain rates, whereas migmatites as the solidus recedes downward during cooling of the crust. In restricted to medium to high-pressure (PN600 MPa) are prevalent in contrast, slow extension brings slowly the solidus to about 27 km to slow extension MCC. the surface over a few tens of my Fig. 3c1. In this case, a large volume of migmatite is advected upward through a quasi-static solidus (blue 4. Finite strain and P-T-t paths region in Fig. 3c1), as rocks are exhumed faster than the solidus. In the latter case, the bulk of deformation accommodating the dome Although the various MCC in Fig. 3 have similar shapes, their exhumation occurs after crystallization, and partial melting is internal deformation differs significantly. This is documented in Fig. 3 restricted to PN600 MPa (≥ ca. 22 km). This contrasted crystallization by contrasted flow paths and contrasted finite strains recorded by a versus exhumation/deformation history suggests that solid-state passive grid. In experiments with no melt, the bulk of the MCC region deformation is dominant in slow extension MCC, whereas weakly is involved in a combination of counter-clockwise rotation (dominant deformed domes are expected in fast extension MCC as crystallization in Fig. 3a), top to the left heterogeneous simple shear, and pure shear and cooling post-date the bulk of exhumation of the dome. The (dominant in Fig. 3b). Counter-clockwise rotation is the consequence

Fig. 5. Comparison of fast McMCC at 25% extension (67.5 km) for various models involving slightly different conditions. a/ Reference model same as in Fig. 3d2. b/ Same as a/ but this model involves a point-like rheological anomaly. c/ In this model the reduction in viscosity occurs within a melt fraction window of 2 to 12%. d/ In this model, the density of the partially molten region is melt-fraction independent. P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 141 of the interplay between asymmetric simple shear and isostasy, which some P-T-t paths show a sudden change from near isothermal produces a horizontal exhumation gradient, as shown by the flow decompression to near isobaric cooling (Fig. 4d) consistent with the paths in Fig. 3a. The top right corner of the grid is caught in the observed change in direction of the particle flow paths. detachment zone, recording a top to the right shearing. Prominent early upward motions and later minor horizontal motion characterize 5. Effects of critical melt fraction and buoyancy the flow path of the lowermost grid nodes (Fig. 3a and b). In melt present experiments, the later horizontal motion is more developed The flow paths of particles in fast McMCC models (Fig. 5a) are and heterogeneous bulk flattening dominates the finite strain of the affected not only by the nature of the rheologic heterogeneity that grid, except in the vicinity of the active segment of the detachment localizes emplacement of the MCC (point vs. fault, Figs. 2 and 5b), but fault, where top to the right simple shear dominates. Away from the also by the fraction of melt in the deep crust and the associated active segment of the detachment, there is a strong contrast between variations in viscosity and density structure. In the experiments the bulk simple shear that dominates the finite strain in melt-absent involving the presence of melt (Figs. 2 and 3c and d), viscosity was fast extension experiments, and the bulk pure shear which dominates dropped between 15 and 30 melt %, but what is the effect of lowering in slow extension melt-present experiments. this melt fraction on the behavior of the McMCC? Aviscosity reduction of P-T-t paths are very sensitive to strain rate but are not affected by the three orders of magnitude between 2 and 12 melt % results in weakening presence or absence of melt (Fig. 4), as long as convection does not affect the partially melted region at shallower depth, and therefore over a the particles. At low extensional strain rate (Fig. 4a and c), P-T-t paths larger volume (Fig. 5c). However, this effect is not sufficient to modify exhibit decompression and cooling along a geothermal gradient in the significantly the large-scale behavior of McMCC (cf. Fig. 5aandc).Inthe range of 20 to 35 °C km−1, with similar shapes in both melt and no-melt model initial condition (Fig. 1), the melt fraction increases nonlinearly experiments. In all experiments, the 20 km marker, located structurally (McKenzie and Bickle,1988), reaching 30% melt at about 8 km below the above the detachment fault (i.e., in the hanging wall), records near solidus. Therefore, a decrease in the critical melt fraction, at which a isobaric heating. This heating is due to the advection of hot material that large viscosity reduction occurs, has no significant rheologic effect. A low occurs preferentially beneath the hanging wall. At higher strain rate critical melt fraction may have significant implications for melt (Fig. 4b and d), most P-T-t paths show an episode of near isothermal migration and local rheology (e.g., Rosenberg and Handy, 2005)that decompression followed by a period of both decompression and cooling are not investigated here, but have little impact on the large-scale along a hotter geothermal gradient (35–65 °C km−1). Deep markers development of McMCC in both fast extension and slow extension cases. have P-T-t paths crossing from the kyanite to the sillimanite stability Another important question concerns the role of buoyancy in the field, whereas shallower markers cross from the kyanite to the upward flow of a partially molten layer. In order to test the buoyancy andalusite stability field. At high strain rate, and when melt is present, effect, we conducted an experiment in which melting was not

Fig. 6. Thor-Odin dome in the Shuswap metamorphic core complex (British Columbia): location of the Shuswap metamorphic core complex (a) and cross-section (b) across the Thor- Odin dome (after Teyssier et al., 2005), (c) comparison of P-T evolution of the migmatite core of the Thor-Odin MCC (thick black line) with P-T-t paths obtained from fast extension experiments. 142 P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 accompanied by density decrease in the post-solidus region. This separating high-grade metamorphic rocks from upper crust), but in experiment (Fig. 5d) confirms that the buoyancy force produces only detail display a wide range of styles that could be linked to the tempo minor differences in the geometry of flow in the partial melt layer (Brun et and duration of extensional deformation, as well as the volume of melt al., 1994; Tirel et al., 2004; Wijns et al., 2005; Gessner et al., 2007; Tirel et involved. The formation of most of the MCC in the northern Cordillera al., 2008). The main differences (cf. Fig. 5a and d) are a slight ballooning (Nevada, USA to British Columbia, Canada) involved crustal melting effect of the partial melt layer and a more continuous channel of high melt and the formation of migmatite domes (McMCC), whereas MCC in fraction above the Moho, and this likely allows a more effective material Arizona and southern California developed in a primarily solid-state transfer from the base of the crust into the MCC. However, upward flow is crust and are dominated by a simple shear detachment zone. determined mainly by a dynamic feedback between extension of the The Ruby Mountain–East Humboldt Range (R–EH) McMCC in the upper crust and the necessity for the low viscosity lower crust to fill the Basin and Range (Nevada) and the Shuswap McMCC in the Canadian zone of extension efficiently (Wdowinski and Axen, 1992). In other terms, Cordillera exemplify some of the contrasting features developing isostasy rather than buoyancy drives exhumation. This conclusion is valid under different conditions of melt and extension rate. The geometry, only when surface extension is the product of kinematic boundary P–T evolution, and extension/exhumation rates of the Shuswap core conditions imposed on the lateral sides of the models. Contrasting results complex, including the Thor-Odin migmatite dome (Fig. 6b) closely can be expected when volume forces, due to lateral variation in match our models of fast extension in the presence of melt (Fig. 3d). In gravitational energy, drive extension. In such a case, space for MCC is this migmatite-cored dome, crystallization of leucogranite and provided by internal redistribution of masses (Rey et al., 2001), in which migmatite leucosome occurred between 60 and 52 Ma (Vander- case the buoyancy of the melted lower crust is of fundamental importance. haeghe et al., 1999; Hinchey et al., 2006), followed by rapid cooling through mica 40Ar/39Ar closure temperatures at 49–48 Ma (Vander- 6. The Shuswap (British Columbia, Canada) and Ruby haeghe et al., 2003; Mulch et al., 2004; Teyssier et al., 2005). The Thor- Mountain–East Humboldt Range (Nevada, U.S.A) as Odin migmatite core is shifted toward the eastern detachment exemples of faster and slower McMCC (Columbia River), suggesting that this fault zone acted as an E- dipping rolling-hinge that separated the cool and thick foreland crust Metamorphic core complexes in the North American Cordillera to the east from the partially molten crust in the hinterland (Teyssier (Fig. 6a) share first-order characteristics (e.g. detachment faults et al., 2005). The P-T-t path of dome rocks records near-isothermal

Fig. 7. Ruby Mountain-East Humboldt Range metamorphic core complex (Nevada, U.S.A.): location of the Shuswap metamorphic core complex (a) and interpretative cross-section (b) across the R-EHR, (c) comparison of P-T evolution of the migmatite core of the R-EHR McMCC (thick black line) with P-T-t paths obtained from slow extension experiments. P.F. Rey et al. / Tectonophysics 477 (2009) 135–144 143 decompression from 800–1000 MPa to b500 MPa at ca. 750 °C years and fit the first-order features of material involved in fast (Fig. 6c; Norlander et al., 2002) under partial-melt conditions, extension models. Although the Ruby Mountain–East Humboldt indicating that melt crystallized at low pressure as in our models Range exhibits an episodic extension history over a relatively long (cf. Fig. 3d). The Thor-Odin example is representative of McMCC in the period of time, it fits in first approximation the thermal and structural northern Cordillera, as well as other core complexes that contain characteristics of slower McMCC. migmatite domes (e.g., Aegean; Himalaya). In contrast, the Ruby Mountain–East Humboldt Range McMCC (Fig. 7a) records a protracted history of extension and exhumation Acknowledgments that may have started as early as Late Cretaceous and continued fi during the Eocene to the mid- to late Oligocene, when the bulk of MCC This work bene ted greatly from the comments of O. Vander- development occurred (e.g. McGrew et al., 2000; Sullivan and Snoke, heaghe, R. Weinberg, T. Spell, C. Wijns and two anonymous reviewers. 2007). Partial melting in the lower crust, synchronous with extension Modeling was supported by AUSCOPE-NCRIS and Computational and exhumation, is evidenced by late Eocene and mid Oligocene Infrastructure for Geodynamics software infrastructure. P. Rey granitic plutons, dikes and sills strongly transposed into the exten- acknowledges a visiting fellowship from the University of Minnesota. sional mylonitic fabric (Wright and Snoke, 1993)(Fig. 7b). In the C. Teyssier and D.L. Whitney's research was partially supported by NSF migmatite dome, metamorphic assemblages record, from ca. 85 Ma to grant EAR-0409776. C. Teyssier was also supported by grant 200021- ca. 55 Ma, a first episode of syn-partial melting decompression and 117694 from the Swiss National Science Foundation. cooling from N900 MPa and 800 °C to 700 MPa and 650 °C, along a path sub-parallel to the kyanite-sillimanite transition (McGrew et al., References 2000) compatible with slow exhumation and crystallization at depth. From ca. 55 to ca. 40 Ma, decompression developed under near constant Arbaret, L., Bystricky, M., Champallier, R., 2007. 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