ARTICLE IN PRESS TECTO-124557; No of Pages 9 Tectonophysics xxx (2009) xxx–xxx Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Dynamic models for metamorphic core complex formation and scaling: The role of unchannelized collapse of thickened continental crust Rebecca Bendick ⁎, Julia Baldwin 1 Department of Geosciences, University of Montana, United States article info abstract Article history: Metamorphic core complexes at collisions between cratons and softer terranes, such as in the northern North Received 15 May 2008 American Cordillera, share a set of characteristic features including spatial and temporal association of Received in revised form 23 January 2009 ductile mid-crustal deformation with brittle normal faulting, spatial coincidence with prior crustal Accepted 19 March 2009 thickening, characteristic spatial scaling and limited duration and extent of deformation. These properties Available online xxxx are reproduced in numerical solutions for gravity-driven collapse of a viscous crustal region under conditions where vertical stress is continuous through thickened lithosphere (rigid, deformable conditions). Such Keywords: Metamorphic core complex solutions allow inversion for effective mechanical properties and crustal geometry from direct observations Crustal flow of aspect ratio and exhumation velocity; in the northern Rockies, core complex geometry is consistent with a Continental dynamics twofold decrease in viscosity of the thickened Cordilleran crustal welt. Gravitational collapse © 2009 Elsevier B.V. All rights reserved. 1. Introduction This study focuses in particular on the northern North American Cordillera (Fig. 1), where metamorphic core complexes (Table 1) share Metamorphic core complexes occur in orogenic belts worldwide a set of common physical characteristics: and offer important opportunities to examine exposures of middle to lower continental crust in regions that have undergone large-scale 1. Crustal exhumation is spatially associated with brittle normal extension, uplift, and surface erosion (Crittenden, 1980; Armstrong, faulting. (Normal faults bound zones of observed ductile 1982). Much previous work on metamorphic core complexes has exhumation.) focused on the mechanisms and timing of detachment faulting and 2. Localization of exhumation is spatially associated with the edge of a exhumation of footwall rocks through integrated structural, petrolo- craton. (Highly exhumed exposures align between the isotopic gic, and geochronologic studies (e.g. Parrish et al., 1988; Hodges and signature of cratonic material at depth and surface exposures of Walker, 1992; House et al., 1997; Foster et al., 2007). continental craton. See Fig. 1.) However, the term ‘metamorphic core complex’ is presently used 3. Localization of exhumation is spatially associated with a thick for a variety of different structures, ranging from low-angle normal crustal welt. (Highly exhumed exposures are associated with initial faults with associated mylonitic shear zones (e.g. Coney, 1980), to crustal thicknesses N50–60 km.) migmatite-cored gneiss domes that record significant near-vertical 4. Exhumation follows crustal thickening and lasts for a limited time. exhumation (e.g. Whitney et al., 2004). We are particularly interested (Peak exhumation appears to last for 10–15 Myr.) in a group of metamorphic core complexes in settings of convergence 5. The length scale of core complex exhumation is limited, and between stiffer and softer continental packages. Examples of this 6. Crustal exhumation is localized to a region of characteristic width specific tectonic setting include the North American Cordillera, the to depth aspect ratio. northeastern Pamir in China (Robinson et al., 2004, 2007), and within the Alpine–Himalaya Belt, including core complexes of the eastern In this paper, we consider the evolution of metamorphic core Mediterranean and central Iran (Lister et al., 1984; Whitney and Dilek, complexes with these characteristics, including the related changes in 1997; Verdel et al., 2007). Extension in these regions has previously the shape and position of crustal rocks. We make the assumption fi been attributed to vertical extrusion due to topographic collapse or that a signi cant part of this process occurs in continental materials fl backarc extension due to slab rollback. that deform continuously and may be investigated by recourse to uid dynamics. Other recent work on tectonic dynamics of continental lithosphere has similarly borrowed tools from fluid dynamics (e.g. England and McKenzie, 1982; Royden, 1996; Beaumont et al., 2004; Flesch et al., 2005) to describe the spatial distribution of both stress ⁎ Corresponding author. Tel.: +1 406 243 5774; fax: +1 406 243 4028. and strain in continents. Unlike these previous efforts, however, we E-mail addresses: [email protected] (R. Bendick), [email protected] (J. Baldwin). develop dynamic solutions that allow vertical variations in strain 1 Tel.: +1 406 243 5778; fax: +1 406 243 4028. and strain rate (in contrast to thin viscous sheets) and coupled 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.03.017 Please cite this article as: Bendick, R., Baldwin, J., Dynamic models for metamorphic core complex formation and scaling: The role of unchannelized collapse of thickened continental crust, Tectonophysics (2009), doi:10.1016/j.tecto.2009.03.017 ARTICLE IN PRESS 2 R. Bendick, J. Baldwin / Tectonophysics xxx (2009) xxx–xxx Fig. 1. Map of the northern Rockies showing the distribution of Eocene metamorphic core complexes, location of the Idaho batholith, the initial 87Sr/86Sr=0.706 line marking the boundary between accreted terranes and craton, the Lewis and Clark fault zone, and the limit of the Sevier thrust belt (modified from Coney, 1980). deformation across crustal boundaries (in contrast to channel flows). materials, including quartz (Brace and Kohlstedt, 1980), feldspars None of the fluid formulations for continents actually imply that the (Zavada et al., 2007), and sedimentary rocks (Schmid et al., 1977; continents are liquid, only that deformation in continental rocks is Zhang et al., 1993). broadly distributed and smoothly varying (hence continuous) at least at the longest length and time scales. Nor do these formulations imply 2. Geological setting of the northern North American that the continental lithosphere is absolutely weak. Successful fluid metamorphic core complexes dynamical descriptions of continental deformation allow effective lithospheric viscosities greater than 1020 Pa s (Bendick et al., 2008), Numerous structures in the northern Cordilleran region of North consistent with results from rock mechanics for common continental America (Fig. 1) have been identified as metamorphic core complexes Table 1 Characteristics of northern Rockies metamorphic core complexes. Name Location Width (km) Max P (kbar) Depth (km) Max T (°C) Metamorphism (Ma) Extension (Ma) Frenchman's Cap B.C., Can. 30–35 12a 40 800a 77–59a 59–53a Valhalla B.C., Can. 20–30 8b 26 820b 70–57c 51–49c Thor-Odin B.C., Can. 10–15 8–10d 26–33 750–800d 75–56e 55–45f Okanogan WA 55 9–10g,h 30–33 700–850g,h 85–70; 61–49 54–47i Kettle Dome/Grand Forks WA/B.C., Can 25–30 5–8j 20 750–850j 89–78; 74–56j 56–51j Priest River ID/WA 20–30 7–11 k 23–33 770–930k 85–55k,l 55–43m,n,o Clearwater ID 30–35 8–11 p,q 26–36 650–750p,q 82–80; 74–72; 64–55p,o 53–46pr Bitterroot ID/MT 30–35 6–8s,t 20–26 650–750s,t 80–75t 55–39t,u Anaconda MT 15–20 4–5v 13–16 600–650v 79o 53–40o Pioneer ID 15–17 3.5w 12 680w 79w 54–45w a Armstrong et al. (1991). b Spear and Parrish (1996). c Gordon et al. (2008). d Norlander et al. (2002). e Carr (1992). f Vanderhaeghe et al. (1999). g Hansen and Goodge (1988). h Harvey (1994). i Kruckenberg et al. (2008). j Laberge & Pattison (2007). k Doughty and Price (2000). l Doughty et al. (1998). m Doughty and Price (1999). n Miller and Engels (1975). o Foster et al. (2007). p Doughty et al. (2007). q Grover et al. (1992). r Burmester et al. (2004). s House et al. (1997). t Foster et al. (2001). u Chase et al. (1983). v Haney (2008). w Silverberg (1990). Please cite this article as: Bendick, R., Baldwin, J., Dynamic models for metamorphic core complex formation and scaling: The role of unchannelized collapse of thickened continental crust, Tectonophysics (2009), doi:10.1016/j.tecto.2009.03.017 ARTICLE IN PRESS R. Bendick, J. Baldwin / Tectonophysics xxx (2009) xxx–xxx 3 on the basis of their structural and petrologic characteristics. These complicated mixed conditions, which strongly affect the spatial include low-angle normal faults bounding a localized region of pattern and magnitude of stress, strain, and strain rate. footwall rocks of higher metamorphic grade, generally characteristic For this investigation, we impose rigid, deformable boundary of middle crustal depths. conditions on an irregular fluidlike lower crustal region. McKenzie Crustal contraction during the Mesozoic and early Cenozoic in et al. (2000) first defined rigid, deformable boundary conditions to western North America resulted from Farallon plate subduction. This investigate lower crustal flow in simple extensional settings. These contraction involved a complex and protracted period of subduction- conditions entail fixed horizontal velocities and balanced (lithostatic) related deformation and plutonism. Crustal shortening during the vertical stresses on horizontal boundaries at the top and bottom of a Sevier orogeny affected a 200-km-wide