The 1.73 Ga Payson Ophiolite, Arizona, USA

Precambrian Ophiolites and Related Rocks Edited by Timothy M. Kusky Developments in Precambrian Geology, Vol. 13 (K.C. Condie, Series Editor) © 2004 Published by Elsevier B.V.

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THE 1.73 GA PAYSON OPHIOLITE, ARIZONA, USA J.C. DANN 90 Old Stow Road, Concord, MA 01742, USA

The 1.73 Ga Payson Ophiolite is a shallow-dipping, layered sequence of coeval gabbro, sheeted dikes, and submarine volcanic rocks, partly disjointed by later intrusion and deformation. The sheeted dike complex is spectacularly exposed as cliffs and water-polished outcrop in many shallow canyons. Gabbro-dike mingling and mutual intrusion attest to rooting of the sheeted dike complex in the underlying gabbro. A stratigraphically continuous zone of intense alteration marks the transition from sheeted dikes to submarine volcanics. A tonalite/dacite suite occurs as rare lavas and as dikes and hypabyssal plutons, mutually intrusive with the basaltic sheeted dikes and gabbro. An older basement complex occurs as roof pendants in gabbro and screens in the sheeted dike complex. An actualistic tectonic model of an intra-arc basin formed by seafloor spreading along an arc-parallel strike-slip fault system explains the origin of the Payson Ophiolite, its emplacement within the arc, and accretion to North America during the ca. 1.70 Ga Yavapai Orogeny.

1. INTRODUCTION The 1.73 Ga Payson Ophiolite (Dann, 1991, 1992, 1997a, 1997b) is about 90 km northeast of Phoenix within the Yavapai-Mazatzal orogenic belt of central Arizona (Fig. 1A). It is one of only a few Early Proterozoic ophiolites known worldwide with a well developed sheeted dike complex and the horizontally layered structure characteristic of Phanerozoic ophiolites and modern oceanic crust (see Moores, 2002, for review). The sheeted dike complex and transitions to overlying volcanic and underlying gabbroic rocks are well exposed as water-polished outcrops in easily accessed canyons (stop one in field trip guide, Karlstrom et al., 1990). Most of the ophiolite remains gently dipping. Fold-and-thrust deformation is localized, and no penetrative fabric occurs in the ophiolite. Even the overlying turbidites have only a locally developed, spaced fracture cleavage. Although low greenschist-grade metamorphism affected the ophiolite, large areas of the gabbro are 95% igneous minerals. The quality of exposure and preservation of primary features facilitated detailed structural mapping and petrological, geochemical, and geochronological analyses of the Payson Ophiolite (PO). The purpose of this paper is summarize the work done and discuss its contribution to our understanding of the mechanics of seafloor spreading, plate tectonics, and continental assembly during the Early Proterozoic. DOI: 10.1016/S0166-2635(04)13002-8

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Fig. 1. (A) Geologic map showing the location of the Payson Ophiolite within the Mazatzal crustal block in the Early Proterozoic of central Arizona (modified from Karlstrom et al., 1990, and Anderson, 1989; ‘T’ is Tonto Creek). (B) Map showing the location of central Arizona within the 1.6–1.8 Ga orogenic belt of North America (from Hoffman, 1989). (C) Tectonostratigraphic columns comparing the Ash Creek and Mazatzal crustal blocks divided by the Moore Gulch shear zone.

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First, we need to feel confident that the complicated map pattern (Fig. 2) actually conceals the layered structure that distinguishes an ophiolite from other crustal sections. Ophiolites are defined by their pseudostratigraphy or horizontally layered structure of mantle tectonite, gabbro, sheeted dikes, and submarine volcanic and sedimentary rocks (Moores, 1982). Importantly, the transitional zones between lithologic layers indicate that the layers were forming simultaneously (unlike real stratigraphy). In addition, the distribution of tonalites, hydrothermal alteration, and chemical sediments record important processes ongoing during development of the crust. In most greenstone belts, deformation and intrusion of late granitoids has dislocated submarine volcanic sequences from their hypabyssal equivalents. What makes the PO special is that this connection is intact. In addition, we need to distinguish between (1) intrusion of a dike swarm into an older plutonic complex and (2) rooting of sheeted dikes in coeval gabbro. These questions were addressed by detailed structural mapping and analysis of the field relations (Dann, 1992). Second, the story of the origin and emplacement of the ophiolite is recorded in its relationship to the rest of the orogenic belt (Dann and Bowring, 1997). Ophiolites commonly occur within terranes or shear zone-bound crustal blocks that record tectonostratigraphic histories and original tectonic settings that are incompatible with neighboring terranes (Fig. 1C). From the comparative tectonostratigraphic histories, the geochemistry of the ophiolitic magma, the geometry of extensional and convergent tectonism, and with reference to modern examples, an actualistic tectonic model can be developed that provides a useful predictive framework for understanding the creation and assembly of Proterozoic crust in the southwestern United States.

2. REGIONAL SETTING The Payson Ophiolite is in the Mazatzal crustal block bound by the Moore Gulch shear zone on the northwest and a belt of post-assembly granites to the southeast (Fig. 1A). This 50–60 km wide crustal block occurs at the eastern end of a 600 km transect of the Proterozoic orogenic belt, exposed in the Transition Zone between the Colorado Plateau and the Basin and Range Province. In central Arizona this orogenic belt consists of submarine volcanic and volcaniclastic rocks that host massive sulfide deposits and are intruded by granitoid plutons. These lithologic associations are interpreted by most workers to represent magmatic arcs (e.g., Anderson, 1989). The magmatic arcs formed over a 40 m.y. period from ca. 1.75 to 1.71 Ga (Bowring et al., 1991), locally involving older crust. From the high estimated rate of crustal growth, the predominance of juvenile magmatic arc rocks, and the juxtaposition of distinct terranes, Karlstrom and Bowring (1988) proposed that the orogenic belt formed by accretion of arc terranes along a convergent plate boundary. Most of the deformation along the block boundaries reflects post-assembly crustal shortening and differential uplift (Bowring and Karlstrom, 1990). Consequently, the role of the block boundaries during the assembly of terranes remains speculative, but it may be particularly important in the origin and emplacement of the Payson Ophiolite.

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Fig. 2. Geologic Map and cross sections of the Payson Ophiolite (contacts outside the ophiolite from Wrucke and Conway, 1987). Sheeted dikes are best exposed in American Gulch (AG), Rattlesnake Canyon, (RC), St. John’s Creek (SJ), and along the east flank of the Mazatzal Mountains (EF). The Larson Spring Formation of the basement complex, intruded and overlain by the ophiolite, are best exposed at the East Verde River (a), Crackerjack Mine (b), Larson Spring (c), Center Creek (d), and Gisela (e).

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3. PAYSON OPHIOLITE The distribution and orientation of dikes reflects the structure of the ophiolite. Sheeted dikes underlie submarine volcanic rocks on the west side of the map (EF, Fig. 2) and overlie gabbro east of the Tertiary valley (AG, RC, Fig. 2). The sheeted dikes dip about 75 degrees to the northeast where gabbro devoid of dikes occurs (‘rv’, Fig. 2). The northeastto-southwest sequence of gabbro, sheeted dikes, and volcanic rocks, combined with the 75 degree northeast dip of the dikes, indicates that the ophiolite pseudostratigraphy dips about 15 degrees to the southwest (cross section, Fig. 2). This reconstruction, based on a perpendicular relationship between dikes and the pseudostratigraphy, is justified by the angular relationships between bedded basaltic volcanic rocks and (1) underlying sheeted dikes (EF, Fig. 2) and (2) bedded felsic volcaniclastic rocks in the basement complex (Dann, 1997a). Parallel to the ophiolite pseudo-stratigraphy, the ca. 1.70 Ga Payson granite intruded its gabbroic roof as a sheet dipping 15–25 degrees to the southwest (Conway et al., 1987). Due to the shallow dip of both the Payson Granite and the ophiolite, the gabbro-norite of Round Valley (‘rv’, Fig. 2) is the deepest level of the ophiolite exposed. The mantle section of the PO remains hidden beneath the Payson Granite. Tertiary normal faults created the sediment-filled valley (Fig. 2) and the complicated map pattern by juxtaposing different levels of the ophiolite pseudostratigraphy. Description of the ophiolite proceeds from gabbro to volcanic rocks with particular emphasis on the transitions that establish the contemporaneity of the dikes with both the gabbro and volcanic section. 3.1. Gabbro Most of the exposed ophiolite is gabbro that forms a pseudostratigraphic layer of plutons beneath the sheeted dike complex. The Round Valley gabbro (‘rv’, Fig. 2) in the northeastern part of the map area is coarse-grained, completely devoid of mafic dikes, and the least altered (1–5%) of all mafic rocks in the ophiolite. The NE-SW cross section shows that the RV solidified about 1.5–2 km below the sheeted dike complex (inset, Fig. 2). Hornblende gabbro mantles the RV, and toward the northwest-trending transition to the sheeted dike complex, it becomes increasingly fractured, altered, and intruded by mafic dikes. Just below the sheeted dike complex, distinct plutons are resolved, based on texture, flow fabrics, and mineralogy, especially the isotropic quartz diorite (‘d’, Fig. 2) that yielded a U/Pb zircon age of 1.73 Ga (Bowring et al., 1991). Gabbro also intruded the sheeted dike complex and the volcanic section as sill-like bodies (e.g., ‘sm’, Fig. 2). All the gabbroic rocks contain hornblende, locally with coarse-grained, ophitic and poikolitic textures (< 4 cm). Elongate plagioclase crystals and folded modal layering define a lineation and/or foliation in the gabbros below the sheeted dike complex. The lineation is primarily orthogonal to the dikes, indicating flow of crystal-rich magma parallel to the direction of extension (Dann, 1997a, Figs. 5, 6). However, the orientation of the layering varies and is locally parallel to the dikes. Structural analysis, based on the orientation of bedding in the basement complex, reveals that the plutonic core of the ophiolite was not tectonically rotated and that magmatic layering with locally steep dips is a primary feature (Dann, 1997a, Fig. 13).

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Rothery (1983) and Greene (1989) reached similar conclusions for the Semail and Lokken ophiolites, respectively. In general, the magmatic layering is less reliable for structural reconstructions than the orientation of sheeted dikes and distribution of the pseudostratigraphy. 3.2. Sheeted Dike Complex The sheeted dike complex is spectacularly exposed in four major canyons as continuous water-polished outcrop, cliffs of parallel slabs of dikes, and hillsides of dikes standing in relief (Fig. 3A). As a result of the overlapping and nearly parallel intrusion, most dikes have well defined chilled margins against other dikes (Figs. 3B, C), and dike splitting and low-angle cross cutting displaced segments of early dikes over tens of meters. Over 600 m of measured section permit an estimate of the proportions of mafic and felsic dikes and screens of gabbro and granitic basement and other features that distinguish one area of sheeted dikes from another (see Dann, 1997a, Fig. 8, and Dann, 1997b, Fig. 3, for examples of sections). The most pronounced difference between the top and bottom of the sheeted dike complex is the 3-fold increase in the average width of dikes (e.g., compare Fig. 4A and Fig. 5E). The depth-thickness relationship inspired a new model for the vertical development of sheeted dike complexes (Dann, 1997b). No complete section from top to bottom of the sheeted dike complex is exposed. The base of the sheeted dike complex and transition to the underlying gabbro is best exposed in the Rattlesnake Canyon area (RC, Fig. 2). The top of the sheeted dike complex and transition to submarine volcanic rocks is only exposed on the west side of the Tertiary valley (EF, Fig. 2). American Gulch (AG, Fig. 2) provides the best display of the intrusive sequence of mafic and dacitic dikes into the basement screens (Figs. 3B–D). Analysis of dike widths places AG at an intermediate level within the sheeted dike complex. These three areas of sheeted dikes form the corners of a triangle, giving the appearance that the sheeted dike complex is not a continuous layer. However, the granitic pluton in the middle of the triangle (SJ, Fig. 2) contains a large roof pendant of 100% sheeted dikes that testify to the original continuity of the layer of sheeted dikes that was eroded off the top of the pluton. 3.3. Transition from Gabbro to Sheeted Dikes Best exposed in the Rattlesnake Canyon area (RC, Fig. 2), the transition from gabbro to sheeted dikes trends northwest, parallel to the trend of the dikes. Over about 100 m, dikes increase from < 50% in gabbro to > 90% with gabbroic screens. The dikes are generally thick and coarse-grained (Figs. 4A, B) with tonalitic dikes up to 15 m thick. Early thick dikes have weakly chilled contacts that locally mingle with gabbroic screens (Fig. 4B). The transition zone contains small dioritic intrusions and locally developed intrusion breccias. Some screens contain a mixture of coarse-grained gabbroic material and finer-grained dike material that is interlayered parallel to a flow foliation/lineation (Fig. 4C). Xenoliths of porphyritic basalt aligned in the flow foliation are identical to dikes cutting the gabbro

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Fig. 3. Sheeted dikes of American Gulch (AG, Fig. 2). (A) View looking northwest down into the gulch. Paler dike (V) is a 3 m thick dacitic dike that is also on outcrop map (D). (B) Water-polished outcrop of sheeted dikes. Basaltic dikes (b) intrude a dacitic dike (x) that intrudes a granitic basement screen (g). (C) One-way chilling (arrows) of basaltic dikes against porphyritic dike (scale bar is 1 cm). (D) Outcrop map of sheeted dikes showing 3 dacitic dikes and 18 basaltic dikes.

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(Fig. 4D). In the gabbro below the sheeted dikes, mafic dikes are pulled apart parallel to the flow foliation, forming trains of enclaves with deformed shapes and mingled contacts. In addition, the gabbro contains conjugate shear bands locally cut by dikes, indicating that the gabbro underwent hot sub-solidus extension consistent with the dikes (see Dann, 1997a, Fig. 6). Gabbro-dike mingling, mutual intrusion, and stretching of xenoliths parallel to the flow foliation in the gabbro indicate that the dikes are rooted in coeval gabbro (see Dann, 1997b, Fig. 6, for summary figure). Flowing gabbroic crystal mush cannibalized the base of the dikes, forming trains of deformed enclaves and mixed dike-gabbro layers along the direction of flow. The transition represents a fluctuating rheological boundary between brittle fracture with dike intrusion and fluid flow of crystal-rich gabbroic magma. This process caused a stepwise decrease in the abundance of dikes with depth, such that the sheeted dike complex bottoms out and dikes are completely absent 1–1.5 km below the sheeted dike complex. Similar features that attest to the dynamic process of dikes rooting in the underlying gabbro occur in many ophiolites (Pederson, 1986; Furnes et al., 1988; Nicholas and Boudier, 1991; Skjerlie and Furnes, 1996). 3.4. Volcanic Section The volcanic section is a thin 400–500 m thick sequence that lies between overlying turbidites (Fig. 5A) and underlying sheeted dikes (Fig. 5E) along the west side of the map area (EF, section Y–Y
, Fig. 2). The section consists mostly of basaltic sheet flows with rare outcrops of pillowed flows. The pillows have a 2–3 cm thick band of vesicles inside well-defined selvages. Flow tops are readily recognizable by large amygdules, breccia, jasper infillings, and interflow sediments (Fig. 5B). Besides basaltic flows, the volcanic section contains an auto-brecciated dacitic flow and associated debris-flow deposits. Clastic interflow sediments include volcanic conglomerates, greywacke, and tuffs. Graded beds and scour-and-fill structures indicate consistent northwest younging (Fig. 5B). Chemical interflow sediments include thick lenses of magnetite-rich banded iron formation, jasper, and chert. Sediments are locally deformed by the weight of overlying flows (Fig. 5B). The base of the volcanic section is intruded by thin basaltic sills and dikes that are distinguished by their cross-cutting relations or vesicle layering parallel to chilled margins. Although the paucity of pillowed flows is unusual for a submarine volcanic section, the thickness of the volcanic section (400–500 m) is typical of many ophiolites (usually < 1 km;

Fig. 4. Base of sheeted dike complex. (A) Two thick mafic dikes (arrows, 4 m wide; map folder at top for scale) in the sheeted dike complex of Rattlesnake Canyon (RC, Fig. 2). (B) Mingled and poorly chilled contact (arrow) between mafic dike and gabbroic screen (gb; pen for scale). (C) Gabbroic screen (gb) in sheeted dikes (arrows point to chills), showing early dike material (dark and fine-grained, between arrows) mingled with, and drawn out along the flow foliation in, the gabbro (1 m across field of view). (D) Porphyritic dike xenoliths elongate parallel to the flow foliation of the gabbro are identical to porphyritic dikes cutting the gabbro (pen for scale).

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Fig. 5. (A) Graded beds of the turbidite sequence overlying the ophiolite (arrow points in younging direction). (B) Interflow clastic sediment, graded from course sand above the amygdaloidal flow top (arrow base) to pale tuff that penetrates crack in overlying flow (arrow tip). (C) Silicified mafic rock of the altered transition from sheeted dikes to volcanic flows (coin for scale). Silica veining (‘s’) outlines pseudo-breccia texture (‘p’). (D) Schematic column showing increasing alteration (white) at the top of the sheeted dike complex. (E) Relatively unaltered sheeted dikes, several 100 m below the volcanic section, with an average width of about 80 cm.

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Moores, 1982) and sections of oceanic crust exposed on the ocean floor (< 500 m; Francheteau et al., 1992). 3.5. Transition from Sheeted Dikes to Submarine Basalt The transition from sheeted dikes to volcanic rocks is marked by a cliff-forming, stratigraphically continuous, siliceous zone. This transitional zone and the overlying volcanic rocks are steeply dipping to overturned along the east flank of the Mazatzal Mountains (EF, section Y–Y
, Fig. 2), where canyons provide cross-sectional exposures. The volcanic section has a sharp lower contact with the massive siliceous zone. In contrast, the sheeted dike complex has a gradational upper contact with the siliceous zone (Fig. 5D). With increasing degrees of alteration, the sheeted dikes merge upwards with the siliceous zone where all primary features are obliterated and the rock takes on a pseudo-breccia texture (Fig. 5C). A few dikes cut the base of the siliceous zone and some bedded, tuffaceous, cherty sediments occur near the top, indicating that this transition zone consists of a mixture of dikes and flows. Although the siliceous transition is internally disjointed by conjugate fractures and shear bands and is the only ophiolitic unit with a crude, nonpenetrative cleavage, the magmatic transition from sheeted dikes to volcanic rocks is intact. The magmatic connection between the dikes and overlying volcanic flows is also indicated by their co-varying geochemistry and phenocryst types (e.g., plag-phyric flows overlie areas of plag-phyric dikes, etc). Although some sills and dikes occur within the volcanic section, the transition from 100% dikes to the volcanic section occurs within about 100 m. The abrupt transition from sheeted dikes to volcanic rocks is characteristic of many well-studied ophiolites (Pallister, 1981; Moores, 1982; Rosencrantz, 1983; Harper, 1984; Baragar et al., 1987) and distinguishes ophiolites from other types of volcanic centers. 3.6. Tonalites and Dacites A suite of tonalitic and dacitic rocks occurs in all three exposed layers of the ophiolitegabbro, sheeted dikes, and volcanic rocks. Rare dacitic flows, breccias, and tuffs are interbedded with the basaltic flows. Tonalitic and dacitic dikes make up about 10% of the sheeted dike complex and locally up to 38%. They are mutually intrusive with, and parallel to, the mafic dikes (Figs. 3B, D). The PO is unusual for the high proportion of dacitic dikes in the sheeted dike complex. These dikes are white to pink in color, and the contrast with the dark green and gray basaltic dikes shows off the cross cutting contacts and makes it easier to reconstruct the intrusive sequence. Besides the mutually crosscutting relations in the sheeted dike complex, composite dikes in the gabbro have co-mingled dacitic and basaltic phases that testify to the coeval intrusion of these two distinct magma types (see Dann, 1997a, Fig. 13). In addition, the tonalite/dacite suite includes small sub-spherical mafic enclaves that indicate mingling and incomplete mixing of basalt in the tonalitic magma. Locally, granitic screens melted and intruded the mafic dikes, suggesting that the abundance of dacitic magma may have been generated from felsic basement within the ophiolite. On

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the other hand, the geochemistry of the upper gabbro and tonalite are complimentary, suggesting that the tonalite represents a residual liquid, filter pressed from the gabbro (Dann, 1992), similar to tonalites in the Karmoy Ophiolite (Pederson and Malpas, 1984). A more detailed petrological and geochemical study is needed to determine the petrogenesis of the variety of tonalites within the PO.

4. BASEMENT COMPLEX The basement complex consists of (1) coarse-grained granitoids, and (2) hypabyssal granite overlain by (3) submarine felsic volcaniclastic rocks of the Larson Spring Formation (Fig. 2). The basement complex is intruded by gabbro and cut by mafic dikes. Few ophiolites contain such a well-defined suite of older felsic rocks, but most of the ophiolite is devoid of roof pendants or screens. The basement complex indicates that the ophiolite developed from extension of older arc crust. It provides a valuable reference frame within the ophiolite, recording a tectonic event that preceded development of the ophiolite as well as the degree of magmatic extension. A northeast-trending belt of coarse-grained, foliated, tonalite and quartz monzodiorite (Fig. 2) occurs as roof pendants in gabbro and is intruded by a swarm of basaltic dikes and small plutons of isotropic porphyritic diorite. The presence of alkali feldspar, low An content of the plagioclase, darker hornblende, and more quartz and biotite distinguishes this suite of rocks from the gabbro and quartz diorite of the ophiolite. Abundant zircons yield a U/Pb age of 1.75 Ga (Dann et al., 1989), making it 20 m.y. older than the ophiolite and one of the oldest rocks in central Arizona. What this pluton intruded is not exposed. Screens in the sheeted dike complex and roof pendants in gabbro define a northeasttrending belt of felsic volcanic rocks, the Larson Spring Formation, and underlying, finegrained, isotropic granitoids (‘a–d’, Fig. 2). Near Larson Spring (‘c’, Fig. 2), the most intact felsic section underlain by hypabyssal granite sits as a block in gabbro, intruded by basaltic dikes (< 20%). Felsic volcaniclastic breccia, beds graded from breccia to porcelainite, and plagio-arenites with scour-and-fill structures indicate that the bedding dips consistently to the northwest. Massive, aphyric, felsic flows, mafic sediments, and chert also occur locally. How much older than the ophiolite these rocks are is not known.

5. HYDROTHERMAL ALTERATION Extensional tectonism generates hydrothermal circulation by facilitating the shallow emplacement of hot magma (dikes) and increasing the fracture porosity. Seafloor hydrothermal alteration and later burial metamorphism produce similar greenschist assemblages in mafic rocks. The effects of seafloor alteration can be distinguished by locating alteration or hydrothermal products that are unambiguously associated with intrusion or eruption of magma. In addition, the location of the most severe alteration is diagnostic of a narrow axial zone of intrusion, the hallmark of seafloor spreading.

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The sheeted dike complex of the PO displays patterns of alteration that are unambiguously associated with intrusion of the dikes. First, the interiors of some dikes with identifiable chilled margins are completely replaced by epidote and quartz. These green epidosites are locally intruded by grey greenschist dikes that retain their igneous textures and whole rock compositions. Second, dikes with a reddish-brown, hematitic alteration near the top of the sheeted dike complex are split by dikes with the more usual greenschist alteration. Third, veins of quartz (with or without sulfides), especially common along the margins of dikes, are also cut by late dikes. Based on the cross cutting relations, the epidosite, hematitic alteration, and quartz veins are temporally associated with intrusion of the dikes. The epidosites are typically found in sheeted dike complexes of ophiolites and are interpreted to require strongly localized, high temperature, fluid fluxes (Gillis and Banerjee, 2000). The most intensely altered unit in the Payson Ophiolite is the siliceous zone that marks the sheeted dike-volcanic transition. Except a few patches of sediment near the top and late dikes near the base of this zone, all original outcrop-scale features are obliterated by quartz-chlorite alteration that locally renders a pseudo-breccia texture (Fig. 5C). Disseminated sulfides are common, and weathering of small patches of gossan creates orange iron straining on some cliff exposures. Silicification and mineralization are concentrated at the transition from sheeted dikes to volcanic rocks in both modern oceanic crust (Alt et al., 1986) and in other ophiolites (e.g., Josephine Ophiolite; Harper et al., 1988). The transition from sheeted dikes to volcanic rocks is the site of eruption on the seafloor. Therefore, the alteration represented by the siliceous zone occurred around vents at the axial zone of seafloor spreading, where fractures open as fissures and permeability is highest. What distinguishes alteration of the PO from other ophiolites is the high degree of silicification to form an erosion-resistant, stratigraphically continuous zone at the dike-volcanic transition. Chemical sediments are interlayered with the volcanic flows and represent the exhalative products of a hydrothermal system that was active during seafloor spreading. A Cu-Pb massive sulfide deposit occurs just above the transition from sheeted dikes to submarine basalts in the southernmost exposure of the PO (Wessels and Karlstrom, 1991) in Tonto Creek (‘T’ on Fig. 1A). Massive sulfide deposits are common components of ophiolites (Gillis and Banerjee, 2000).

6. GEOCHEMISTRY All components of the 1.73 Ga Payson Ophiolite—submarine basalts, sheeted dikes, gabbro, and tonalite—as well as the 1.75 Ga basement complex have geochemical signatures of magmatic arc rocks (Dann, 1991, 1992). These include light rare earth element (LREE) and large-ion lithophile element (LIL) enrichment and relative high-field strength element (HFSE) depletion, typical of arc rocks (Pearce et al., 1984). The basaltic rocks plot in the ‘arc’ or ‘suprasubduction zone’ field in all tectonic discrimination diagrams (Th-HfTa, Ti-Cr, Cr-Y, etc.). Analyses of mafic dikes define a tholeiitic fractionation trend of

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increasing FeO, TiO2 , P2 O3 , and V with decreasing MgO, typical of arc tholeiites. Hydrous igneous minerals (hornblende, biotite) in the gabbros and clinopyroxene-controlled fractionation of the dikes indicate that the parental magma was hydrous, consistent with generation above a subduction zone. Nd isotopic analyses indicate the influence of an older LREE-enriched component (Dann et al., 1993). The ophiolite has the geochemical and isotopic features of the high Ce/Yb suite of arc magmas (cf. Hawkesworth et al., 1993). Sheeted dikes may form during rifting of arcs, rifting of continental crust, or rifting of volcanic islands (e.g., Hawaii (Walker, 1987) or Canary Islands (Stillman, 1987)). However, the geochemistry alone indicates that the PO formed during an extensional phase in the evolution of a magmatic arc.

7. DEFORMATION OF THE OPHIOLITE Four stages of deformation are recorded by structures of the PO. First, rotation of the basement complex occurred prior to development of the ophiolite. Then, crustal extension guided the intrusion of basaltic magma, culminating in seafloor spreading. Structures that formed during intrusion of the basaltic magma provide important clues about the tectonic setting. Third, the ophiolite and overlying rocks are affected by two Early Proterozoic episodes of coaxial convergent deformation, the ca. 1.70 Ga Yavapai orogeny (D2 ) and the ca. 1.67 Ga Mazatzal Orogeny (D3 ). An earlier period of deformation (D1 ) only affected terranes west of the Moore Gulch Fault (Figs. 1A, C). Finally, Tertiary extensional faulting created the sediment-filled valley that cuts through the middle of the map area (Fig. 2). The bedded rocks of the Larson Spring Formation occur as narrow screens in the sheeted dike complex in Center Creek (‘d’, Fig. 2). The orientation of bedding defines an angular unconformity beneath the volcanic section of the ophiolite, which exposed granite on the pre-ophiolite paleosurface (see Dann, 1997a, Fig. 12). This angular relationship, the consistent orientation of bedding in the roof pendants, and repetition of the same lithologies from one roof pendant to another suggests that blocks of the basement complex rotated along listric normal faults prior to development of the ophiolite. Fault-bound domains of sheeted dikes with dips diverging 30 degrees from adjacent domains indicate block rotations along normal faults in the RC area. Outcrops of the overlying Cambrian Tapeats Formation are only rotated 5–10 degrees by Tertiary normal faults. As a result, the larger block rotations may reflect normal faults active during development of the ophiolite. Normal faults and rotated blocks occur along the mid-ocean ridges and in well-exposed ophiolites. Rotated dikes define grabens in the Troodos Ophiolite (Varga and Moores, 1985), and in the Josephine Ophiolite, entire crustal sections were rotated as much as 50 degrees relative to the overlying sediments and underlying Moho (Harper, 1984). Proving their syn-ophiolite origin, vertical dikes cut the rotated sections of sheeted dikes in these examples. This relationship has not yet been found in the PO. The most prominent fold of the sheeted dike complex occurs in RC area (‘s’, Fig. 2). The dikes and foliation in gabbroic screens rotate about 75 degrees clockwise at they approach, and end at, a poorly exposed, northeast-trending boundary with a younger granite. On

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an outcrop scale, dextral shear zones and faults are intruded by dikes. Consequently, this boundary may have been a dextral strike-slip shear zone active during development of the ophiolite. A system of northeast-trending strike-slip shear zones, including the boundaries of the Mazatzal block, is consistent with arc-parallel strike-slip faults that commonly occur within island arcs. Fold-and-thrust structures in the Mazatzal Mountains east of the Tertiary valley (Fig. 2) record two episodes of convergent deformation, D2 and D3 , separated by the unconformity at the base of the Mazatzal Group (section Y–Y
, Fig. 2). Overall NE-trending structures, in addition to the northeasterly trend of Yavapai-Mazatzal orogenic belts across North America (Fig. 1C), indicate the presence of a NE-trending convergent margin, subduction zones, and island arcs during the main phase of crustal assembly. In the Mazatzal block, the ca. 1.70 Ga Yavapai Orogeny produced the northeast-trending syncline in the turbidites overlying the ophiolite (‘f’ and section Y–Y
, Fig. 2). Crustal thickening drove uplift, subaerial exposure, and erosion, marked by an unconformity that truncated the fold in the turbidites and, to the south, cut down into the sheeted dikes. The unconformity is closely associated with eruption of the ca. 1.70 Ga, subaerial, Red Rock Rhyolite and intrusion of hypabyssal equivalents and large sheets of granite (i.e., Payson Granite, Fig. 2). Only subvolcanic facies are preserved in the map area (Fig. 2). Siliciclastic sedimentation of the Mazatzal Group covered the unconformity. Deposition of these rocks in a foreland setting suggests that the earlier Yavapai Orogeny involved accretion of the Mazatzal block to North America. Coaxial with the Yavapai structures, the Mazatzal Orogeny produced a foreland system of thrust faults and related folds in the Mazatzal Group, defining the D3 -phase of convergent deformation. Despite the deformation of the bedded sequences east of the Tertiary valley, the plutonic core of the exposed ophiolite was not folded.

8. TECTONOSTRATIGRAPHIC ANALYSIS The tectonostratigraphic history of the Mazatzal block is recorded in three distinct units: (1) the 1.75 Ga basement complex, (2) the 1.73 Ga PO and overlying basin-filling sedimentary and submarine volcanic sequences (1.73–1.71 Ga), and (3) 1.70 Ga subaerial rhyolite and related granite and later fluvial to shallow-marine siliciclastic sediments (Figs. 1C, 2). The 1.75 Ga granitoids of the basement complex and the 1.73 Ga PO are incompatible with the tectonostratigraphic history (Fig. 1C) of the adjacent Ash Creek block and other blocks to the northwest (Fig. 1A). The Ash Creek block records submarine arc volcanism and northwest-trending D1 deformation prior to intrusion of the 1.735 Ga Cherry Batholith (Fig. 1C; Karlstrom and Bowring, 1991). Northwest-trending dikes or other evidence of rifting while the PO was forming are lacking. Likewise, the basement complex shows no indication of the northwest-trending D1 deformation. Juvenile Nd isotopic signatures of the Ash Creek block contrast with evidence for a LREE-enriched component in all rocks of the Mazatzal block (Dann et al., 1993). This period of incompatibility between adjacent blocks suggests that movement along Moore Gulch shear zone juxtaposed the Ash Creek and Mazatzal crustal blocks. The lack of evidence for a subduction zone or low angle fault

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that juxtaposed the two crustal blocks is best explained by a strike-slip boundary. Late dip-slip movement along Moore Gulch shear zone juxtaposes rocks from different crustal depths and obscures early fabrics. The PO is overlain by a volcano-sedimentary sequence, characterized by a lack of stratigraphic continuity across the Mazatzal block due to both facies changes and structural imbrication. In the map area, the basalts of the PO are directly overlain by dacitic volcaniclastic breccia with lenses of jasper and then a thick sequence of turbidites (Fig. 5A). Ash beds within the turbidites have U/Pb zircons ages of ca. 1.72 Ga (Dann et al., 1989). West of the map area (‘H’, Fig. 2), andesitic flows and coarse volcaniclastic breccias are interbedded with, and overlain by, turbidites and pelitic sediments with ash beds. These relations indicate that andesitic arc volcanoes erupted before and during turbidite deposition (Anderson, 1989) and on, or adjacent to, the PO. Finally, three granodiorite plutons (Fig. 2) intruded the PO at ca. 1.71 Ga (Conway et al., 1987). Anderson (1989) mapped a sequence of slates across the Moore Gulch shear zone, suggesting an overlap and contiguity of the two crustal blocks. In addition, ca. 1.70 Ga Yavapai deformation is recorded on both sides of the shear zone. Consequently, the Ash Creek and Mazatzal blocks must have been juxtaposed prior to the main phase of convergent deformation. After Yavapai deformation, the Ash Creek and Mazatzal blocks underwent differential uplift, probably reflecting different crustal profiles established early in their development. This difference is best appreciated by noting that the Mazatzal crustal block is unique, not only for the PO but also for preserving at least 4 unconformities and 3 transitions from plutonic to coeval volcanic rocks. Apparently, the Mazatzal block was uplifted less than adjacent blocks, suggesting less over-thickening during convergent deformation. The unique character of the crustal section probably began with the origin and emplacement of the ophiolite.

9. ORIGIN AND EMPLACEMENT OF THE PAYSON OPHIOLITE Ophiolites commonly originate in marginal basins above subduction zones because this tectonic setting predisposes them to be incorporated into continental crust during accretion of arcs or collision of continents. The formative tectonic setting and mechanics of emplacement are closely related, a theme that is important to any tectonic model of ophiolites. Seafloor spreading produced the horizontal layered structure of the PO as indicated by (1) the laterally extensive sheeted dike complex, (2) rooting of the sheeted dikes in coeval gabbro, (3) the abrupt transition to submarine lavas, and (4) the distribution of hydrothermal alteration and its exhalative products. The supra-subduction zone signature of the mafic rocks and older and younger arc lithologies implies that seafloor spreading took place within a magmatic arc. Within a 40 m.y. period, the basement complex developed within an arc, extensional tectonics rifted the arc and opened an intra-arc basin by seafloor spreading, arc volcanics and derived turbidites filled the basin, and arc granitoids intruded the basin floor. In modern arc settings, seafloor spreading creates both intra-arc and backarc submarine basins.

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The D2 and D3 convergent deformations affecting the Mazatzal block reflect a northeast-trending convergent margin that controlled the assembly of arc terranes and/or collisional orogenesis. Northwest-trending dikes of the PO formed by northeast-southwest extension, parallel to the convergent boundary prior to continental assembly. The schematic block diagram in Fig. 6A shows the Mazatzal and Ash Creek blocks prior to the ca. 1.70 Ga D2 Yavapai orogeny. In a Cretaceous back-arc basin represented by the Rocas Verdes ophiolite (de Wit and Stern, 1981), dikes are parallel to, and extension was perpendicular to, the convergent boundary. So, if the PO formed above a northeast-trending subduction zone, a simple back-arc model does not fit. Alternatively, arc-parallel extension occurs in modern arcs along arc-parallel strike-slip faults. For example, the Marinduque intra-arc basin in the Philippines developed from a pull-apart structure to seafloor spreading along the arc-parallel, strike-slip Philippine fault zone (Sarewitz and Lewis, 1991). This small basin, about the size of the Mazatzal block, contains a fossil axial-spreading center orthogonal to the strike-slip faults (Fig. 6B). Volcanoes rise from the basin floor. Turbidites are pouring in and interfingering with the volcanic debris. Strike-slip faults were active during seafloor spreading. When the basin crust is finally uplifted and exposed, we probably would see gabbro, sheeted dikes, and submarine volcanics overlain by turbidites and, like the PO, screens and roof pendants of older arc crust. In addition, we might see evidence for strike-slip shear zones within the ophiolite. Since the ophiolite remains above the subduction zone after its formation, we might expect to see arc plutons intruding the basin crust. As the basin is transported along the fault, the ophiolitic crust will be juxtaposed with arc crust that formed 100’s of km away in the same arc (Sarewitz and Lewis, 1991), probably with a contrasting tectonostratigraphic history. The Philippine arc is a collage of terranes including fragments of older deformed crust and several ophiolites, juxtaposed along major faults with up to 1000 km of displacement. An important feature of this model is that this juxtaposition occurs prior to the main phase of convergent deformation that accretes the arc to the continent. The Payson Ophiolite may be the oldest example of this mode of ophiolite generation and emplacement, attesting to the complex evolution of Early Proterozoic magmatic arcs leading up to the assembly of continental crust. High estimated crustal growth rates, indicated by large areas of juvenile crust like Yavapai-Mazatzal orogenic belt, suggest that continental assembly is episodic. Patchett and Chase (2002) estimate a 16% probability for margin-parallel strike-slip movement > 400 km that could effectively concentrate juvenile crust in small regions, giving the impression of higher than actual crustal growth rates. Direct evidence for early strike-slip movement along terrane boundaries that are reactivated during convergent deformation and post-assembly differential uplift is inherently difficult to recover. As a result, only by piecing together the tectonostratigraphic histories of terranes can the role of strike-slip tectonics in the assembly of continental crust be appreciated. Further analysis of the Payson Ophiolite and associated rocks is needed to better understand plate tectonics and the timing and mechanics of continental assembly during the Early Proterozoic.

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Fig. 6. (A) Schematic model of Mazatzal and Ash Creek crustal blocks at the time of formation of the Payson Ophiolite (ca. 1.73 Ga) and prior to D2 deformation of the ca. 1.70 Ga Yavapai Orogeny. The contrast in tectonostratigraphies requires an active boundary to juxtapose the distinct terranes. Arc-parallel extension in a step-over zone within a system of arc-parallel strike-slip faults rifted older arc crust and culminated in seafloor spreading and formation of an intra-arc basin. The basin was a locus of deposition within the arc. Transported along the arc by strike-slip movement, the basin was juxtaposed with the Ash Creek block prior to convergent deformation. (B) Similar in size to the Mazatzal block, the Marinduque intra-arc basin formed by seafloor spreading along the Philippine strike-slip fault zone in the Philippine arc (modified from Sarewitz and Lewis, 1991).

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REFERENCES Alt, J.C., Honnerez, J., Laverne, C., Emmermann, R., 1986. Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry, evolution of seawater-basalt interactions. Journal of Geophysical Research 91, 10309–10335. Anderson, P., 1989. Proterozoic plate tectonic evolution of Arizona. In: Jenny, J.P., Reynolds, S.P. (Eds.), Geologic Evolution of Arizona. Arizona Geological Society Digest 17, 17–148. Baragar, W.R.A., Lambert, M.B., Baglow, N., Gibson, I., 1987. Sheeted dikes of the Troodos ophiolite, Cyprus. In: Halls, H.C., Fahrig, W.F. (Eds.), Mafic Dike Swarms. Geological Association of Canada Special Paper 34, 257–272. Bowring, S.A., Karlstrom, K.E., 1990. Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States. Geology 18, 1203–1206. Bowring, S.A., Karlstrom, K., Chamberlain, K.R., Dann, J.C., 1991. The tectonic evolution of central Arizona. Geologic Society of America Abstracts with Programs 23 (4), A7. Conway, C.M., Karlstrom, K.E., Silver, L.T., Wrucke, C.T., 1987. Tectonic and magmatic contrasts across a two-province Proterozoic boundary in central Arizona. In: Davis, G.H., VandenDolder, E.M. (Eds.), Geologic Diversity of Arizona and Its Margins: Excursions to Choice Areas. Arizona Bureau of Geology Special Paper 5, 158–175. Dann, J.C., 1991. Early Proterozoic ophiolite, central Arizona. Geology 19, 590–593. Dann, J.C., 1992. Origin and emplacement of the Early Proterozoic Payson ophiolite, central Arizona. Ph.D. dissertation. Washington University, St. Louis, MO, p. 340. Dann, J.C., 1997a. Pseudostratigraphy and origin of the Early Proterozoic Payson Ophiolite, central Arizona. Bulletin of the Geological Society of America 109, 347–365. Dann, J.C., 1997b. Branching sheeted dikes and seafloor spreading within an Early Proterozoic intraarc basin. Journal of Geophysical Research 102 (24), 917–924, 929. Dann, J.C., Bowring, S.A., Coleman, D.S., 1993. Geochronologic, isotopic, and structural constraints on the evolution of an Early Proterozoic intra-arc basin, central Arizona. Geological Society of America Abstracts with Programs 25 (5), 237. Dann, J.C., Chamberlain, K.R., Bowring, S.A., 1989. An Early Proterozoic ophiolite, central Arizona: U-Pb constraints on its structure and development. Geological Society of America Abstracts with Programs 21 (6), A23. Dann, J.C., Bowring, S.A., 1997. The Early Proterozoic Payson ophiolite and the Yavapai-Mazatzal orogenic belt, central Arizona. In: de Wit, M.J., Ashwall, L. (Eds.), Tectonic Evolution of Greenstone Belts. In: Oxford Monographs on Geology and Geophysics, vol. 35, pp. 781–790. de Wit, M.J., Stern, C.R., 1981. Variations in the degree of crustal extension during formation of a back-arc basin. Tectonophysics 72, 229–260. Francheteau, J., Armijo, R., Cheminee, J.L., Hekinian, R., Lonsdale, P., Blum, N., 1992. Dyke complex of the East Pacific Rise exposed in the walls of the Hess Deep and the structure of the upper oceanic crust. Earth and Planetary Science Letters 111, 109–121. Furnes, H., Pederson, R.B., Stillman, C.J., 1988. The Leka Ophiolite Complex, central Norwegian Caledonides: field characteristics and geotectonic significance. Journal of the Geological Society of London 145, 401–412. Gillis, K.M., Banerjee, N.R., 2000. Hydrothermal patterns in supra-subduction zone ophiolites. In: Dilek, Y., Moores, E., Elthon, D., Nicolas, A. (Eds.), Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program. Geological Society of America Special Paper 349, 283–297.

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Greene, T., 1989. Magmatic evolution of the Lokken SSZ ophiolite, Norwegian Caledonides: relationships between anomalous lavas and high-level intrusions. Geological Journal 24, 251–274. Harper, G.D., 1984. The Josephine ophiolite, northwestern California. Geological Society of America Bulletin 95, 1009–1026. Harper, G.D., Bowman, J.R., Kuhns, R., 1988. A field, chemical, and stable isotope study of subseafloor metamorphism of the Josephine ophiolite, California-Oregon. Journal of Geophysical Research 93, 4625–4656. Hawkesworth, C.J., Gallagher, K., Hergt, J.M., McDermott, F., 1993. Mantle and slab contributions in arc magmas. Annual Review of Earth and Planetary Sciences 21, 175–204. Hoffman, P.E., 1989. Precambrian geology and tectonic history of North America. In: Bally, A.W., Palmer, A.R. (Eds.), The Geology of North America—An Overview. Geological Society of America, The Geology of North America A, pp. 447–512. Karlstrom, K., Bowring, S.A., 1988. Early Proterozoic assembly of tectonostratigraphic terranes in southwestern North America. Journal of Geology 96, 561–576. Karlstrom, K.E., Bowring, S.A., 1991. Styles and timing of Early Proterozoic deformation in Arizona: constraints on tectonic models. In: Karlstrom, K.E. (Ed.), Proterozoic Geology and Ore Deposits of Arizona. Arizona Geological Society Digest 19, 1–10. Karlstrom, K.E., Doe, M.F., Wessels, R.L., Bowring, S.A., Dann, J.C., Williams, M.L., 1990. Juxtaposition of Proterozoic crustal blocks: 1.65–1.60 Ga Mazatzal orogeny. In: Gehrels, G.E., Spencer, J.E. (Eds.), Geologic Excursions through the Sonoran Desert Region, Arizona, and Sonora. Arizona Geological Survey Special Paper 7. Moores, E.M., 1982. Origin and emplacement of ophiolites. Reviews of Geophysics and Space Physics 20, 735–760. Moores, E.M., 2002. Pre-1 Ga (pre-Rodinian) Ophiolites: Their tectonic and environmental implications. Bulletin of the Geological Society of America 114, 80–95. Nicholas, A., Boudier, F., 1991. Rooting of the sheeted dike complex in the Oman ophiolite. In: Peters, T., Nicholas, A., Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of Oceanic Lithosphere. Kluwer Academic, Dordrecht, The Netherlands, pp. 39–54. Pallister, J.S., 1981. Structure of the sheeted dike complex of the Samail ophiolite near Ibra, Oman. Journal of Geophysical Research 86, 2661–2672. Patchett, P.J., Chase, C.G., 2002. Role of transform continental margins in major crustal growth episodes. Geology 30, 39–42. Pearce, J.L., Lippard, S., Roberts, S., 1984. Characteristics and tectonic significance of suprasubduction zone ophiolites. In: Kokelaar, B.P., Howells, M.F. (Eds.), Marginal Basin Geology: Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basin. Blackwell Scientific Publications, Oxford, England, pp. 77–94. Pederson, R.B., 1986. The nature and significance of magma chamber margins in ophiolites: examples from the Norwegian Caledonides. Earth and Planetary Science Letters 77, 100–112. Pederson, R.B., Malpas, C.A., 1984. The origin of oceanic plagiogranites from the Karmoy ophiolite, Western Norway. Contributions to Mineralogy and Petrology 88, 36–52. Rosencrantz, E., 1983. The structure of sheeted dikes and associated rocks in the North Arm massif, Bay of Islands ophiolite complex, and the intrusive process at oceanic spreading centers. Canadian Journal of Earth Sciences 20, 787–801. Rothery, D.A., 1983. The base of a sheeted dyke complex, Oman ophiolite: implications for magma chambers at oceanic spreading axes. Journal of the Geological Society of London 140, 287–296.

References

93

Sarewitz, D.R., Lewis, S.D., 1991. The Marinduque intra-arc basin, Philippines: basin genesis and in situ ophiolite development in a strike-slip setting. Geological Society of America Bulletin 103, 597–614. Skjerlie, K.P., Furnes, H., 1996. The gabbro-dyke transition zone demonstrated on Tviberg, SolundStavfjord ophiolite complex. Geological Magazine 133, 573–582. Stillman, C.J., 1987. A Canary island dyke swarm: Implications for the formation of oceanic islands by extensional fissural volcanism. In: Halls, H.C., Fahrig, W.F. (Eds.), Mafic Dike Swarms. Geological Association of Canada Special Paper 34, 243–255. Varga, R.J., Moores, E.M., 1985. Spreading structure of the Troodos ophiolite, Cyprus. Geology 13, 846–850. Walker, G.P.L., 1987. The dike complex of Koolau volcano, Oahu: Internal structure of a Hawaiian rift zone. U.S. Geological Survey Professional Paper 1350, 961–993. Wessels, R.L., Karlstrom, K.E., 1991. Evaluation of the tectonic significance of the Proterozoic Slate Creek shear zone in the Tonto Basin area. In: Karlstrom, K.E. (Ed.), Proterozoic Geology and Ore Deposits of Arizona. Arizona Geological Society Digest 19, 193–210. Wrucke, C.T., Conway, C.M., 1987. Geologic map of the Mazatzal Wilderness and contiguous roadless areas, Gila, Maricopa, and Yavapai counties, Arizona. United States Geological Survey OpenFile Report 87-664, scale 1:48,000.