Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic fi eld gradients: Upper Granite Gorge, Grand Canyon, USA Gregory Dumond† Kevin H. Mahan‡ Michael L. Williams Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Karl E. Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA ABSTRACT pressures recorded at peak temperatures, subhorizontal shortening and subvertical and intensity of late-stage thermal spikes due extension during crustal thickening. The Paleoproterozoic orogen of the south- to local dike emplacement. High-precision western United States is characterized by ΔPT “relative” thermobarometry confi rms Keywords: Grand Canyon, P-T-t-D path, conti- a segmented, block-type architecture con- lateral temperature variations on the order nental crust, rheology, channel fl ow. sisting of tens of kilometer-scale blocks of of 100–250 °C with little to no variation in relatively homogeneous deformation and pressure. The Upper Granite Gorge thus INTRODUCTION metamorphism bounded by subvertical high- represents a subhorizontal section of low- strain zones. New fi eld, microstructural, and ermost middle continental crust (~0.7 GPa). Proterozoic rocks of southwestern North petrologic observations combined with pre- Results imply that the entire ~70-km-long America are part of a >1000-km-wide orogenic viously published structural and geochrono- transect decompressed from ~0.7 to ~0.3– belt that records the progressive southward logical data are most consistent with a tec- 0.4 GPa levels as one large coherent block in growth of Laurentia (Karlstrom et al., 2001) tonometamorphic history characterized by the Paleoproterozoic. (Fig. 1). An enigmatic characteristic of the belt a clockwise, looping pressure-temperature The transect represents a 100% exposed is its block-type architecture, i.e., blocks with (P-T) path involving: (1) initial deposition fi eld laboratory for understanding the het- internally homogeneous deformational and met- of volcanogenic and turbiditic supracrustal erogeneity and rheologic behavior of lower- amorphic characteristics are separated by sub- rocks at ca. 1.75–1.74 Ga, (2) passage from most middle continental crust during oro- vertical high-strain zones from blocks with dif- <12 km (below pressures equivalent to the genesis. Hot blocks achieved partial melting ferent characteristics (Karlstrom and Bowring, aluminosilicate triple point) to ~25 km conditions during penetrative subvertical 1988). This distinctive architecture has been depths (~0.7 GPa) between ca. 1.70 and fabric development. Although these blocks investigated for over twenty-fi ve years, with 1.69 Ga, (3) decompression back to ~12 km were weak, large-scale horizontal channel particular focus on issues involving growth, sta- depths (0.3–0.4 GPa) by ca. 1.68 Ga, and fl ow was apparently inhibited by colder, bilization, reactivation, and the general character (4) a protracted period of near-isobaric cool- stronger blocks that reinforced and helped of middle continental crust (Karlstrom and Wil- ing (ca. 200–250 Ma). The general geometry preserve the block-type architecture. Devel- liams, 2006). Consequently, there is now great of this looping P-T path is similar for rocks opment of dramatic lateral thermal gradi- potential for use of this natural fi eld laboratory across the entire traverse; however, signifi - ents and discontinuities without breaks in to answer fi rst-order questions about middle to cant differences in peak temperatures are crustal level is attributed to: (1) spatially lower crustal processes. In particular, to what recorded (~500 to >750 °C). Notable varia- heterogeneous advective heat fl ow delivered extent does the block-type architecture refl ect tions along the transect are also primarily by dense granitic pegmatite dike complexes the original accretionary geometry of the oro- thermal in nature and include differences in and (2) local transcurrent displacements gen as opposed to the result of subsequent upper the temperature of the prograde history (i.e., along block-bounding high-strain zones over to middle crustal tectonism? How susceptible is early andalusite versus kyanite), equilibrium an ~15–20 m.y. time interval. Exhumation of the middle continental crust to fl ow as a channel the transect from 25 to 12 km depths is inter- during orogenesis (e.g., Beaumont et al., 2004)? preted to refl ect erosion synchronous with And, how does rheology of continental crust †Corresponding author e-mail: gdumond@geo. penetrative development of steeply dipping evolve through time (e.g., Klepeis et al., 2004)? umass.edu. ‡Current address: Mail Stop 100-23, Division of NE-striking foliations and steeply plung- The answers to these questions have important Geological and Planetary Sciences, California Institute ing stretching lineations, consistent with an implications for our understanding of the evolv- of Technology, Pasadena, California 91125, USA. orogen-scale strain fi eld involving NW-SE ing strength and behavior of continental crust GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 202–220; doi: 10.1130/B25903.1; 12 fi gures; Data Repository item 2007007. 202 For permission to copy, contact [email protected] © 2006 Geological Society of America Metamorphism in middle continental crust, Upper Granite Gorge, Grand Canyon and their high-strain zone boundaries with an emphasis on understanding the character, rheol- WYOMING ogy, and exhumation of orogenic middle con- PROVINCE tinental crust (Figs. 2 and 3). Previous work CBCB North SGSG established the general structural and geochro- Pacific America FM-LMFM-LM MMMM nologic framework for the region (Ilg et al., Ocean 1996; Hawkins et al., 1996; Ilg and Karlstrom, Atlantic FCFC IRIR Fig.1 Ocean MOJAVE 2000). This paper focuses on a suite of samples PROVINCE N HSHS with semipelitic to pelitic bulk compositions and utilizes detailed microstructural observa- 1000 km tions, high-resolution electron microprobe X-ray mapping, published petrogenetic grids, BCBC Proterozoic shear zones and quantitative absolute and relative thermo- with thrust sense barometry to construct detailed P-T-D histories YAVAPAI NENE Precambrian exposures PROVINCE for each tectonic block. Data from each of the blocks, along with published geochronologi- CRC SCSC R Grand FOFO cal data, demonstrate that a single, composite looping P-T-t-D path can be constructed for the GNGN Canyon PLPL RTRT Fig.2 PTPT entire Upper Granite Gorge. The results repre- sent a new synthesis of the metamorphic evolu- BWBW o tion of Precambrian basement in the Grand Can- MSMS MBMB SHSH 35 N MGMG yon. New techniques for reducing uncertainty in MTM MAMA T SCSC thermobarometry are applied to evaluate P-T FPFP MAZATZAL differences between blocks with maximum CFCF PROVINCE precision (i.e., Worley and Powell, 2000). The present-day exposure represents a near-isobaric ~0.7 GPa level (~25 km paleodepths) of conti- nental crust, exhumed to 0.3–0.4 GPa levels as N a single, relatively coherent tectonic block by ca. 1.68 Ga. 100 km 109o W GEOLOGIC BACKGROUND Figure 1. Map of exposed Proterozoic rocks in southwestern Unites States with major prov- inces and shear zones and location of Grand Canyon (modifi ed after Karlstrom and Wil- The Upper Granite Gorge of the Grand Can- liams, 2006). CB—Cheyenne belt; FM-LM—Farwell Mountain–Lester Mountain shear yon is divided into six lithotectonic blocks, sepa- zone; SG—Skin Gulch shear zone; FC—Fish Creek–Soda Creek shear zone; MM—Moose rated by NE-striking, steeply dipping, high-strain Mountain shear zone; IR—Idaho Springs–Ralston shear zone; HS—Homestake shear zone; zones (Figs. 2 and 3) (Ilg et al., 1996; Hawkins BCG—Black Canyon; NE—Needle Mountains; BW—Big Wash shear zone; GN—Gneiss et al., 1996). From southeast to northwest, these Canyon shear zone; CR—Crystal shear zone; MS—Mountain Spring shear zone; MB— blocks have been named: Mineral Canyon, Mesa Butte fault; CF—Chaparral fault; SH—Shylock shear zone; MG—Moore Gulch Clear Creek, Trinity Creek, Topaz Canyon, fault; MA—Mazatzal thrust belt; SC—Slate Creek shear zone; FP—Four Peaks shear Tuna Creek, and Walthenberg Canyon (Figs. 2 zone; SP—Spring Creek shear zone; FO—Fowler Pass shear zone; PL—Plomo thrust; and 3) (Hawkins, 1996). The tectonic features RT—Rincon thrust belt; PT—Pecos thrust; MT—Manzano thrust belt. of the Upper Granite Gorge are attributed to an early event of thrusting (D1), isoclinal folding, and penetrative NW-striking fabric development during orogenesis and the subsequent stabiliza- metamorphic thermal and baric gradients are (S1), followed by progressive subhorizontal con- tion and preservation of continental lithosphere. important prerequisites for exploiting these “fi eld traction (D2) and production of a NE-striking, Field transects of tilted crustal sections or laboratories.” The Upper Granite Gorge of the steeply dipping foliation (S2) associated with large exposures of exhumed middle to lower Grand Canyon is a 100% exposed cross section upright to overturned folds within blocks and continental crust represent important laboratories of six crustal blocks along an ~70-km-long tran- subvertical high-strain zones between blocks for understanding the character and evolution of sect. The cross section is interpreted to represent (Ilg et al., 1996; Ilg and Karlstrom, 2000). Four deformation, magmatism, and metamorphism at the mid-level of 30- to 40-km-thick continental granite pegmatite dike complexes occur within different crustal levels (e.g., Proterozoic rocks crust at the culmination of the ca. 1.72–1.68 Ga the six-block transect and locally intrude across of northern New Mexico: Grambling, 1986; the Yavapai orogeny (Ilg et al., 1996), interpreted as some of the high-strain zone boundaries. From Kapuskasing Uplift: Percival and West, 1994; the a consequence of accretionary tectonism involv- southeast to northwest, these are the Cotton- Fiordland-Westland orogen: Klepeis et al., 2003; ing juvenile, island-arc terrane assembly of thin wood, Cremation, Sapphire, and Garnet pegma- the East Athabasca granulite terrane: Mahan and (15- to 25-km-thick) crustal fragments (Bowring tite complexes (Fig.