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Pdf/12/3/900/4092675/900.Pdf 900 by Guest on 24 September 2021 Research Paper Research Paper GEOSPHERE Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, GEOSPHERE; v. 12, no. 3 northwestern Cordillera (USA) doi:10.1130/GES01262.1 Robert B. Miller1, Stacia M. Gordon2, Samuel Bowring3, Brigid Doran1, Noah McLean3*, Zachary Michels1*, Erin Shea1*, and Donna L. Whitney4 11 figures; 3 tables; 1 supplemental file 1Department of Geology, San Jose State University, One Washington Square, San Jose, California 95192-0102, USA 2Department of Geological Sciences and Engineering, University of Nevada-Reno, 1664 N. Virginia Street, Reno, Nevada 89557, USA 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue MIT Building 54, Cambridge, Massachusetts 02139, USA CORRESPONDENCE: robert .b .miller@ sjsu .edu 4Department of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, USA CITATION: Miller, R.B., Gordon, S.M., Bowring, S., Doran, B., McLean, N., Michels, Z., Shea, E., and Whitney, D.L., 2016, Linking deep and shallow ABSTRACT basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, crustal processes during regional transtension in an and then covered by ca. 49 Ma Teanaway basalts and intruded by associated exhumed continental arc, North Cascades, north- The North Cascades orogen (northwestern USA) provides an exceptional mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin western Cordillera (USA): Geosphere, v. 12, no. 3, p. 900–924, doi:10.1130/GES01262.1. natural laboratory with which to evaluate potential temporal and kinematic subsided rapidly. Extension directions from these dikes and from Eocene dikes links between processes operating at a wide range of crustal levels during col- that intruded the Cascades core are dominantly oblique to the overall trend of Received 6 September 2015 lapse of a continental arc, and particularly the compatibility of strain between the orogen (275°–310° versus ~320°, respectively) and to the northwest-south- Revision received 9 February 2016 the upper and lower crust. This magmatic arc reached a crustal thickness of east to north-south ductile flow direction in the Skagit and Swakane rocks. Accepted 27 April 2016 ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional This discordance implies that coeval extensional strain was decoupled be- Published online 11 May 2016 transtension was marked by magmatism, migmatization, ductile flow, and tween the brittle and ductile crust. Strain orientations at all depths in the exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core Cascades core contrast with the approximately east-west extension driven by coeval with subsidence and rapid deposition in nonmarine basins adjacent orogenic collapse in coeval metamorphic core complexes ~200 km to the east. to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part the larger of two regions of exhumed deep crust with Eocene cooling ages in from dextral shear along the plate margin and from along-strike gradients in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced crustal thickness and temperature. mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to INTRODUCTION 53 Ma; the youngest orthogneisses overlap 40Ar/39Ar biotite dates, compati- ble with rapid cooling. Gently to moderately dipping foliation, subhorizontal Collisional orogens and some contractional continental magmatic arcs orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional have gross similarities in their structure, including thickened crust (50–70 km) domains, and strong stretching parallel to lineation of hinges of mesoscopic and broad plateaus that have zones of partially molten lower to middle crust folds in the Skagit Gneiss Complex are compatible with transtension linked (e.g., Tibet and Altiplano-Puna; Nelson et al., 1996; Schilling et al., 2006; Ward to dextral-normal displacement of the Ross Lake fault zone, the northeastern et al., 2014). Studies of exhumed orogenic crust and models for lateral and boundary of the Cascades core. The other deeply exhumed domain, the 9–12 verti cal crustal flow have most commonly been formulated based on colli- kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging sional orogens, such as the Himalayas (e.g., Law et al., 2006, and references lineation and gently to moderately dipping foliation, which are associated therein). Much less is known about the deep levels of continental arcs, which with top-to-the-north noncoaxial shear. This gneiss is separated from overly- contain the rocks that record the thermal, rheological, and mechanical tran- ing metamorphic rocks by a folded detachment fault. The Eocene Swauk and sition of an orogen from contractional crustal thickening to extension and/or Chumstick basins flank the southern end of the Cascades core. In the Swauk transtension. For example, in the North American Cordillera, more research has concentrated on deep crustal flow and exhumation of the hinterland *Current addresses: McLean: Department of Geology, University of Kansas, Lindley Hall, 1475 meta morphic core complexes (e.g., MacCready et al., 1997; Vanderhaeghe Jayhawk Blvd. Room 120, Lawrence, Kansas 66045; Michels: Department of Geoscience, Uni- For permission to copy, contact Copyright versity of Wisconsin, 1215 W Dayton Street, Madison, Wisconsin 53706; Shea: Department of and Teyssier, 1997; Teyssier et al., 2005; Gervais and Brown, 2011) than on the Permissions, GSA, or [email protected]. Geological Sciences, University of Alaska, Anchorage, Alaska 99508. Meso zoic arc system to the west (Fig. 1). Similarly, lower to middle crustal flow © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 3 Miller et al. | Linking crustal processes Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/900/4092675/900.pdf 900 by guest on 24 September 2021 Research Paper 124° reached thicknesses of ≥55 km (Miller and Paterson, 2001) in the Late Creta- 51° F r a ceous, and a wide range of Cretaceous crustal levels (0 to ≥40 km) is exposed. se r F It is interpreted as the western margin of a proposed Late Cretaceous to Eo- au l cene orogenic plateau, the eastern margin of which is marked by metamorphic t CBTS core complexes (Whitney et al., 2004). Eocene migmatization, plutonism, and 121° exhumation of some of the deepest (8–12 kbar) rocks of the Cascades crystal- 50° Pa line core were in part coeval with motion on dextral strike-slip and oblique-slip sa y faults, formation of nonmarine, transtensional basins, and intrusion of exten- te n Fault sive basaltic to rhyolitic dike swarms (e.g., Haugerud et al., 1991; Gordon et al., 2010a; Eddy et al., 2016). This orogen thus offers an exceptional opportunity to evaluate links between contemporaneous deep and shallow crustal processes during regional transtension. Furthermore, the record of how different crustal NWCS RL levels evolved provides a view of the construction and collapse of a thick con- F E tinental arc that may be analogous in some respects to the Cenozoic central CFB Andes (Western Cordillera and Altiplano) (e.g., Scheuber and Reutter, 1992; Beck and Zandt, 2002), where deep crust has not been exhumed. C N h 48° e l We integrate a large body of different types of data to arrive at a synthe- a 122° n Bl sis of processes occurring from the middle crust to the surface during a time En o c ALASKA-ALEUTIAN t k iat interval largely centered on the Eocene (60–45 Ma). We address the potential RANGE SC 50 km Faul interplay of deformation, metamorphism, partial melting, and magmatism at F Wenatchee Block t different depths, including the temporal and dynamic relations of these pro- COAST cesses to exhumation of arc orogenic crust, formation of nonmarine basins, RANGE and intrusion of dike swarms. Particular emphasis is placed on how strain was IDAHO BATHOLITH partitioned at different depths in the crust during transtension and whether the upper crust was decoupled from the deeper, ductilely flowing lower crust 500km (e.g., Tibetan Plateau, Royden et al., 1997; Altiplano-Puna, Husson and Semere, SIERRANEVADA 2003; Gerbault et al., 2005; Ouimet and Cook, 2010). Overview of North Cascades Geology PENINSULAR RANGES The Coast Plutonic Complex and its southeast extension, the crystalline core of the North Cascades (Cascades core), form a >1500-km-long plutonic Figure 1. Generalized map of Mesozoic and Paleogene western North American Cor di- and metamorphic belt that represents an exhumed Cretaceous and Paleogene lleran arc plutons and metamorphic core complexes. Inset emphasizes distribution of magmatic arc (Figs. 1 and 2) (e.g., Armstrong, 1988; Tabor et al., 1989; Miller metamorphic rocks (purple) and plutons (orange) in the Cascades core and southern Coast belt. The Coast belt thrust system (CBTS), eastern Cascades fold belt (ECFB), north- et al., 2009a). In the southern Coast Mountains and North Cascades, arc plutons west Cascades thrust system (NWCS), and reverse shear zones in the Cascades core are intrude small late Paleozoic to Cretaceous oceanic and arc terranes (Journeay also shown. The dextral Straight Creek–Fraser fault (SCF) displaces the Cascades core and Friedman, 1993). The Cascades core includes the Chelan Mountains ter- from the main part of the Coast belt. The Entiat fault, Pasayten fault, and Ross Lake fault zone (RLF) are also major high-angle faults. The Cascades core is divided by the Entiat rane, Nason terrane, and Swakane terrane. The Chelan Mountains terrane con- fault into the Chelan and Wenatchee blocks, which have different thermal histories. sists of the Napeequa and Cascade River–Holden units, and metasupracrustal rocks in the Skagit Gneiss Complex are also commonly considered part of this terrane (Fig.
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