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GEOSPHERE Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension, Romero Wash–Tecolote Ranch GEOSPHERE; v. 13, no. 2 area, southeastern Arizona doi:10.1130/GES01381.1 Daniel A. Favorito and Eric Seedorff 24 figures; 4 tables; 3 supplemental files Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA

CORRESPONDENCE: dfavorito@email .arizona .edu ABSTRACT alteration and sparse quartz veins are present without observed potassic CITATION: Favorito, D.A., and Seedorff, E., 2017, alteration. A barren porphyry dike (65.9 ± 0.7 Ma, U-Pb zircon) cuts the Romero Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension, Laramide structures that are exposed along much of the Laramide por- Wash fault, limiting the upper age of the fault. Similar dikes nearby are sericiti Romero Wash–Tecolote Ranch area, southeast phyry copper province of southeastern Arizona have been dismembered and cally altered, suggesting that alteration postdates shortening, similar to inter- ern Arizona: Geosphere, v. 13, no. 2, p. 577–607, tilted by superimposed Cenozoic normal faults, such that the overall style pretations reached at the nearby porphyry copper systems of Ray, Resolution, doi:10.1130/GES01381.1. of shortening (e.g., thin-skinned, low-angle thrusts versus basement-cored, and Kelvin-Riverside. moderate-angle reverse faults) is unclear pending a compelling reconstruction Received 19 June 2016 Revision received 7 December 2016 of superimposed extension. This study integrates new geologic mapping in Accepted 27 January 2017 the Romero Wash–Tecolote Ranch area with earlier work and utilizes palin- INTRODUCTION Published online 17 March 2017 spastic reconstructions of structures related to extension and shortening to determine the geometry of structures in this segment of the Laramide oro- Porphyry copper deposits form in magmatic arcs that mostly develop in genic belt and their relationship to local products of magmatism and hydro- compressional tectonic settings, but the detailed temporal and spatial relation- thermal alteration. ships between shortening and development of porphyry deposits are not clear. Geologic mapping indicates that multiple generations of Cenozoic normal Locations where both the timing of porphyry deposit formation and reverse faults can be identified through crosscutting relationships. Near both ends of faulting are known are few. Certain deposits globally seem to postdate short- the study area, the oldest synextensional strata dip vertically, indicating that ening (e.g., Barton et al., 2005; Nickerson et al., 2010), whereas others appear the area has been tilted 90° eastward during extension. Structural reconstruc- to have been emplaced during or before shortening (e.g., Perelló et al., 2010; tions of normal faults reveal two reverse faults within the area, the Romero Piquer et al., 2015). Separate questions are whether reverse faults and associ- Wash fault and the Tecolote fault. Once restored, both faults originally dipped ated folds play a role in the location of porphyry deposits and the geometry in opposite directions at moderate angles and demonstrate clear evidence of the intrusions and alteration patterns. It has been proposed that intrusions for associated fault-propagation folds. As a result of 90°E tilting by Cenozoic related to porphyry deposits form in regional transfer zones of reverse faults normal faulting, the Romero Wash fault became an overturned reverse fault, (Hill et al., 2002; Gow and Walshe, 2005; Kloppenburg et al., 2010), but other lo- whereas the Tecolote fault became an apparent normal fault. Constraints from cations such as in hanging-wall anticlines or along fault planes may be plausi the surface geology and forward models indicate that the Tecolote fault has ble (Niemeyer and Munizaga, 2008). The relationship between shortening and ~2.8 km of displacement on a fault that had an initial dip of 55°E, whereas porphyry deposit generation has implications for exploration strategies. the Romero Wash fault has ~0.75 km of displacement on a fault that had an With additional work, the nature of shortening, i.e., primarily the geometry, initial dip of 58°W. Considering their relative displacements and orientations, magnitude, and timing of structural features, also could be characterized well in the Romero Wash fault is interpreted to be a backthrust to the Tecolote fault. the Laramide porphyry copper belt of Arizona because extension has exposed These results are consistent with the interpretation that Laramide shorten- the Laramide crust at a wide range of crustal depths (Barton et al., 1988; See- ing in southeastern Arizona was dominated by basement-cored uplifts. Based dorff et al., 2008). Shortening in the North American Cordillera during the Lara on reconstruction of a cross section, the amount of Laramide shortening (e = mide orogeny (ca. 80–50 Ma) consisted of both thin-skinned, low-angle thrust

Δl/li) in the study area was 1.8 km or 20%, and the total amount of subsequent faults and basement-cored uplifts bounded by variably vergent, moderate-

Cenozoic extension (e = Δl/li) was 14.6 km or 200%. angle reverse faults (DeCelles, 2004). Shortening in the Sevier belt of southern For permission to copy, contact Copyright Two stocks of granodiorite, with associated porphyry dikes and related Nevada and central Utah is of thin-skinned, thrust belt geometry (Armstrong, Permissions, GSA, or [email protected]. hydrothermal alteration, occur within the study area. Sericitic and propylitic 1968; DeCelles and Coogan, 2006), whereas the foreland area of the central

Rocky Mountains of Wyoming and Colorado, and perhaps New Mexico and 111°W 110°45′W G A l p o a c Chihuahua, Mexico, contains basement-cored uplifts (Brown, 1988; Hamilton, 188 b h e H e 60 M i t N l n l 1988; Haenggi, 2002; Seager, 2004; Erslev and Koenig, 2009). The character s s of Laramide shortening is least understood in southeastern Arizona. This is Miami where Cenozoic extension and associated sedimentation have resulted in dis- Globe 10 km 60 memberment, tilting, burial, and local erosion of earlier products of Laramide 70 P deformation (Dickinson, 1991). It is also an area where numerous porphyry i n a l copper systems were forming broadly contemporaneous with shortening Superior M t n s (Titley, 1982). Interpreted Laramide features in this area include many base- 60 Resolution ment-cored uplifts (Davis, 1979; Dickinson, 1991), a few thin-skinned thrusts (Dickinson, 1991; Waldrip, 2008), and local tectonite fabrics (Bykerk-Kauffman 33°15′N and Janecke, 1987). Thus, the structural style of deforma tion appears to be

inhomogeneous within the region, perhaps varying with time or in space. D 77

117 r M i e Although the overall timing of the Laramide orogeny has been broadly p s c a p l characterized, only a few reverse faults or related fabrics in Arizona have tight i n M g t Ray n S s r age constraints (Bykerk-Kauffman and Janecke, 1987; Krantz, 1989; Waldrip, p e r i Riv Gila n 2008; Nickerson et al., 2010). In addition, the magnitude of shortening in the g T M o region also is poorly constrained, in part due to the complications of Cenozoic t r n t s i Kelvin Kearny extension. l l a

This study describes the geology of the Romero Wash–Tecolote Ranch area M Christmas t Riv n er a 2 il (~220 km ), located in the southern Tortilla Mountains west of the San Pedro s 117 G Valley (Fig. 1). The results presented here are based on new field work, with 33°N emphasis on structure and alteration, as well as incorporating results of pre- Winkelman vious maps of Krieger (1974a, 1974b) and Keith (1983). We show that Cenozoic Sa extension has tilted Laramide-age and older rocks in the study area 90° to the Study n east, such that the two main Laramide reverse faults now are an apparent Area Dudleyville P

d normal fault and an overturned reverse fault. Structural reconstructions indi- d

r r o cate that shortening produced a basement-cored uplift bounded by moder- o 77

ately dipping reverse faults with opposing vergence. New radiometric dates

and crosscutting relationships constrain the age of reverse faulting. Mapping N i n R t i revealed two small-scale patterns of hydrothermal alteration associated with Flagst e a v y e - r porphyry dikes, which postdate shortening and may represent the fringe of a S i x larger porphyry system. There is a lack of a close spatial relationship between 32°45′N H Phoenix i l l s hydrothermal alteration and reverse faulting, and alteration patterns exhibit a 79 Mammoth general lack of structural influence. Tucson San Manuel 77 REGIONAL TECTONIC AND GEOLOGIC SETTING Figure 1. Location map of study area (red dashed box) showing towns (green circles), The Romero Wash–Tecolote Ranch area is located within a highly extended significant copper deposits and/or districts (copper stars), major roads (bold black portion of the southeastern Basin and Range province where mid- to late lines), rivers (blue lines), and mountain ranges (gray regions). Black rectangle in map of Cenozoic extension resulted in dismemberment of the Laramide magmatic Arizona designates area of the figure. arc (Dickinson, 1991). Laramide magmatism resulted in numerous porphyry copper systems in southeastern Arizona, making the region one of the largest Manuel–Kalamazoo (Fig. 1). The center of the study area is ~80 km north of copper provinces in the world (Titley, 1982). In many places, the porphyry sys- Tucson and 120 km east-southeast of Phoenix. Nearby towns include Dudley tems were dismembered and tilted by subsequent Cenozoic extension (Wilkins ville and Winkelman, both located ~7 km to the west. and Heidrick, 1995; Maher, 2008; Nickerson and Seedorff, 2016). Nearby major Crystalline basement rocks in the southern Tortilla Mountains consist of copper districts include Ray, Superior, Globe-Miami, Christmas, and San Proterozoic Pinal Schist that was deposited at ca. 1.7 Ga (Meijer, 2014), which

subsequently was intruded by various plutons, including the Ruin and Ora documented (Bykerk-Kauffman and Janecke, 1987). Magmas were emplaced cle Granites at 1.4 Ga (Conway and Silver, 1989; Dickinson, 1991). Following a throughout the Laramide time interval in southeastern Arizona (e.g., Reynolds period of erosion, rocks of the Proterozoic Apache Group, consisting mostly et al., 1986). As in most arc settings, magma composition generally became of fine siliciclastic and lesser carbonate rocks, were deposited on the beveled more felsic with time (e.g., Lang and Titley, 1998). Local sedimentary rocks surface. Next, the Troy Quartzite was deposited, followed by the emplacement and mafic Laramide magmatism of the Williamson Canyon volcanics predated of dikes, sills, and sheets of diabase and eruptions of basaltic lava at ca. 1.1 Ga shortening (Willden, 1964). Magmatism associated with porphyry copper de- (Wrucke, 1989; Bright et al., 2014). Diabase primarily intruded as sills along posits ranges in age from ca. 75–60 Ma near the study area (e.g., Seedorff stratigraphic horizons in Proterozoic strata and as subhorizontal sheets in the et al., 2005b) but extends to younger ages farther south and east (e.g., Barra upper ~1 km of the underlying crystalline basement, regardless of basement et al., 2005; Leveille and Stegen, 2012; Stegen et al., 2016). In the few examples lithology or fabric (Howard, 1991). After a hiatus of a half-billion years, Paleo- with geologic constraints, these porphyry systems appear to postdate shorten- zoic carbonate and lesser siliciclastic rocks were deposited unconformably ing (Manske and Paul, 2002; Barton et al., 2005; Nickerson et al., 2010). Finally, over Proterozoic strata. two-mica granites were emplaced from the late Laramide to the mid-Cenozoic The Laramide orogeny, commonly defined as spanning the interval from (e.g., Gehrels and Smith, 1991; Fornash et al., 2013). ca. 80 Ma to ca. 50 Ma (e.g., Coney, 1976), has been interpreted to be the re- Following an extended period of tectonic quiescence, the subducting slab sult of tectonic basal traction on the lithosphere during a time of subhorizon- underneath the North American Cordillera steepened from its previously sub- tal subduction (Bird, 1998). The style of Laramide shortening in southeastern horizontal orientation, allowing for a new influx of magmatism from 34–15 Ma Arizona appears to be variable (Fig. 2). Basement-cored uplifts bounded by (Dickinson, 1991). Because there was no more plate coupling beneath south- moderate-angle reverse faults (Davis, 1979; Lawton and Olmstead, 1995) and eastern Arizona after this time, crustal thinning could commence. The major- thin-skinned thrust faults (Waldrip, 2008) are reported, with the dominant style, ity of extension occurred from 25–15 Ma, and synextensional sediments were or spatial and temporal variations in style, yet to be determined. Ductile fea- deposited during this time in basins created by normal faulting (Dickinson, tures related to Laramide shortening, such as tectonite fabrics, have also been 1991; Gawthorpe and Leeder, 2000). Tilting associated with normal faulting is recorded by inclined sequences of sedimentary rocks that commonly display 111° W 110° W fanning-upward dips, indicating growth during slip on normal faults (Maher, Wal Globe 2008). By late Miocene, modern basins began to accumulate sediment that is n N Canyonut Flagsta flat or gently dipping (Scarborough and Peirce, 1978). Recent dissection of this basin fill has been accompanied by development of piedmont surfaces, stream Favorito and Seedorff -Geosphere Phoenix terraces, and soils of variable maturity. Part 1: DESCRIPTION OF ROCK TYPES Tucson Part 2: METHOD OF CROSS SECTION RECONSTRUCTION AND VALIDATION Florence 33° N Winkelman Part 1: GEOLOGIC UNITS Sa ord DESCRIPTION OF ROCK TYPES Hot Springs

Proterozoic and Paleozoic Rocks J L i Canyon ohnny ttle Dragoon- Nor The pre-Cenozoic stratigraphy of the study area generally consists of a thin Basement rocks within the study area include the Pinal Schist and Ruin Granite (Dickinson, 1991). Pinal Schist crops out in the eastern half of the study area (Fig. 3) and section of Proterozoic clastic rocks overlain by thinner sections of Paleozoic Ap th Chiricahua consists primarily of metavolcanic rhyolite to andesite (Krieger, 1974a; Keith, 1983). The unit carbonate and Mesozoic volcanic rocks that have been intruded by various has a maximum crustal thickness of 2 km in exposures along Swingle Wash. The large majority Edgar Canyon Ly ache of basement exposures consist of Ruin Granite, a porphyritic granite distinguished by large K- on Hills igneous bodies ranging in age from Proterozoic to as young as Cenozoic (Figs. feldspar crystals up to 5 cm long set in a coarsely crystalline groundmass of quartz, plagioclase, E Pass and biotite. Aplite, pegmatite, and alaskite porphyry dikes of similar age are present within the m 3 and 4). Important Laramide intrusions include diorite, Smith Granodiorite, pi Tucson Ruin Granite, and a fine-grained alaskite phase is present near the (untilted) top of the pluton re-Santa Rita Little Rincon (Krieger, 1974a). and Rattler Granodiorite (Fig. 3). The total thickness of pre-Cenozoic strata is The Apache Group was deposited nonconformably on basement and consists of quartzite, ~1 km in the study area, and a complete section is only observed west of Jim siltstone, and lesser carbonate rocks (Shride, 1967; Wrucke, 1989). The Apache Group consists of the Pioneer Formation, Dripping Springs Quartzite, and Mescal Limestone (Fig. 4). In some 32° N Drag Thomas Wash (Fig. 3). areas, the Mescal Limestone is overlain by basaltic lava flows that are overlain by the Troy Hars Quartzite. Proterozoic diabase sills averaging 150 m thick intruded primarily into the siltstone o Cenozoic rocks within the study area consist dominantly of synextensional unit of the Dripping Springs Quartzite and as subhorizontal sheets into underlying basement rock Whetstone-Mustanon Swisshel h sedimentary rocks that span from Oligocene to Pliocene and locally range within a kilometer of the unconformity (Fig. 3, 5). The diabase, dated at ~1.1 Ga (Wrucke, 1989; aw Cr Bright et al., 2014), is dark gray-green, commonly fine-grained at chilled margins, and medium- Huac in thickness from 0 to 1.5 km. These rocks include the Oligocene to Miocene to coarse-grained with ophitic to subophitic texture near the interior of the intrusions (Wrucke, 1989). The most abundant minerals are plagioclase, clinopyroxene, and olivine. m eek huca Cloudburst Formation, Miocene San Manuel Formation, Miocene to Pliocene Paleozoic rocks are only exposed in the western section of the map (Fig. 3) and consist g primarily of siltstone of the Cambrian Abrigo Formation and carbonate rocks of the Martin Nogales Bisbee Quiburis Formation, and less extensive Cenozoic dikes and other possible in- Devonian Formation and Mississippian Escabrosa Limestone (Bryant, 1968; Krieger, 1974b). trusions. Stratigraphic horizons here serve as important geologic markers for 50 km structural reconstructions, and dated horizons offer temporal constraints on 1Supplemental Text. Please visit http://doi .org/10 1 .1130/GES01381 .S1 or the full-text article on www Figure 2. Map of Laramide reverse faults of southeastern Arizona with data from Krantz the ages of faulting. Supplemental Text , Part 1, contains descriptions of the .gsapubs.org to view the Supplemental Text. (1989) and Dickinson (1991). Study area shown by red dot. various rock types.

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Mesoproterozoic Troy R anc h Tertiary rock avalanche deposits Overturned normal fault, stick and ball on Yd Tca Yt Quartzite down-dropped block (normal fault rotated Map eyer through horizontal) Location M Tertiary and/or Cretaceous Mesoproterozoic Apache KTu Ya Ya Xp Group 50 undivided igneous bodies Overturned reverse fault, teeth on uplifted Mesoproterozoic Ruin Cretaceous Smith Granodiorite Yr block (reverse fault rotated through horizontal) Xp Ksg Granite 35 Apparent reverse fault (normal Ranch Yr Cretaceous Rattler Granodiorite Paleoproterozoic Pinal Schist fault rotated through vertical) Kr Xp Wash Apparent normal fault (reverse Tsm Kw Cretaceous Williamson Canyon Syncline trace with Volcanics fault rotated through vertical) plunge direction 0 km 5 km w E A Qa Tq KTu Ya A′ Tsm Tsm 3 km Ya Yt Tchu Yd Tchu Yr Pz Tchl

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Cenozoic stratigraphy STRUCTURAL FRAMEWORK Pliocene Quiburis Formation San Manuel Formation Proterozoic Shear Zones iocene M A pervasive east-northeast–trending quartz ± biotite fabric in Ruin Granite upper member Hackberry Wash facies of has been mapped northwest of the study area in the Kelvin-Riverside area Cloudburst Formation (Schmidt, 1971). Proterozoic diabase dikes crosscut the foliation (Schmidt, lower member 1971), and similar features in Ruin Granite have been recorded in the Winkel- Oligocene man, Crozier Peak, and Black Mountain quadrangles (Krieger, 1974a, 1974b, Proterozoic - Mesozoic stratigraphy 1974d). This deformation was interpreted by Schmidt (1971) to be Proterozoic in age and the result of differential movement within the crystallizing igneous body of Ruin Granite during the final phase of emplacement. 0.9 volcanic rocks

0.8 Mesozoic Laramide Structure

Prior workers in the region have recognized the difficulty of distinguishing Escabrosa Limestone 0.7 Laramide reverse faults from low-angle normal faults in east-central Arizona upper member (Willden, 1964; Krieger, 1974a, 1974b; Richard and Spencer, 1998b; Maher, Martin Formation lower member 2008). Previously identified reverse faults within the study area include the m) 0.6 aleozoic P Romero Wash fault. Krieger (1974a, 1974c) interpreted the fault to be a flat Abrigo Formation

ness (k section of an east-vergent, thin-skinned thrust that had been subsequently 0.5 tilted. Dickinson (1991) proposed that this fault may correlate with a similar upper member structure to the southeast, near the mouth of Aravaipa Creek (Young et al., Troy Quartzite 2009). Just north of the study area near Ray (Fig. 1), the Walnut Canyon thrust 0.4 (Fig. 2) places Pinal Schist over Proterozoic and Paleozoic strata as young as lower member Naco Formation (Keith, 1986; Richard and Spencer, 1998b). A fold is present Mescal Limestone in the hanging wall of the fault as an overturned east-vergent anticline with 0.3 erage stratigraphic thick a north-trending fold axis. The fault is interpreted by Richard and Spencer Av siltstone member Apache Group (1998a) as a series of flats and ramps, connecting with the Telegraph Canyon thrust to the west. Evidence for east-vergent Laramide reverse faults and asso 0.2 diabase Dripping Spring Quartzite ciated folds in Paleozoic and Mesozoic strata is also observed farther north in arkose member the Superior district (Manske and Paul, 2002). To the northwest of the study 0.1 Barnes Conglomerate Member area, in the Christmas quadrangle, several reverse faults and related folds oterozoic

Pr upper member have been identified (Willden, 1964). As in previous examples, an east-north- Pioneer Formation east vergence direction is suggested. 0 Scanlan Conglomerate Member

Cenozoic Structure Ruin Granite Cenozoic extension within southeastern Arizona is characterized by north-northwest–trending normal faults of varying dip associated with tilt- block homoclines (Dickinson, 1991; Maher, 2008). Synextensional sedimentary Pinal Schist rocks with fanning-upward sequences (e.g., Cloudburst Formation and San Manuel Formation) were deposited in the hanging walls of these normal faults. Figure 4. Stratigraphic column for the southern Tortilla Mountains. Based on data from Regions that have undergone tilting due to extension can be subdivided into Krieger (1974a, 1974b) and Dickinson (2002). tilt domains, which are broad areas with rocks tilted roughly in the same direc-

tion and by similar amounts (Maher, 2008). In the northern Tortilla Mountains Adobe Illustrator® was used to generate modern-day cross sections. Bed- (Fig. 1), Nickerson et al. (2010) attributed the observed 90° of eastward tilting to ding measurements were projected along strike into sections at the same be the result of multiple sets of down-to-the-west nearly planar normal faults, eleva tion. Sequential unfaulting and untilting of Cenozoic normal faults were with one set of down-to-the-east faults. A similar expression of extension is performed using Adobe Photoshop®. Pre-Cenozoic cross sections were then seen near Ray, where sets of down-to-the-west normal faults are interpreted forward-modeled with the Midland Valley MoveTM structural modeling soft- to have also tilted rocks steeply to the east (Maher, 2008), and near Resolu- ware kit, using modern surface contacts and dips as constraints. Adobe Illus- tion, where faults have tilted rocks moderately to the east (Manske and Paul, trator® was then used to model erosion surfaces and synextensional depos- 2002; Maher, 2008). In addition to tilted blocks, normal fault-related folds have its. Stereonet 9 (Allmendinger et al., 2012) was used to generate and contour been interpreted in the region of study. Howard and John (1997) interpreted equal-area stereonet plots of structural data and to calculate fold axis orienta- the Tecolote and Jim Thomas synclines, both located within the study area, tions. Supplemental Text (see footnote 1), Part 2, contains further explanation possibly to be the result of forced folds in cover strata that occurred as a direct of reconstruction methods. result of Cenozoic normal faulting. Two samples from the study area were collected for U-Pb radiometric age dating. Igneous zircons were analyzed using the laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) method at the University of METHODS Arizona LaserChron Center (see Appendix).

This study began with a compilation of previous maps of the area by Krieger (1974a, 1974b), Keith (1983), and Dickinson (2002). Geology of an area DESCRIPTION OF STRUCTURES from Sample Wash north to Bee Wash (Fig. 3) was primarily mapped using the Anaconda paper- and mylar-based mapping method (Brimhall et al., The structures described below are ordered by age, from oldest to young- 2006) at scales ranging from 1:5000 to 1:10,000. Separate overlays were est. Faults that crop out in the study area are divided into descriptively simi used for lithology, structure, and alteration. Observations on alteration of lar sets of faults, which for convenience are numbered from 1 to 5 based felsic and mafic mineral sites, magnetite stability, veins, and ore minerals on crosscutting relationships, similar orientations, and similar spatially were recorded on the alteration overlay. The structural overlay focused on associated folds (Fig. 3). Key data related to each set of faults are tabulated gathering new bedding orientations, accurate fault traces, fault orientations, in Table 1. and kinematic indicators such as slickenlines. Specific smaller areas in the vicinity of Indian Camp Wash and Tecolote Ranch (Fig. 3) were mapped briefly in order to determine key geologic relationships. Positions of geo- Shear Zones logic measurements, sample sites, and photographs were acquired using a Garmin GPSMAP 64s. Locations of geologic contacts were determined and Shear zones and associated foliation are found together within the Horse recorded using a topographic map with a contour interval of 30 m and a Hills, but foliated rocks without associated shear zones persist at least 1.5 km custom GPS grid array. Mapped contacts were plotted over ArcGIS satellite to the south and 3 km to the west of the Horse Hills (Fig. 5). Both features images, and satellite images were used to extend mapped contacts into in- strike northeast, dip steeply to the southeast or northwest, and affect Ruin accessible areas. Granite and related aplite dikes. Local diabase dikes are not deformed. Foli Preexisting names for topographic and geologic features have been used ated granite has aligned biotite and elongated quartz grains. Ruin Granite as much as possible, but new names have been created as needed for refer- proximal to shear zones commonly displays moderate to intense chlorite ence. The newly named topographic features are Bee Wash and Cow Fence alteration with stable K-feldspar and quartz. In several areas, shear zones Wash (Fig. 3), and the newly named geologic features include the Dirt, Sand, appear to crosscut and displace Ruin aplite and pegmatite dikes (Fig. 5). Cholla, Tecolote, Eagle Wash, Tarantula, Agave, Beehive, Diorite, Northern Because crosscutting bodies of diabase are undeformed, this deformation Ridge, Southern Ridge, Bee Wash, and Horse Hills faults (Fig. 3). is interpreted to be Proterozoic in age, as others have also concluded (e.g., Due to the nature of weathering of local host rocks, primarily of the Ruin Schmidt, 1971). Granite, many key faults are poorly exposed, and few direct fault plane measurements could be made. Structure contour maps and three-point problems were used to determine the orientations of most faults. Some fault segments Fault Set 1 and Spatially Associated Folds solely involving Ruin Granite were partially mapped in ArcGIS using satellite imagery where fault gouge was apparent. These data were subsequently Fault set 1 includes faults that do not cut Cenozoic sedimentary rocks, and field checked. all of these faults have evidence for fault-related folds.

TABLE 1. CHARACTERISTICS OF FAULTS IN THE ROMERO WASH–TECOLOTE RANCH AREA Present-day Sedimentary rocks with growth Yo ungest rocks cut and Exposed crosscutting relationships Fault set Examples of named faults Strike direction dip of faults relationships to faults of this set offset by faults of this set with faults of older sets Notes 5 (youngest) Smith Wash, Northern Primarily NNW Primarily ENE, Quiburis FormationQuiburis FormationFaults cut and offset faults of Ridge, Southern Ridge steep sets 1, 2a, 3 4Horse Hills, Cowhead Well, NNW WSW, steep Quiburis FormationSan Manuel Formation, Faults cut and offset faults of Eagle Wash possibly Quiburis Formation sets 2a, 2b, 32 3Hackberry, Swim Hole NNE WSW, moderateSan Manuel FormationUpper member of Cloudburst Faults cut and offset faults of Jim Thomas syncline is Formation set 2b; no spatial overlap in hanging wall of with faults of sets 1, 2a Hackberry fault 2b Dirt, Sand Primarily NE, shallowLower member of Cloudburst Cretaceous volcanic rocksNo spatial overlap with faults of Restricted to NW portion Formation set 1 of study area 2a Beehive, DioriteEto SSE Primarily S to SW, None observedWilliamson Canyon volcanics Faults cut and offset faults of Restricted to NE portion moderate or Smith Granodiorite set 1 of study area 1 (oldest) Romero Wash, Bee Wash, NNW and NE NNE and NW, None observedRomero Diorite N.A. (oldest set) Associated fault Tecolote moderate propagation folds (Tecolote syncline) Abbreviation: N.A.—not applicable. 1Age relationship between fault sets 2b and 2a is uncertain. 2Crosscutting relationship observed in Putnam Wash quadrangle (Dickinson, 2000).

Romero Wash Fault System Wash fault, there is little to no variation in bedding orientation (Fig. 10), suggesting that there is no folding here—only tilting. In the Bee Wash area, Apache This fault system is located along the eastern side of the study area (Figs. Group strata strike N-S and dip either upright east, vertical, overturned west, 3, 5, and 6) and includes the Romero Wash fault and Bee Wash fault (Table 1). or doubly-overturned east, i.e., rotated over 270° (Figs. 6, 9B, and 9C). This In the Romero Wash area (Fig. 5), the Romero Wash fault strikes ~N25°W, dips change in bedding orientation occurs locally around the Romero Wash and 28°–36°E, and places older Ruin Granite on younger Apache Group and dia- Bee Wash faults, as shown in roughly E-W cross sections (Figs. 11 and 12), in- base. From Cow Fence Wash to Sample Wash, Romero Diorite intrudes along dicating the presence of fault-related folds. Stereonet analyses of the southern the fault. The eastern dike contact is intrusive, and the western contact is either Bee Wash area indicate rocks have been folded about an axis of N18°E 37° and brecciated or covered by alluvium. In the Romero Wash area, breccia along this the average interlimb angle is 74° (Fig. 9C). Only one outcrop with an obvious contact occurs in the Romero Diorite and rocks in the footwall of the fault and fold was found in the Pioneer Formation (Fig. 13). Fold axes here averaged is characterized by angular clasts up to tens of cm in diameter surrounded by N26°E 38°, closely matching stereonet results. clay fault gouge (Fig. 7). Locally, this zone of breccia and gouge is ~5–10 m wide. A map-scale syncline also has been observed southwest of Flying UW These relationships indicate that the western dike contact is the fault plane. Ranch (Fig. 3). It is unclear, however, whether this is a fault-related fold be- Structure contour mapping of this surface (Fig. 8) shows that the Romero Wash cause the present geometry of the syncline (Krieger, 1974a; Keith, 1983) pre- fault is approximately planar. Due to relative offsets of diabase and Apache dicts a reverse fault dipping to the west, but no direct evidence for such a fault Group just southeast of Bee Wash, the Romero Wash fault is interpreted to con- was observed. tinue northward through the Bee Wash area (Figs. 3 and 6). Here, the Romero Wash fault strikes N25°W to N, dips 55°–60°E, and places older Apache Group and diabase on younger Apache Group and diabase. In rare exposures of the Tecolote Fault and Spatially Associated Folds fault within this area, there is a zone of fault breccia and gouge roughly 1 m wide. At Cow Fence Wash (Fig. 5), an unaltered Smith Granodiorite porphyry The Tecolote fault (Table 1) is located east of Tecolote Ranch (Fig. 3) and oc- dike, with a U-Pb zircon age of 65.87 ± 0.69 Ma, crosscuts the Romero Wash curs in relatively flat topography with sparse exposure. The fault strikes N40°E fault; therefore, the Romero Wash fault must be Laramide or older. to N10°E and dips 41°W. The Tecolote fault places younger Apache Group and The Bee Wash area (Fig. 6) is also the only location where the Bee Wash diabase on older Ruin Granite. The fault cuts a dacite porphyry dike and an fault is exposed. This fault has a similar orientation to the Romero Wash fault unclassified intermediate to felsic dike that had intruded Apache Group strata locally and also places older rocks on younger rocks. in the hanging wall (Fig. 3). In the rare locales of good exposure (Fig. 14), a In the Romero Wash area, Apache Group strata dip vertically to steeply to zone of fault breccia and gouge roughly 10–15 m wide is present. Clasts here the east (Figs. 9A and 5). Along E-W cross sections that contain the Romero are commonly angular and several tens of cm in diameter.

81 Horse Hill Horse

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87 83 u 85 Y 85

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85 85

dsb

72 30 36

31 63 Yd

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Yp Romero Kd Yd Y Wash fault

Ksg

80 65.9 + 0.7 Ma 80

dsa

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U-Pb zir 65

U-Pb zir Yd as R h o a

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60 75 Ym

35 35 45 35 Qs

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26 25

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25 50 B ′

Ts 20

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m 20 m Yd T TKr Yd Yd Ts Kh d TKu Kh a Ksg Qa l Ym Yr Ym Qs chu Xp Kd Qt Yp Yd Tq Kr Ys Yt Yr 60 15 m sb sa ss 30 a 70 d Map S Map Units dik rhyodacite di Cr Cr Cr andesit Cr Granodiorit Cr Te Te hornblende diorite Granodiorit Te Te Te Quaterna Quaterna Quaterna Wa Conglomerat Granit Mesopr Granite aplit mylonitic Ruin Granit Formation Springs Ark Springs Siltstone Limestone Quar Mesopr Schist Pa Mesopr Mesopr Mesopr Conglomerat Mesopr Mesopr Mesopr Mesopr Mesopr Mesopr rt rt rt rt rt etaceous etaceous Rattler etaceous pr etaceous hornblende etaceous Smith leopr sh facies of Wa U-Pb zir Contact es, undivided St Over (teeth on uplif bedding Estimated stri St St bedding St dir Normal fault with dip ia ry ia ry ia ry ia ry ia ry ri ke ri ke ri ke ri ke tzit ection sh e ymbols Upper Member e and/or Cr and/or Cr Quiburis Formatio San Manuel Formatio oter oter oter oter oter oter oter oter oter oter oter oter turned e of and dip and dip and dip ry ry ry con age dat ve talu s alluvium soil e e ose ozoic Pinal ozoic Scanlan ozoic Ruin Ro ozoic Ruin ozoic ozoic Pioneer ozoic Barnes ozoic Dripping ozoic Dripping ozoic Mescal ozoic Tr ozoic diab e Cloudburst Formatio ke e e ophyritic rt me ro re s ical bedding ke ve ted block of of of etaceous etaceous

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GEOSPHERE | Volume 13 | Number 2 Favorito and Seedorff | Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension 584 Research Paper

110° 52′ 30″ W 110° 52′ W 110° 51′ 30″ W Map Units Kdi N Tchu Tsrt Qal Quaternary alluvium

Yr Qt Quaternary talus Ydss Qt

30 ″ N 85 Yd 80 Qs Quaternary soil

32° 59 ′ Tq Tertiary Quiburis Formation 30 Tertiary rhyolitic to dacitic tuff Xp 65 Tsrt 60 Ydss Tertiary Upper Cloudburst Tchu Formation D 55 80 Yp 65 25 Ydsa TKu Tertiary and/or Cretaceous dikes, 78 63 Qs undivided 47 80 52 49 D′ Kr Cretaceous Rattler Granodiorite 56 80 Yr 52 75 Kdi Cretaceous Diorite 50 80 Yd 77 Yd 80 Yd Mesoproterozoic diabase Qal lt 45 Mesoproterozoic Troy u sb a Yd Yt Quartzite f 25 Xp Qs h 60 50 t 85 s l Mesoproterozoic Mescal a u a Ym f Limestone W h o h s r s a Mesoproterozoic Dripping e a 25 45 W Ydss m Qt Springs Siltstone W o Ydsa R e Mesoproterozoic Dripping 60 e Ydsa B Springs Arkose Tchu

Mesoproterozoic Barnes 67 Qt Yp Ydsb Conglomerate 58 60 45 Ydsb Mesoproterozoic Pioneer N Figure 6. Geologic map of the Bee Wash Yp Formation 35 area. Based on new mapping and previ- TKu e

32° 59 ′ Mesoproterozoic Scanlan Yr e

t Ys ous work by Krieger (1974a), Cornwall and

73 l B Conglomerate 36

u Yp Kr a 85 Krieger (1975), Keith (1983), and Dickinson 54 f 40 Mesoproterozoic Ruin Yd Yr 63 57 e Granite

l (2002). Location for this figure is shown in

o Qs 83

H Figure 3. Ydsa Paleoproterozoic Pinal Schist 51 84 Xp 42 57 m Xp 63 56 i Qal

60 w 75 72 Ydsb 48 52 S Qt 70 60 Yd Ydss C Ys 51 Ydsa 51 Map Symbols 38 70 68 80 53 80 85 81 70 Strike and dip of bedding 40 80 25 Agave fault67 Qt 48 65 Strike and dip of overturned 60 30 71 68 Tchu bedding ee B Ydsa W sh 39 a 65 Strike and dip of 73 45 70 65 Ydss 60 doubly-overturned bedding 49 Ys 57 58 Ym Strike of vertical bedding Bee 88 Yd 75 hiv 44 65 52 Yp e 53 Strike and dip of foliation f 67 50 a 74 53 56 Ydss u l 63 68 t 61 70 Strike of vertical foliation 59 64 40 55 Kr Qt 38 50 60 Contact 60 79 70 64 50 Yr 54 30 Kr Normal fault with dip 21 50 85 direction

Overturned normal fault 85 79 30 ″ N

60 Yd Xp 50 Overturned reverse fault 32° 58 ′ 60 65 C′ Wash

Yr Tchu Yp 70 Ydsa Ydsa Yp Ydss Kr 60 Ym Yt 0.5 km Kdi 70 65 65 75

3300 3450

Dip: 30°

Dip: 30° 30 ″ N 32° 56 ′

330

Figure 7. Photograph of fault breccia within the Romero Diorite located approximately 1 km south of Romero Wash along the Romero Wash fault. Dip: 28° Figure 8. Structure contour map of the Romero Wash fault (red) between Cow Fence Wash and Sample Wash. Contour interval is 30 ft. Kd

Folds associated with the Tecolote fault affect rocks of the Apache Group

345 3300

on the northwestern side of the fault (Fig. 3). Here, the strike of bedding 0 ranges from N-S to E-W and dips north upright, east upright, vertical, and overturned west (Figs. 3 and 9D). The axis of the fold calculated by stereonet N analysis is N20°W 50° (Fig. 9D). The calculated axial plane is N48°E 70°W, 32° 56 ′ and the interlimb angle is 70°. The axial plane was estimated by assuming a simple bisector of the two relatively planar fold limbs. The Ruin Granite– Dip: 31° Apache Group contact is clearly folded (Fig. 3), and the dacite dike appears

to be folded. 110° 50 ′ 30 ″ W Fault Set 2a

50 The major faults within fault set 2a are the Beehive fault and the Agave 34 fault (Table 1). The majority of faults within this set occur east of Eagle Wash in the northeastern portion of the study area (Figs. 3 and 6). Major faults of this set primarily strike S30°E to E-W, dip ~35°–65°S, and place older rocks Dip: 36° on younger rocks or vice versa. Some faults of set 2a are mantled by the upper member of the Cloudburst Formation (Figs. 3 and 6), so faults of this Kd set were active before deposition of the upper member of the Cloudburst Formation. No synextensional deposits appear to be directly related to this 200 m 600 ft fault set.

A N B N C N D N

N18°E, 37° N29°W, 34° N26°E, 38° N21°E, 50°

Pole to bedding Fold axis Calculated axis from bedding 2% contour interval 2% contour interval Mean fold axis 1% contour interval 3% contour interval n = 100 n = 59 n = 213 n = 21

Figure 9. Lower-hemisphere equal-area stereonet plots of poles to bedding and fold axes for (A) Romero Wash area (Fig. 5); (B) Bee Wash area north of line of latitude 32°59′30″N (Fig. 6); (C) Bee Wash area south of line of latitude 32°59′30″N (Fig. 6); and (D) Tecolote syncline (Fig. 3). A plane of best fit to bedding poles is shown in nets where folding is present. Plots were generated with the program Stereonet 9 (Allmendinger et al., 2012).

WSW ENE

Rome ro W ash

Horse Hills fault faul t

B Tsm Figure 10. Geologic cross section across the Kd Tchu Romero Wash area along line of section BB′ Yp Yr shown in Figure 5, with no vertical exagger- Ydss Yd B′ ation. Projected locations of measured bed- Kd Kr Yp ding attitudes are shown near the surface. Ydsa Yd dss Yr Y Ym Yt Yd Tchu Yr Ydsb

Ca Me

Dm 0.5 km Yd Tchl

0.5 km

Romer W Tsm? E Bee

o W Wash fault ash fault

Swim Hole fault

Figure 11. Geologic cross sections across C the southern Bee Wash area along line of section CC′ shown in Figure 6, with no vertical exaggeration. Projected locations Ys Kr of measured bedding attitudes are shown Yd Ym near the surface. C′ Yr Yd Yd Yp Ydss Yp Ydsa Ym Yt Kr Yr Xp 0.5 km Ys

0.5 km

WSW Bee ENE

Wash fault

Swim Hole fault Figure 12. Geologic cross sections across D the northern Bee Wash area along line of Yr section DD′ shown in Figure 6, with no vertical exaggeration. Projected locations of measured bedding attitudes are shown Ydsa near the surface. Yd Yp D′ Xp Yd Yr

0.5 km Ydss Ys Ydsa Ym Ydsb Yd

0.5 km

Figure 14. Photograph of the Tecolote fault where an intermediate to felsic dike is cut by Figure 13. Photograph in the southwestern Bee Wash area 200 m north of section CC′, showing and currently lies above Ruin Granite. folded shale beds of the Pioneer Formation involved in an originally east-vergent anticline in the hanging wall of the Romero Wash fault. Its presently synformal geometry was produced by tilting associated with later normal faults. Beds are highlighted with dashed purple lines. Doubly-overturned beds dipping SSE are on the left, and overturned beds dipping NW are on Fault Set 3 the right. Image is facing N60°E. The Hackberry and Swim Hole faults constitute fault set 3 (Figs. 3 and 6; Table 1). These faults strike N20°–30°W, and the Swim Hole fault dips ~25°– Fault Set 2b 33°W. No exposures of the Hackberry fault in the field area were adequate to obtain a reliable attitude on the fault surface, although to the north, Dickinson Fault set 2b consists of the Dirt and Sand faults (Table 1), which occur (1995, 1996, 2001) documented dips ranging from 55° to 30°. Consequently, west of Jim Thomas Wash. These faults primarily strike north and place a structure contour map was made for the fault based on the intersection of younger Apache Group and diabase on older Ruin Granite (Fig. 3). The pre- topography with the trace of the fault for a length of ~1 km, which yields a dip cise location of these faults is uncertain due to a majority of their contacts of ~28°W (Fig. 15) The Swim Hole fault places younger Apache Group and dia being granite-on-granite in topographic lows with significant alluvial cover. base on older Pinal Schist, and the Hackberry fault places structurally higher Because of this, dips are also uncertain, but they are estimated to be ~20°E Ruin Granite intruded by diabase sheets on structurally lower Ruin Granite because of the attitudes of similar faults only a few km to the north near without diabase. Ripsey Wash (Nickerson et al., 2010, p. 308–309). The Dirt fault and Sand fault appear to be crosscut by a N-S–trending fault that likely belongs to fault set 4 (Fig. 3). Restoration of offset along this younger fault indicates Fault Set 4 that the Dirt fault and Sand fault are likely separate, parallel faults, with the Dirt fault being structurally higher. Exposures and associated crosscutting Major faults within set 4 include the Cowhead Well, Eagle Wash, and Indian relationships are not definitive, but it appears that the faults of set 2b were Camp faults (Fig. 3; Table 1). These faults generally strike N40°W to N-S and active during or in part before all of the deposition of the lower member of typically dip 50–60°W. The Cowhead Well fault, despite its moderate dip, is the Cloudburst Formation. This is supported by similar strikes between these grouped into this fault set because it cuts the Camp Grant fault, a fault that faults and lowest Cloudburst strata as well as the ~70° cut-off angle between most resembles those of set 3, located to the southeast in the Putnam Wash these features. Similar relationships are observed just north of the field area quadrangle (Dickinson, 2000). The Camp Grant fault is, in turn, regarded as the (Nickerson et al., 2010). northern continuation of the San Manuel fault (Hansen, 1983; Dickinson, 2000).

110° 55′ W Fault Set 5

N Primary faults within set 5 include the Smith Wash and Lopez Ranch faults. These faults strike N15°W to N and dip steeply to the east (Fig. 3; Table 1). The

Southern Ridge and Northern Ridge faults may be related to the Smith Wash

′ 110° 54 110°

fault tipping southward (Fig. 3).

″

W Cenozoic Folds

3330 The Jim Thomas syncline, located along Jim Thomas Wash (Fig. 3), is an example of a Cenozoic fold (Dickinson, 2002). It primarily involves units as young as the San Manuel Formation, with dips of strata ranging from hori- 15″ N Dip: 31° zontal to dipping gently to moderately to the east or west. The fold measures

32° 57′ 10 km along its axis and consistently trends north-northwest along the strike of the Hackberry fault. The southern end of the syncline plunges gently to the

3375 3450 north, whereas the northern end, which is beyond the study area, plunges to the south. The estimated axial plane is oriented almost due north and dips 85°W. The interlimb angle is ~120°.

Figure 15. Structure contour map of the VEINS, ALTERATION, AND MINERALIZATION Hackberry fault between line of section AA′ (Fig. 3) and the westernmost ex- Styles and Distribution of Alteration and Veins posure of Cretaceous diorite. Contour interval is 15 ft. 3525 3450 3375 Hydrothermally altered rocks crop out in the vicinity of Smith Wash in two main areas, both of which are proximal to Smith Granodiorite stocks (Fig. 16). The primary host rock for alteration is Ruin Granite, with a smaller areal extent of alteration in the Smith Granodiorite. The two main alteration styles

N present are sericitic and propylitic, both of which are characteristic of the ° upper levels of porphyry-style copper systems (e.g., Seedorff et al., 2005a).

32° 57′ Dip: 24 There are local quartz veinlets, but no clear evidence for potassic alteration was observed. Sericitic alteration within the field area is characterized by quartz-pyrite 3450 veins with sericitic envelopes that are clearly related to Smith Granodiorite porphyry dikes that are of Laramide age. These veins range from 1 to 2 mm to a few cm wide, occur in abundances ranging from roughly 2–20 veins per meter, strike roughly E-NE, and dip steeply N or S (Fig. 17A).

South of Smith Wash (Fig. 16), sericitic alteration is confined to a broad 3450 area of veins around a stock of unaltered Smith Granodiorite. Porphyry dikes that appear to emanate eastward from this stock are moderately sericitically altered or fresh. North of Smith Wash (Fig. 16), sericitic alteration is confined to a narrow region of veins around and within a stock of Smith Granodiorite and related porphyry dikes. A breccia pipe is located within the western portion of 200 m this stock, and it contains subangular cobble- to boulder-sized clasts of Smith 600 ft Granodiorite that are intensely sericitized and appear to have been rotated. Rare quartz veinlets occur in clasts of the breccia.

110° 52’ 30” W 110° 51’ 30” W N

N 69.8 ± 2.4 Ma U-Pb Zircon 32° 58’ N 32° 57’

65.9 ± 0.7 Ma U-Pb Zircon

Strong sericitic Moderate chlorite Moderate epidote Fault U-Pb zircon age date Moderate sericitic Weak chlorite Weak epidote

Weak sericitic 1 km

Figure 16. Map of the distribution of hydrothermal alteration within the Smith Wash area. Location for this figure is shown in Figure 3.

A N B N

N29°W, 34°

Figure 17. Lower-hemisphere equal-area stereonet plots of poles to veins throughout the study area. (A) Quartz-sericite- pyrite veins; (B) calcite-siderite veins. Plots were generated with the program Stere- onet 9 (Allmendinger et al., 2012).

2% contour interval 1.5% contour interval n = 65 n = 134

Propylitic alteration within the field area is characterized by K-feldspar Crosscutting Relationships between Faulting and Alteration (relict), epidote, chlorite, and hematite. Associated veins include carbonate, epidote, and chlorite veins. In general, this alteration is present distal to the Within the main area of alteration, the Diorite and Southern Ridge faults two centers of sericitic alteration (Fig. 16). Calcite-siderite veins occur through- crosscut altered rocks. Most minor faults within Figure 7 likely offset alter- out the study area, primarily near faults, and may be a more distal expres- ation, but these features were not mapped in detail. An exception to this is sion of propylitic alteration. These veins range from several mm to 1 m in located west of the southern stock of Smith Granodiorite, where propylitic width, strike E-NE, and dip steeply N or S. The similar orientation of the calcite- alteration appears to be concentrated along a minor E-W–striking fault. Near siderite veins to sericitic veins indicates that both types of veins may be part of Cow Fence Wash (Figs. 5 and 16), a barren Smith Granodiorite porphyry the same hydrothermal system (Fig. 17). crosscuts the Romero Wash fault. This porphyry dike is similar, if not identical, in original composition to sericitically altered porphyry dikes a few hundred meters north. This suggests that the Romero Wash fault predates hydrother- Copper Mineralization mal alteration.

Primary copper mineralization has only been observed in a small number of locations. Chalcopyrite either occurs in veins in diorite, in calcite-siderite U-Pb GEOCHRONOLOGY veins, or rarely in sericitically altered Ruin Granite. Oxidized copper minerals such as malachite and chrysocolla have been observed in the Ruin Granite and Table 2 shows results of U-Pb zircon geochronology for one sample from Pioneer Shale near the Romero Wash fault and in diabase where it is cut by this study and one sample collected and analyzed by S.E. Runyon (2016, per- the Smith Granodiorite. Near Smith Wash, copper-oxide minerals occur in sonal commun.), both of which are located in the eastern half of the study area clasts of Ruin Granite that are hosted by Smith Granodiorite igneous breccia (Figs. 5 and 16). Sample Ksg-p of Smith Granodiorite porphyry was chosen for and in northwest- striking faults. One occurrence of exotic copper conglomer- dating because it cuts the Romero Wash fault. U-Pb ages were obtained from ate was found near the southern Smith Granodiorite stock. Here, pebble-sized microanalysis of zircons by the laser ablation–inductively coupled plasma– clasts of Ruin Granite are cemented by copper oxide. mass spectrometry (LA-ICP-MS) method at the University of Arizona.

TABLE 2. U-Pb AGES OF IGNEOUS ROCKS IN ROMERO WASH–TECOLOTE RANCH AREA

Latitude Longitude Age 2σ Sample (N) (W)Rock type Local host of intrusion Structural relationships (Ma) (Ma) CommentsSource Ksg-p 32°56.818′ 110°51.200′ Smith Granodiorite Ruin Granite and Apache GroupCrosscuts Romero Wash 65.9 0.7 Inherited core age of S.E. Runyon (2016, porphyry fault 1414 ± 29 Ma personal commun.) Ksg-s 32°58.104′ 110°52.134′ Smith Granodiorite Ruin Granite and dioriteNo known relationship to 69.8 2.4 Inherited core age of This study structures 1442 ± 16 Ma

Sample Ksg-p yielded an age of 65.9 ± 0.7 Ma, and sample Ksg-s yielded an Cenozoic Normal Fault Generations age of 69.8 ± 2.4 Ma (Table 2). These ages confirm that the Smith Granodiorite is a Laramide intrusion. The normal faults that crop out in the field area can be grouped into four separate generations using crosscutting relationships, and an additional generation is interpreted to be present based on relationships exposed north of INTERPRETATIONS the field area (Table 3). Faults belonging to sets 2b, 3, 4, and 5 are Cenozoic normal faults because they cut all Laramide features (Fig. 3), have Cenozoic Identification of Laramide Faults synextensional sedimentary rocks in their hanging walls, display normal stratigraphic separation, and, in many exposures, cut and offset Cenozoic rocks or The Romero Wash fault, which belongs to fault set 1 (Table 1), is cut by have Cenozoic rocks in their hanging walls. The initial dip of each fault and the a Laramide dike (Fig. 5), making it a pre-Cenozoic structure. The Tecolote amount of tilting associated with each generation of faults can be estimated fault is included in fault set 1 primarily on the basis of similar style of asso- using structural reconstructions of faults and the angular relationship between ciated folds (i.e., overturned beds). Using these relationships, the Tecolote synextensional strata and faults of various generations (Fig. 18). The initial dip fault is also inferred as a pre-Cenozoic structure. Because the only other of a fault is determined by the cut-off angle between the fault and the oldest known period of pre-Cenozoic deformation within the region occurred related synextensional stratum. An exception to this is the second generation during the early Mesoproterozoic, and the Romero Wash and Tecolote of faults (Table 3); not one example of this generation of faults crops out in the faults cut late Mesoproterozoic diabase, these faults are inferred to be of study area (discussed below). The amount of tilting produced by a given fault Laramide age. generation is inferred to be roughly equal to the range of dips of synexten- Faults belonging to fault set 2a (Table 1) may also be Laramide faults. sional strata associated with that fault generation. These faults cut rocks as young as Smith Granodiorite (or Williamson Canyon volcanics), and some are mantled by the upper member of the Generation 1 Oligocene to Miocene Cloudburst Formation (Figs. 3 and 6). This suggests these faults may be related to the deposition of the lower Cloud- The first generation of Cenozoic normal faults consists of faults from set burst Formation (i.e., fault set 2b). However, once restored, these faults 2b (Table 3), because faults of sets 1 and 2a are Laramide faults (see above). have opposite dips and dissimilar strikes compared to those of fault set The normal faults of generation 1 generally lack crosscutting relationships with 2b (discussed later), indicating these fault sets are not related, and thus younger faults, as only one fault from set 4 (see generation 4 below) cuts faults fault set 2a is not related to the lower Cloudburst Formation using this of set 2b (Fig. 3). As noted earlier, the crosscutting relationships within the reasoning. field area are not definitive, but a reasonable hypothesis is that faults of set 2b Because fault set 2a is likely older than the upper Cloudburst, and not re- were active during or in part before all of the deposition of the lower member lated to the Oligocene lower Cloudburst, this fault set must be pre-Cenozoic of the Cloudburst Formation and controlled the half-grabens in which the sedi because the Cloudburst marks the beginning of Cenozoic extension within ments of the lower member of the Cloudburst Formation accumulated. The the study area (Dickinson, 1991). These faults may be late Laramide in age, Dirt and Sand faults of set 2b are overturned normal faults (i.e., they initiated because they clearly postdate fault set 1 (Laramide) and cut many, if not all, as west-dipping normal faults but have been rotated through horizontal and Laramide units. In order to understand the initial orientations and configura- now dip east), with estimated present-day easterly dips of ~20°. Considering tion of these Laramide faults, the effects of Cenozoic normal faulting must be that the oldest strata within the lower member of the Cloudburst Formation restored. Hence, an interpretation of Laramide structures follows the interpre- dip 90°E, this implies a bedding-to-fault angle, and ultimately a fault initiation tation of the Cenozoic structural evolution below, i.e., addressing faults of sets angle, of 70° (Fig. 18). The net amount of tilting associated with this fault gen- 2b through 4. eration is 40°E (Fig. 18).

TABLE 3. CURRENT AND RESTORED ORIENTATIONS OF TERTIARY NORMAL FAULTS1 Predominant dip Estimated net tilting Expected direction Maximum Normal fault Fault Synextensional of synextensional associated with of concurrent tilting Present Present offset Restored Restored Characterization generation set sedimentary unit sedimentary rocks2 each generation of fault blocks Fault strike3 dip3 (km) strike dip of fault 5 (youngest) 5 Quiburis Formation 5–0°E >5°W Westward Smith Wash N10°W60°EN.A.N.A.N.A. Normal fault 4 4 Quiburis Formation 10–0°E ~10°E Eastward Indian Camp N10°W60°WN. A. N11°W70°W Normal fault Wash Horse Hills N20°W60°W0.5 N20°W70°W Normal fault Eagle Wash N16°W54°W1.7 N16°W64°W Normal fault Cowhead Well N35°W40°W1.9 N33°W50°W Normal fault 3 3 San Manuel 30–10°E ~20°E Eastward Hackberry N25°W28°W1.5 N23°W58°W4 Normal fault Formation Swim Hole N20°W29°W1.9 N20°W59°W Normal fault 2 N.A. Upper member 50–30°E ~20°E Eastward Tortoise5 N20°W10°W2.7 N20°W60°W Normal fault of Cloudburst Formation 1 (oldest) 2b Lower member 90–50°E ~40°E Eastward Sand N5°E 20°E 1.3N29°W72°W Overturned of Cloudburst normal fault Formation Dirt N5°E 20°E N.A. N29°W72°W Overturned normal fault Abbreviation: N.A.—not applicable. 1Restorations based on untilting about an axis of N20°W; minor faults and faults with no reliable strike and dip are excluded. 2Eastward dips of synextensional strata are not included because they appear to be the result of folding associated with normal faulting, not just tilting. 3Faults with variable strike and dip are averaged. 4Using average orientations closest to line AA′ (Fig. 3). 5This fault inferred; see text.

Generation 2 (see generation 1). Likewise, faults of set 3 in the study area likely bounded the half-grabens in which the sediments of the San Manuel Formation accu- Evidence from stratigraphic relationships and the structure north of the mulated (see generation 3 below). There are no synextensional faults exposed field area indicates the presence of a second generation of faults, which was in the field area, however, that account for deposition of growth beds of the active after generation 1 and before subsequent generations (Table 3). As upper Cloudburst Formation (Table 3), which are bound above and below noted above, faults of set 2b were active before deposition of the upper mem- by thick sections exhibiting homoclinal dips that therefore lack evidence of ber of the Cloudburst Formation and probably bound half-grabens that were fault-related growth (Maher et al., 2004). Consequently, faults of an interme- forming during deposition of the lower member of the Cloudburst Formation diate generation are presumed to have been involved during the deposition

W E Figure 18. Diagram of each normal fault generation (colored) 0° with associated synextensional formations. Average modern 40 Net tilting: 90°E 50°E 10° Quiburis 90° Fo dips of each fault generation and modern range of dips for ° 10°E rmation 10°E synextensional formations are given. The cut-off angle (red) Lowe 4 20 30°E 20° between the oldest synextensional strata and the modern F r 65° 55°W o Cloudburs ° rmation fault dip is the inferred angle of initiation for a given fault gen- 50° 30°E San Manuel Formation eration. The range of dips for synextensional strata associ- E Upper Cloudburst 70° Generation t ated with a given fault generation (black bold) is equal to the Formation amount of tilting produced by that given fault generation. The 29°W 3 values presented are based on observed attitudes of Cenozoic 59° ration Gene 20°E strata and normal faults within the map area, coupled with ration Gene 1 60° the structural reconstructions, except for generation 2. Gen- 10°W eration 2 is based on an assumed cut-off angle of 60°, which 2 Generation implies a dip of 10°W (see text for discussion).

of the Cloudburst Formation in the study area and can be inferred on an old cause the Smith Wash fault is a prominent range-bounding fault. Tilting published seismic-reflection survey in a region nearby (figs. 1–12 of Barton associated with this fault set appears to be minimal because these faults et al., 2005). are steeply dipping at 60°, as expected for the initiation angle for normal Beds near the base of the upper Cloudburst Formation in the map area faults (Table 3). have quite variable dips, commonly between 30°E and 50°E, and to the north near Hackberry Wash dip ~50°E (Maher et al., 2004). Assuming a bedding- to-fault angle of ~60° (i.e., possibly the expected angle of an initially steeply Amount of Net Cenozoic Tilting west-dipping fault) with beds at the base of the upper Cloudburst Formation that currently dip ~50°E, then the present-day attitude of second-generation The principal evidence for the net amount of tilting during Cenozoic faults might be expected to be ~10°W, as discussed further in the reconstruc- time is the orientation of the oldest synextensional strata, i.e., rocks of the tion below. A set of faults with similar orientation is observed a few kilometers Oligocene to Miocene Cloudburst Formation. The attitudes of the underly- to the north near Ripsey Wash (Nickerson et al., 2010, p. 308–309). The net ing pre-Cenozoic strata in most places are similar to those of the Cloudburst amount of tilting associated with this fault generation is 20°E (Fig. 18). The Formation (Fig. 3). Therefore, the pre-Cenozoic rocks in those places were primary fault within this generation is the Tortoise fault (Fig. 19E; Table 3). subhorizontal at the time the basal beds of the lower Cloudburst Forma- tion unit were deposited, i.e., at the inception of extension when faults of Cenozoic normal fault generation 1 initiated. West of Jim Thomas Wash Generation 3 (Fig. 3), the lower Cloudburst Formation strikes ~N20°W and dips as steeply as vertical. At the eastern side of the field area, just north of Romero Wash, The faults of set 3, the Hackberry and Swim Hole faults, constitute the faults the upper member of the Cloudburst Formation strikes ~N10°W and dips as of this generation (Table 3). Faults of generation 2 cut and offset faults of gen- much as 85°W (Table 2). Together, these two locales indicate the field area eration 1, and faults of generation 2 are cut by faults of generation 3. As noted has been tilted ~90° to the east during Cenozoic extension. The axis of net earlier, both faults clearly cut and offset and thereby postdate the upper mem- rotation is ~N20°W because the majority of tilted Cenozoic strata strike in ber of the Cloudburst Formation, and the San Manuel Formation is in the hang- this direction. ing walls of both the Swim Hole and Hackberry faults (Fig. 3). Therefore, the Hackberry and Swim Hole faults probably controlled the half-grabens in which the sediments of the San Manuel Formation accumulated. The Swim Hole fault Structural Reconstructions dips ~29°W, and bedding in the San Manuel Formation in the hanging wall of the Swim Hole fault dips ~30°E, which defines a bedding-to-fault angle of ~59° Choice of Cross Sections, Goals, and Assumptions (Fig. 18). The Hackberry fault displays similar relationships. The net amount of tilting associated with this fault generation is 20°E (Fig. 18). A regional section along AA′ (Fig. 3) has been reconstructed in order to determine the possible Cenozoic structural evolution of the study area (Fig. 19). Reconstructions along BB′ (Fig. 20) and CC′ (Fig. 21), considerably Generation 4 shorter sections by comparison, draw on the results from the reconstruction along AA′. The main goal of each reconstruction is to make a stepwise This generation includes all faults from set 4 (Table 3). They consistently dip restoration of the effects of Cenozoic normal faulting and associated tilting, steeply west. Faults within this generation, such as the Eagle Wash fault, make ultimately to determine the orientation and distribution of Laramide struc- a bedding-to-fault angle of ~65° with beds of the Quiburis Formation (Fig. 18). tures. Without evidence to the contrary, the unexposed portions of faults Because the majority of these faults dip to the west, typically at 55°–60°, and here are assumed to be approximately planar or curviplanar projections of the Quiburis Formation ranges in dip from flat to 10°E, this fault generation is the exposed portions, following observations made in the field and seismic inferred to have an overall net tilt of 10° E (Fig. 18). evidence from earthquakes on numerous normal faults around the world (e.g., Jackson and White, 1989). The reconstructions of normal faults as- sume rigid-body deformation in order to avoid complexity, even though Generation 5 active normal faults display hanging-wall and footwall flexure (e.g., Stein et al., 1988; Roberts and Yielding, 1994). In addition, without data to the con- This fault generation is composed of the primarily steeply east-dipping trary, pure dip-slip displacement is assumed, even though faults can have faults of set 5 (Table 3). These faults are interpreted to be the youngest due components of oblique slip and rotation within the fault plane itself (e.g., to relationships observed to the northwest (Nickerson et al., 2010) and be- Seedorff et al., 2015).

GEOSPHERE | Volume 13 | Number 2 Favorito and Seedorff | Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/577/1001750/577.pdf 595 by guest on 02 October 2021 on 02 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/577/1001750/577.pdf Research Paper Figure 19. Structural reconstruction along line of section AA erosion that produces the modern cross section. restoration. In (C)–(G), faults that have moved and underwent tilting are bold, and the locations of future faults are dashed. Step (H) involves no faulting but demonstrates apparent dip of Tecolote fault in restored cross section is slightly steeper than the true restored dip reported in Table 4 because the strike of the fault rotates during the that in (A) reverse fault tip shown as red dot, and area of tri-shear deformation defined by dashed red lines. Fault propagation to slip ratio for Tecolote 2.0. In (B), fault = than the lower Cloudburst Formation is shown by bold black line. Erosion surfaces are represented by dotted lines, and eroded rocks for a given time are transparent. Note 3 km 0 km H G 0 km A F 3 km E D

Choll

′ moving forward in time from Laramide time (before shortening) to modern. Modern topography for rocks older a fault B

Sand fault

Ea ? gle Wa

sh fa

ult colo t

Tar fa e

l an t faul tu Hack la t Romero b e rr y W fault ash fault T or tois e f ault

Horse Hill

s fault C

Swim

Hole faul

GEOSPHERE | Volume 13 | Number 2 Favorito and Seedorff | Characterization and reconstruction of Laramide shortening and superimposed Cenozoic extension 596 Research Paper

WSW ENE

WSW ENE Figure 20. Structural reconstruction along line of section BB′. (A) Restored geologic cross section along line of section BB′ during Laramide time, before reverse faulting. Reverse fault tip shown as red dot, and area of tri-shear deformation defined by dashed red lines. (B) Geologic cross section after reverse faulting and intrusion of the Rattle Granodiorite and before Ceno zoic extension. Fault propagation to slip ratio = 1.5. Modern topography is shown by bold black line and is dashed where Cenozoic rocks presently crop out. Horse Hills fault 1 km

ash fault W o present-day surface Romer

1 km

Constraints and Uncertainties The amount of tilting associated with each generation of faults was determined by restoring the bedding of the oldest synextensional sedimentary Section AA′ was chosen for a structural reconstruction because it inter- rocks associated with each fault to horizontal (Fig. 18). Faults that have been sects the Tecolote syncline and includes most sets of faults and key geologic involved in structural reconstructions have qualitative uncertainty values for markers. The section is oriented normal to the strike of most faults and of both the amount of offset and amount of tilting. The uncertainty in the amount Proterozoic to Cenozoic strata (Fig. 3). The Sand fault of generation 1 (see of offset is largely based on the presence of reliable contact markers, and above) has been projected into the cross section from the northwestern corner tilting uncertainty is based on the degree of exposure and the accuracy and of the map area (Fig. 3). The Swim Hole fault has also been projected from consistency of dips of faults and their associated synextensional sedimentary north of the line of section (Fig. 3). Continuous, E-W–striking Proterozoic to strata. The Swim Hole, Horse Hills, and Romero Wash faults have an esti- Laramide age dikes within the vicinity of AA′ provide proof for the absence of mated uncertainty in offset of ±5%, the Bee Wash, Hackberry, and Dirt faults additional significant faults at the present surface across the majority of the ±20%, Cholla and Tecolote faults ±30%, and the Eagle Wash, Tarantula, and cross section (Fig. 3). The exposure of a thick sill of diabase near the center of Tortoise faults ±50%. The estimated uncertainty in the amount of tilting for the section (Fig. 3) provides a key constraint on the reconstruction, as it has to Cenozoic fault generations is fair for generations 2 and 3 (±20%) and good restore to relatively shallow levels within the Ruin Granite (see above). This for generations 1 and 4 (±10%). The estimated uncertainty in the net tilting is indicates that this block is not part of the deeper levels of the Ruin Granite. only ±5%. The amount of offset on faults has been estimated by measuring the offset The challenge of dealing simultaneously with both Laramide shortening of geologic contacts. In several cases, displacements are estimated from fault and Cenozoic extension also complicates the assignment of uncertainties in exposures north of the line of section where the same faults cut rocks with structural reconstructions. An iterative approach was used to produce a viable reliable markers, such as near Hackberry Wash and the Tea Cup–Grayback plu- cross-sectional reconstruction (Supplemental Text Material [see footnote 1], ton (e.g., Seedorff and Maher, 2003; Barton et al., 2007; Nickerson et al., 2010). Part 2). This results in a degree of internal circularity within the reconstruction Faults that have geologic markers that offer tight constraints in the field area because the extensional structure and the pre-extensional structural model on the amount of offset include the Romero Wash, Dirt, Swim Hole, Hackberry, are not mutually independent (e.g., Pape et al., 2016). Thus the regional re- and Horse Hills faults. Faults with poor constraints include the Eagle Wash, construction shown here is not unique; nonetheless, it provides a testable Tarantula, and Cholla faults. hypothesis.

A WE

Figure 21. Structural reconstruction along line of section CC′. (A) Restored geologic cross section along line of section CC′ during Laramide time, before reverse faulting. Reverse fault tip shown as red dot, and area of tri-shear deformation defined B WEby dashed red lines. (B) Geologic cross section after reverse faulting and intrusion of the Rattle Granodiorite and before Cenozoic extension. Fault propagation to slip ratio = 3.4 for the Romero Wash fault present-day surface and 4.8 for the Bee Wash fault. Modern topography is shown by bold black line.

Wash fault Bee Swim Hole fault Wash fault ro

1 km Rome

1 km

Results of Stepwise Reconstruction of Normal Faults Cenozoic and Laramide faults that lack exposures of offset markers (Tables 3 and 4). The initial and final stages of reconstruction of the Cenozoic normal

A structural reconstruction of Cenozoic normal faults along a regional line faults indicate a total of 14.6 km or 200% extension (e = Dl/li) of the study area of section AA′ (Fig. 3) indicates that the study area has been progressively dis- during the Cenozoic. The fifth generation of Cenozoic normal faults is ignored membered and tilted eastward by four separate generations of normal faults because these faults do not intersect the line of section AA′. The reconstruc- (Fig. 19; Table 3). This is directly supported by the geometric relationships be- tion presented here closely resembles Proffett’s (1977) reconstruction of the tween faults and related synextensional deposits observed in the field (Fig. Yerington district, Nevada, where rocks had been rotated steeply westward by 18). In addition, the restoration provides estimates of the amount of offset for multiple sets of crosscutting normal faults.

Fault Nomenclature: Original versus Present-Day component of dip but have been rotated through vertical and currently have a steep easterly component of dip). Map relationships previously discussed The presence of multiple generations of Cenozoic normal faults, with large indicate these faults are likely late Laramide in age. Once restored, major faults amounts of net tilting (~90°E), produces complications in interpreting earlier within fault set 2a strike roughly S27°E and dip 65°NNE (Table 4) and appear deformation. Indeed, even the descriptive nomenclature for faults can be confus- to be normal faults. This dip direction is nearly opposite of the restored orien- ing because of the extreme amount of net tilting achieved by Cenozoic extension. tations of faults belonging to fault set 2b (Table 3), further indicating that these In the terminology we employ, a fault is overturned if it has been rotated two sets are not related. through horizontal, which (1) reverses the dip direction of the fault, (2) reverses the slip direction of the fault, but (3) maintains the relative ages of units above Laramide Reverse Faults and below the fault. Therefore, a normal fault rotated through horizontal becomes an overturned normal fault, and a reverse fault rotated through hori- The structural reconstruction along section AA′ (Fig. 19) demonstrates the zontal becomes an overturned reverse fault. In contrast, we use the modifier kinematic evolution of the study area from the late Laramide to present day; apparent for faults that have rotated through vertical. After rotating through showing how rocks here underwent 90° of eastward rotation through a series vertical, (1) the dip direction of the fault reverses, (2) the slip direction of the of down-to-the-west normal faults. Using these results regarding the structural fault reverses, and, most importantly, (3) the relative ages of units above and history of the region, detailed sections along BB′ at Romero Wash (Fig. 20) and below the fault are also reversed. Because of the third characteristic, a fault CC′ at Bee Wash (Fig. 21) were structurally reconstructed, even though only that originally was a normal fault becomes an apparent reverse fault, and a one normal fault generation is exposed at the surface in each cross section. fault that originally was a reverse fault becomes an apparent normal fault. The Reconstructions along these sections, combined with results from section AA′, maps presented here use symbols that are consistent with this terminology are used to gain further insight into the geometry of Laramide reverse faults (see legends of Figs. 3, 5, and 6). within the study area. Whether a fault is normal or reverse ordinarily is regarded as a descriptive Tecolote fault. Presently, the Tecolote fault is an apparent normal fault be- classification. To use the traditional classification directly, however, requires cause it places younger Apache Group rocks on older Ruin Granite (Fig. 3). Once the assumption that the present-day orientation is similar to the original one. restored to its original orientation (Fig. 19; Table 4), the Tecolote fault is a west- In this study area, that assumption clearly is false; so the more complicated no- vergent Laramide reverse fault that places older rocks on younger rocks, strikes menclature described above is needed. This nomenclature effectively acknowl roughly north-northeast, and dips moderately at ~55°E. Structural reconstruc- edges both the original and present-day orientations of the fault and thus tions indicate ~2.8 km of displacement and 2.5 km of vertical uplift (Fig. 19). The

2Supplemental File 1. Please visit http://doi .org/10 contains a component of structural interpretation. The implicit component of Tecolote syncline, located to the west of the Tecolote fault (Fig. 3), is interpreted .1130/GES01381.S2 or the full-text article on www interpretation to classify structures here is analogous to using an overturned to be a fault-propagation fold in the footwall of the fault. Once the fold is rotated .gsapubs.org to view the Supplemental File 1. bedding symbol on an outcrop that lacks a facing indicator but confidently can 90°E, the eastern limb of the fold is overturned, and the western limb is nearly be considered overturned because of broader-scale geologic relationships. horizontal (Fig. 22; Supplemental Files 12 and 23). Because the basement-cover interface is clearly folded (Fig. 3), the reverse fault tip is interpreted to have initi- Laramide Normal Faults ated deep in the basement ~1.8 km below the interface (Fig. 19A), as indicated by forward models. In addition, the modern orientation of the axis of the fold, N22°E The faults of set 2a, located on the eastern side of the study area, pres- 50° (Fig. 9D), is nearly identical to fault-related folds associated with the Romero ently strike almost east-west, and some of them, although normal faults, are Wash fault near Bee Wash, which had an average fold axis of N18°E 37° (Fig. 9C). now apparent reverse faults (i.e., they initiated as normal faults with a westerly This suggests these folds formed in the same compressional regime.

TABLE 4. CURRENT AND RESTORED ORIENTATIONS OF LARAMIDE FAULTS1 Present Present Offset Restored Restored Fault Fault set strike dip (km) strike dipModern characterization of faultOriginal characterization of fault Beehive 2a S15°E 50°S >1.0 S26°E64°N Apparent reverse faultDown-to-the-NNE normal fault Diorite 2a S13°E 47°S >0.2 S28°E66°N Apparent reverse faultDown-to-the-NNE normal fault Romero Wash 1N25°W2 32°E2 0.75 N17°W58°W Overturned reverse faultEast-vergent backthrust Tecolote 1N10°E3 41°W3 2.8 N04°E55°E Apparent normal faultWest-vergent reverse fault 1Restorations based on untilting 90°W about an axis of N20°W; minor faults and faults with no reliable strike and dip are excluded. 3Supplemental File 2. Please visit http://doi .org/10 2Using average orientation of the Romero Wash fault in the Romero Wash area. .1130/GES01381.S3 or the full-text article on www 3Using orientation of northernmost exposure of the Tecolote fault. .gsapubs.org to view the Supplemental File 2.

NNW SSE At Bee Wash, the Romero Wash fault and Bee Wash fault presently dip ~60°E (Fig. 6). Reconstruction along section CC′ (Fig. 21) indicates ~0.62 km of maxi- Tsm mum displacement for the Romero Wash fault in this location, ~0.41 km for the Bee Wash fault, and an original dip of 30°W for both faults. Both faults appear Yt Yr to pinch out to zero displacement to the north. Fault-related folds appear to be associated with both faults here because bedding orientation changes prox- Ym imal to both faults in a pattern expected for fault-propagation folds. As with Yds the Tecolote fault, the basement-cover interface is folded here, as indicated by

bedding orientations in the Pioneer Formation. The folds here are more open, Yp

and this is likely due to the smaller amount of slip on the Romero Wash fault

compared with the Tecolote fault. Structural reconstructions along CC′ indicate

Te the reverse fault tip initiated ~350 m beneath the basement-cover interface for co lo the Romero Wash fault and ~120 m beneath for the Bee Wash fault (Fig. 21). Yd te f The regional reconstruction along section AA′ indicates that the amount au l t of Laramide shortening (e = Dl/li) in the study area was 1.8 km or 20% (Fig. 19). 1 km Relationship between Magmatism and Reverse Faulting 1 km Emplacement of Laramide intrusive rocks in the study area generally post- Figure 22. Cross section through a 3D model of the Tecolote syncline that has been dates reverse fault displacement. Structural reconstruction along section BB′ tilted 90°W. Laramide and/or Cenozoic intrusions have been removed for simplicity. (Fig. 20) indicates that the Rattler Granodiorite crosscuts the Romero Wash Tecolote syncline is located near Tecolote Ranch in Figure 3, and a cross section through the tilted fold is shown in Figure 19. Supplemental File 1 (see footnote 2) is a simpli- fault. From Cow Fence Wash, moving southward, the laccolith gradually fied 3D model of the Tecolote syncline and Tecolote reverse fault as they appear today. pinches out over a distance of ~1.5 km (Fig. 5). If the gradual pinching out is Supplemental File 2 (see footnote 3) is a 3D model of the structures after they have also assumed in the orthogonal direction, i.e., structurally above and below, been rotated 90°W, restoring them to their original orientations. Both files are viewable using Midland Valley MoveViewer; available at www.mve.com/software/moveviewer. the laccolith would continue to a depth of ~1.5 km. If the reverse fault had cut the laccolith, it would have been repeated within the footwall of the fall, but it is not (Figs. 5 and 10), indicating that the laccolith likely cuts the fault. Similar Romero Wash fault system. Presently, the Romero Wash fault is an over- reasoning for the same relationship is seen in the reconstruction along section turned reverse fault that places older Ruin Granite on younger Apache Group CC′ (Fig. 21). Other igneous units that crosscut the Romero Wash fault are the but with a down-dip sense of displacement (Fig. 10). The Romero Wash fault Smith Granodiorite porphyry and rhyodacite porphyry (Fig. 5). Smith Grano system is interpreted to have a strike length of 11 km and may have accommo- diorite porphyry also cuts the Tecolote fault, but the Tecolote fault cuts a dacite dated the most displacement where it is buried under Cenozoic cover, just east porphyry within the fault-bounded footwall Tecolote syncline (Tables 3 and 4). of Smith Wash (Fig. 3). Reconstruction along section AA′ (Fig. 19) indicates that This dacite porphyry appears to be folded, which is expected because it is cut the Romero Wash fault is a backthrust to the Tecolote fault due to their oppos- by the Tecolote fault. ing vergence directions and because the Romero Wash fault has significantly The only igneous unit with a geometry that appears to be directly affected less displacement than the Tecolote fault. Once restored to its original orienta- by reverse faulting is the Romero Diorite (Figs. 5 and 10), which intrudes along tion (Fig. 20), the Romero Wash fault is an east-vergent Laramide reverse fault the Romero Wash fault. Because the Romero Diorite is brecciated along its that places older rocks on younger rocks, strikes roughly north-northwest, and western contact, which is the Romero Wash fault surface, the unit is inter- dips moderately to the west. preted to have intruded between slip events on the fault. All other igneous At Romero Wash, the Romero Wash fault presently dips on average 32°E. units appear to have no relationship to the geometry of reverse faults or to the Reconstruction along section BB′ (Fig. 20) indicates ~0.75 km of displacement geometry of related folds. and an original dip of 58°W. Here, the Romero Wash fault has a consistent bedding cut-off angle of ~55°, and there is no evidence for fault-related folding Age of Reverse Faulting (Fig. 9A). South of Romero Wash, displacement on the Romero Wash fault appears to approach zero between Sample and Swingle Washes. However, south Laramide igneous rocks provide age constraints on reverse faulting in the of this area, the Romero Wash fault splits into two reverse faults that appear to study area. Previous studies have dated various local igneous units employing continue southward under Cenozoic cover (Fig. 3). K-Ar and fission-track methods, and this study adds new U-Pb zircon age dates

as a more reliable method for determining ages of crystallization. Because the fabrics that indicate east-vergent movement, but their relationship to the two Romero Wash fault is interpreted to be a backthrust of the Tecolote fault (Fig. aforementioned, more shallow styles of shortening, is uncertain. 19B), the ages of those two faults should be similar. Crosscutting Smith Gran- The Romero Wash and Tecolote faults are both examples of moderate- odiorite Porphyry (Fig. 5) brackets the Romero Wash fault to a minimum age of angle Laramide basement-cored uplifts. Currently, the Romero Wash fault is 65.9 ± 0.7 Ma (U-Pb zircon). The dacite porphyry dike near Tecolote Ranch ap- an overturned reverse fault, whereas the Tecolote fault is an apparent normal pears to be folded and cut by the Tecolote fault. This unit has an age of 73.1 ± fault (Fig. 3; Table 4). Once displacement and tilting on Cenozoic normal faults 2.1 Ma (K-Ar hornblende). These ages constrain the age of local reverse fault- are restored, the Tecolote fault in its original orientation is a regional-scale, ing to ca. 66–73 Ma assuming the reverse faults are related and thus formed west-vergent reverse fault with 2.8 km of displacement and 2.5 km of verti- at the same time. cal uplift (Fig. 19B). The Romero Wash fault is interpreted to be a backthrust to the Tecolote fault, with opposing vergence and 0.75 km of displacement. Relationship between Alteration and Reverse Faulting Seismic-reflection data, map relationships, and drill-hole piercements have led authors to interpretations with reverse faults and subsidiary backthrusts Geologic evidence indicates that alteration appears to postdate reverse in the Front Range of Colorado (Erslev et al., 2004) and in the Rattlesnake faulting within the study area even though there is no indication of Laramide Mountain area of Wyoming (Erslev, 1993), both of which are classic locales of faults or folds near the two centers of alteration. Alteration is interpreted to Laramide deformation. Backthrusts have also been documented in the Sierras postdate reverse faulting because Smith Granodiorite porphyry dikes cut the Pampeanas in Argentina, a modern analog to the Rocky Mountain foreland, Romero Wash fault (Figs. 5 and 16). Even though the dike that cuts the fault is where García and Davis (2004) interpreted blind backthrusts on the basis of unaltered, mesoscopically identical dikes only a few hundred meters away are map evidence. moderately sericitically altered (Fig. 16). Because the presently east-dipping Both the Tecolote and Romero Wash faults demonstrate clear evidence Romero Wash fault projects under cover to the east (Fig. 3), alteration appears for fault-propagation folds (Figs. 19B, 21, and 22). This style of thick-skinned to be hosted within the footwall of the reverse fault. This likely aided in its deformation might be expected for this local region because the basement preservation, just as the Tecolote syncline in the footwall of the Tecolote fault is homogeneous and stiff, dominated by plutonic rocks, and the cover rock was preserved. sequence is thin, only ~1 km thick (e.g., Mitra et al., 1988; García and Davis, 2004). The results of this study are consistent with Davis’s (1979) conclusion DISCUSSION that shortening within southeastern Arizona was accommodated primarily by basement-cored uplifts with associated fault-propagation folds. Styles of Laramide Reverse Faults in Southeastern Arizona The Walnut Canyon thrust is located just northwest of this study area, where we have interpreted basement-cored uplifts to be the shortening style. One The structural style of reverse faults across southeastern Arizona appears might not expect two highly contrasting styles of shortening to be only 20 km to be variable, as thin-skinned thrusts and basement-cored uplifts have been apart, perhaps even along strike. Thus, is Walnut Canyon an outlier of thin- documented. Waldrip (2008) interpreted the Kelsey Canyon and Hot Spring skinned shortening in the midst of basement-cored uplifts (Fig. 2), or, in this faults in the southern Galiuro Mountains (Fig. 2) to be low-angle thrusts that area of complex extension, might Walnut Canyon be a basement-cored uplift? are part of a regional overthrust system. Evidence for this includes hanging Other reverse faults within the region may also deserve further study. Be- wall flat-footwall ramp geometries, relatively low footwall cut-off angles (<30°), cause porphyry deposits form in the shallow crust, understanding the configu- and transposed Cretaceous strata that suggest burial under a thick thrust ration of Laramide reverse faults may allow for the discovery of new deposits sheet. Another possible example of thin-skinned deformation is the Walnut in the region. In addition, these faults may aid in the interpretation of known Canyon thrust (Richard and Spencer, 1998a, 1998b), located near Ray (Fig. 2). deposits because post-ore reverse faults may cut and offset ore bodies, and In contrast, farther to the south, within a large area centered on the Whetstone pre-ore faults may focus mineralization. Mountains (Fig. 2), Davis (1979) interpreted that shortening was accommodated by variably vergent, moderate-angle reverse faults associated with base- Folding in the Porphyry Copper Belt ment-cored uplifts. A prominent example of these faults is the Huachuca fault, which dips at 60° once restored to its original orientation. Other significant Even though the majority of folds observed within the porphyry copper belt reverse faults are located within the Little Rincon Mountains, Santa Rita Moun- of Arizona appear to be related to Laramide reverse faults, some are likely re- tains, Chiricahua Mountains, and the Christmas and Superior mining districts lated to Cenozoic normal faults. Understanding the geometry and differences (Fig. 2), but the style of shortening at these places is not well known. Deeper between these two types of fault-related folds is the key to interpreting the expressions of shortening are documented in the Santa Catalina Mountains structural evolution of a given area and also to determining its exploration (Fig. 2), where Bykerk-Kauffman and Janecke (1987) dated Laramide tectonite potential. Moderate-angle reverse faults can create fault-propagation folds,

where bedding in the footwall and hanging wall are commonly overturned. (Nickerson et al., 2010), Resolution (Manske and Paul, 2002), and Ray (Barton Normal faults can create drag, reverse drag, roll-over, and transverse folds. et al., 2005; Maher, 2008). Evidence for porphyry formation following short- All of these types of extensional folds tilt strata gently to moderately and, ening also is observed in the central Andes of Chile at Los Pelambres (Perelló if not subsequently tilted, have a clear and distinct lack of overturned beds et al., 2012). Nonetheless, there also are several examples of porphyry systems (Schlische, 1995). The largest and most prominent fold within the study area emplaced during and prior to shortening. Examples of Chilean porphyries em- is the Tecolote syncline. Howard and John (1997) suggest that the syncline placed during shortening include the Centinela district (Perelló et al., 2010), may be a steeply north-tilted drape fold of cover rock on the down-to-the-west the Rio Blanco-Los Bronces district (Piquer et al., 2015), and the Potrerillos Tecolote fault, which they interpret as a normal fault within a detachment sys- porphyry (Olson, 1989; Marsh et al., 1997; Niemeyer and Munizaga, 2008). tem. They also claim the Tecolote syncline is coaxial with the southern Jim Deposits formed prior to shortening events include the Cadia district of New Thomas syncline. Dickinson (2002) built upon this idea, proposing that the South Wales, Australia (Holliday et al., 2002), the Kerr-Sulphurets-Mitchell dis- Tecolote and Jim Thomas synclines may reflect flat-ramp detachment geom trict (Kirkham and Margolis, 1995; Bridge et al., 1996) and the Gibraltar deposit etry on the Hackberry fault. (Bysouth et al., 1995) of British Columbia, and the Aitik deposit, Sweden (Wan- Structural reconstructions from this study indicate that the Tecolote fault is hainen et al., 2003). a reverse fault with associated fault-propagation folds (Fig. 19). The Tecolote Within the Romero Wash–Tecolote Ranch area, barren porphyry dikes syncline, once tilted back 90°W to remove the effects of Cenozoic extension, crosscut reverse faults (Figs. 5 and 16). Sericitically altered dikes of identical consists of a horizontal western limb and an overturned eastern limb bounded original composition crop out near these barren dikes (Fig. 16), indicating that by the Tecolote fault (Figs. 3 and 22). The overturned limb is inconsistent with alteration postdates reverse faulting. This is consistent with the relationships normal fault-drape folding, and map patterns of bedding attitudes suggest in the three nearby Laramide examples discussed above. In addition, neither that the Jim Thomas syncline is not coaxial with the Tecolote syncline. The the location nor the geometry of the stocks of Smith Granodiorite and their present-day axial plane of the Tecolote syncline is N48°E 70°W, whereas the associated alteration patterns appear to have been influenced by any preexist Jim Thomas syncline axial plane averages almost due north and dips 85°W ing structure. In all examples documented to date in the Laramide porphyry (Fig. 3). The strikes of the two planes are ~50° apart from one another, further copper belt of Arizona, porphyry generation and associated hydrothermal al- indicating that the two folds probably formed through different processes and teration postdates reverse faulting at the point of observation. However, future are not related. In addition, the interlimb angles differ for the two folds: 70° for studies may find examples in Arizona with the opposite relative ages—as in the Tecolote syncline and 120° for the Jim Thomas syncline. The Jim Thomas Chile—because compression and magmatism in arc settings broadly over- syncline may be a drag fold associated with the Hackberry fault, as suggested lap in time. by Dickinson (2002). Dickinson proposed that strata were tilted eastward prior Understanding the relationship between shortening and porphyry gener- to drape folding, which would help explain the synformal shape observed, ation should prove useful for further exploration in a mature region such as and that drape folding resulted in the westward dip of the eastern limb of the Laramide porphyry belt of Arizona. Until there are more examples of these the syncline. This is a reasonable interpretation for the study area because relationships, the dominant patterns remain elusive. all synextensional sedimentary rocks are expected to be east-dipping due to tilting on down-to-the-west normal faults, and any drape folds that would form CONCLUSIONS on down-to-the-west faults would tilt strata westward. In addition, the relatively large interlimb angle of the Jim Thomas syncline supports this claim. Cenozoic normal faulting in the Tecolote Ranch and Romero Wash area has The evidence provided here for the Tecolote and Jim Thomas synclines being provided both an opportunity and a challenge in studying the style of Lara- separate structures is also inconsistent with Dickinson’s (2002) interpretation mide deformation and its relationship to porphyry generation in southeastern that these two structures may reflect flat-ramp detachment geometry of the Arizona. Extension here resulted from displacement and tilting on three gen- Hackberry fault. erations of down-to-the-west normal faults that were mapped at the surface, a fourth generation of down-to-the-west normal faults that is not exposed at Relationship between Shortening and Porphyry Copper Generation the surface but is indicated by stratigraphic and structural evidence, and a fifth generation of down-to-the-east normal faults. The bedding orientations of the There are few examples where time-space relationships between shorten- oldest synextensional strata present, the lower member of the Hackberry Wash ing and porphyry copper generation can be demonstrated, with the scarcity facies of the Cloudburst Formation, indicate that the region underwent ~90° of of examples in part due to a lack of detailed observations. Three locales in eastward tilting. Based on a reconstruction of a cross section through the study

Arizona contain evidence for such relationships, and they are all located within area, the total amount of Cenozoic extension (e = Dl/li) was 14.6 km or 200%. 50 km of the study area (Fig. 1). Reverse faulting appears to have predated Reverse faults in the field area include the Romero Wash and Tecolote faults. porphyry generation and related hydrothermal alteration at Kelvin-Riverside The Romero Wash fault is cut by Laramide dikes, indicating that it is Laramide

in age. Structural reconstructions indicate that these faults were originally Sample Preparation moderate-angle reverse faults that together, resulted in 2.5 km of structural uplift. The Tecolote fault appears to be the fault with the greatest displacement, Zircon crystals were extracted from samples by traditional methods of crushing and grinding, followed by separation with a Wilfley table, heavy liquids, and a Frantz magnetic separator. Sam- and the Romero Wash fault has been interpreted as a backthrust of the Tecolote ples were processed such that all zircons were retained in the final heavy-mineral fraction. A large fault due to their opposing vergence directions and difference in amount of split of these grains (generally 200 grains) was incorporated into a 1-inch epoxy mount together offset. Both faults have clear evidence for fault-propagation folds, of which the with fragments of our Sri Lanka standard zircon. The mounts were sanded down to a depth of ~20 microns, polished, imaged, and cleaned prior to isotopic analysis. largest example is the Tecolote syncline. Once restored to its original orientation, this fold appears to have an overturned limb bounded by the Tecolote fault, further evidence that it is related to reverse faulting. The amount of Lara Analytical Methods and Reporting Procedures mide shortening (e = Dl/l ) in the study area was 1.8 km or 20%, followed by a i U-Pb geochronology of zircons was conducted by laser ablation–multicollector–induc- small amount of late Laramide extension. The geology of the area is consistent tively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center with the interpretation that shortening in southeastern Arizona was dominated (Gehrels et al., 2006, 2008). The analyses involve ablation of zircon with a New Wave UP193HE by basement-cored uplifts. Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of 30 microns. The ablated material is carried in helium into the plasma source of a Nu HR ICPMS, which is Laramide hydrothermal alteration within the study area is associated with equipped with a flight tube of sufficient width that U, Th, and Pb isotopes are measured simul- crystallization of Smith Granodiorite porphyry dikes and related stocks. Alter- taneously. All measurements are made in static mode, using Faraday detectors with 3 × 1011 238 232 208 206 204 202 ation types include both sericitic and propylitic, with propylitic alteration occur- ohm resistors for U, Th, Pb- Pb, and discrete dynode ion counters for Pb and Hg. Ion yields are ~0.8 mv per ppm. Each analysis consists of one 15-second integration on peaks ring distal to sericitic alteration. Copper occurrences are limited and typically with the laser off (for backgrounds), 15 one-second integrations with the laser firing, and a 30 consist of copper-oxide minerals, with few instances of primary copper min- second delay to purge the previous sample and prepare for the next analysis. The ablation pit eralization. The lack of exposed potassic alteration, a common feature of por- is ~15 microns in depth. For each analysis, the errors in determining 206Pb/238U and 206Pb/204Pb result in a measure- phyry copper systems, indicates the alteration patterns observed may repre- ment error of ~1%–2% (at 2-sigma level) in the 206Pb/238U age. The errors in measurement of sent the fringe of a larger porphyry system. In one location, a barren porphyry 206Pb/207Pb and 206Pb/204Pb also result in ~1%–2% (at 2-sigma level) uncertainty in age for grains dike crosscuts the Romero Wash fault, and similar dikes nearby are sericitically that are >1.0 Ga, but are substantially larger for younger grains due to low intensity of the 207Pb signal. For most analyses, the crossover in precision of 206Pb/238U and 206Pb/207Pb ages occurs altered, indicating alteration postdates reverse faulting. Overall, there is no at ca. 1.0 Ga. The 204Hg interference with 204Pb is accounted for by the measurement of 202Hg clear evidence in the study area that Laramide structures strongly influenced during laser ablation and subtraction of 204Hg according to the natural 202Hg/204Hg of 4.35. This Hg the location or geometry of porphyry intrusion and hydrothermal fluid flow. correction is not significant for most analyses because our Hg backgrounds are low (generally ~150 cps at mass 204). Common Pb correction is accomplished by using the Hg-corrected 204Pb and assuming an APPENDIX. LASER ALBATION–INDUCTIVELY COUPLED initial Pb composition from Stacey and Kramers (1975). Uncertainties of 1.5 for 206Pb/204Pb and PLASMA–MASS SPECTROMETRY U-Pb ZIRCON 0.3 for 207Pb/204Pb are applied to these compositional values based on the variation in Pb isotopic composition in modern crystal rocks. GEOCHRONOLOGIC METHODS Inter-element fractionation of Pb/U is generally ~5%, whereas apparent fractionation of Pb isotopes is generally <0.2%. In-run analysis of fragments of a large zircon crystal (generally every Sample Descriptions fifth measurement) with known age of 563.5 ± 3.2 Ma (2-sigma error) is used to correct for this fractionation. The uncertainty resulting from the calibration correction is generally 1%–2% (2s) for Ksg-s both 206Pb/207Pb and 206Pb/238U ages. Concentrations of U and Th are calibrated relative to our Sri Lanka zircon, which contains ~518 ppm of U and 68 ppm Th.

Sample Ksg-s is a Smith Granodiorite porphyry dike that crosscuts the Romero Wash fault just south of Cow Fence Wash (Fig. 5). It was chosen for dating to place an upper age limit on Results the Romero Wash fault. It contains 40% phenocrysts in a light- to medium-gray, fine-grained groundmass. The phenocryst assemblage consists of 60% plagioclase, 0.5–4 mm; 10% K-feldspar, Results are shown in Table 2. Uncertainties shown in this table are at the 1-sigma level and 0.5–1 mm; 15% biotite/chlorite, 1–2 mm; and 15% hornblende, 2–4 mm. Hydrothermal alteration include only measurement errors. Inheritance was tested in the samples by examining both the and weathering have resulted in feldspars being largely replaced by sericite and clay and horn- core and tips of each zircon where possible. The resulting interpreted ages are shown on weighted blende and biotite being replaced by chlorite. mean diagrams using the routines in Isoplot (Ludwig, 2008) (Fig. A1). The weighted mean diagrams show the weighted mean (weighting according to the square of the internal uncertainties), Krd the uncertainty of the weighted mean, the external (systematic) uncertainty that corresponds to the ages used, the final uncertainty of the age (determined by quadratic addition of the weighted Sample Krd is a Romero Diorite dike that intrudes along the Romero Wash fault and appears mean and external uncertainties), and the mean square of weighted deviates (MSWD) of the data to have been brecciated by fault movement. It was chosen for dating to determine the age of the set. Figure A2 shows concordia diagrams for the two analyzed samples. Romero Wash fault. The texture is primarily fine grained, and it contains 5% phenocrysts, of which Sample Ksg-p has an interpreted age of crystallization of 65.9 ± 0.7 Ma. Crosscutting relation- are mostly potassium feldspar measuring 1–2 mm. The groundmass consists primarily of plagio ships indicate that sample Krd cannot be Proterozoic in age. Thus the reported age is interpreted to clase laths, with lesser amounts of biotite, chlorite, and sparse quartz. Hydrothermal alteration be the age of inherited zircon grains, which is consistent with the widespread Proterozoic country and weathering have resulted in feldspars being largely replaced by sericite and clay, and biotite rock within the study area. The precise age of crystallization for sample Krd is unknown, but it is being replaced by chlorite. likely also Laramide in age.

data-point error symbols are 2σ data-point error ellipses are 2σ 82 0.014 Sample Ksg-p Sample Ksg-p

78 0.013

74 0.012 e 70 U 0.011 23 8 / Pb

Best ag 66 0.010 206

62 0.009

58 Mean = 65.87 0.69 [1.0%] 2σ 0.008 Wtdby data-pt errs only, 0 of 33 rej. MSWD = 1.19, probability = 0.21 54 (error bars are 2σ) 0.007 0.03 0.05 0.07 0.09 0.11 0.13 207Pb/235U

data-point error symbols are 2σ Sample Krd data-point error ellipses are 2σ 1750 0.30 Sample Krd

1600 0.28 1650

0.26 e 1550

U 1400

23 8 0.24 / Best ag Pb

1450 20 6 0.22

1200 0.20 1350

Mean = 1444 14 [0.95%] 2σ 0.18 Wtdby data-pt errs only, 0 of 30 rej. 1250 MSWD = 1.2, probability = 0.18 0.16 (error bars are 2σ) 1.82.2 2.63.0 3.43.8 4.24.6 207Pb/235U Figure A1. Weighted mean diagram for U-Pb dates. Each red bar represents a single spot analysis, and the mean is shown as a green line. Only results for spots within the tips of Figure A2. Concordia diagrams for U-Pb dates. Only results for spots within the tips of zircon zircon grains are shown for sample Ksg-p. MSWD—mean square of weighted deviates. grains are shown for sample Ksg-p.

ACKNOWLEDGMENTS Coney, P.J., 1976, Plate tectonics and the Laramide orogeny, in Woodward, L.A., and Northrop, Financial support for this study was provided by the Lowell Institute for Mineral Resources at S.A., eds., Tectonics and Mineral Resources of Southwestern North America (Kelley Vol- the University of Arizona, a Society of Economic Geologists Graduate Student Fellowship spon- ume): New Mexico Geological Society Special Publication 6, p. 5–10. sored by Newmont Mining Corporation, a Geological Society of America Graduate Student Re- Conway, C.M., and Silver, L.T., 1989, Early Proterozoic rocks (1710–1615 Ma) in central to south- search Grant, and the University of Arizona’s Spencer Titley scholarship. Max Larkin, Sean O’Neal, eastern Arizona, in Jenney, J.P., and Reynolds, S.J., eds., Geologic Evolution of Arizona: Simone Runyon, Eduardo Garcia, and Ben Schumer assisted with field mapping. Mark Barton, Roy Arizona Geological Society Digest 17, p. 165–186. Johnson, Jason Mizer, and Simone Runyon are thanked for their geologic insights, Steve Lingrey Cornwall, H.R., and Krieger, M.H., 1975, Geologic map of the Kearny quadrangle, Pinal County, for feedback on reconstructions, and Kojo Plange for his assistance in interpreting U-Pb results. Arizona: U.S. Geological Survey Quadrangle Map GQ-1188, scale 1:24,000, text 9 p. We appreciate the constructive comments of Jon Spencer and Tim Lawton, and especially the Davis, G.H., 1979, Laramide folding and faulting in southeastern Arizona: American Journal of useful, detailed comments of Associate Editor Terry Pavlis. Science, v. 279, p. 543–569, doi:10.2475/ajs.279.5.543. DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A: American Journal of Science, v. 304, p. 105–168, doi:10 .2475 /ajs.304.2.105. 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