TECTONICS, VOL. 23, TC1019, doi: /2002tc001454, 2004

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1 TECTONICS, VOL. 23,, doi: /2002tc001454, 2004 Miocene extension and extensional folding in an anticlinal segment of the Black Mountains accommodation zone, Colorado River extensional corridor, southwestern United States Robert J. Varga, 1 James E. Faulds, 2 Lawrence W. Snee, 3 Stephen S. Harlan, 4 and Lori Bettison-Varga 1 Received 28 August 2002; revised 14 October 2003; accepted 14 November 2003; published 25 February [1] Recent studies demonstrate that rifts are characterized by linked tilt domains, each containing a consistent polarity of normal faults and stratal tilt directions, and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults. The mid-miocene Black Mountains accommodation zone of southern Nevada and western Arizona is a well-exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains. In the southeastern part of this zone near Kingman, Arizona, east dipping normal faults of the Whipple tilt domain and west dipping normal faults of the Lake Mead domain coalesce across a relatively narrow region characterized by a series of linked, extensional folds. The geometry of these folds in this strike-parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity. Synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other. Opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions, whereas large fault overlaps produce elongate folds parallel to faults. Analysis of faults shows that the folds are purely extensional and result from east/northeast stretching and fault-related tilting. The structural geometry of this portion of the accommodation zone mirrors that of the Black Mountains accommodation zone more regionally, with both transverse and strike-parallel antithetic segments. Normal faults of both tilt domains lose displacement and terminate within the 1 Department of Geology, College of Wooster, Wooster, Ohio, USA. 2 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada, USA. 3 U.S. Geological Survey, Denver, Colorado, USA. 4 Department of Environmental Sciences, George Mason University, Fairfax, Virginia, USA. Copyright 2004 by the American Geophysical Union /04/2002TC accommodation zone northwest of Kingman, Arizona. However, isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the Lake Mead domain began to form between 1 m.y. to 0.2 m.y. prior to east dipping faults of the Whipple domain. The accommodation zone formed above an active and evolving magmatic center that, prior to rifting, produced intermediate-composition volcanic rocks and that, during rifting, produced voluminous rhyolite and basalt magmas. INDEX TERMS: 8109 Tectonophysics: Continental tectonics extensional (0905); 8010 Structural Geology: Fractures and faults; 9350 Information Related to Geographic Region: North America; 9604 Information Related to Geologic Time: Cenozoic; 3640 Mineralogy and Petrology: Igneous petrology; KEYWORDS: accommodation zones, extension, Basin and Range, normal faults, rift zones, argon-argon. Citation: Varga, R. J., J. E. Faulds, L. W. Snee, S. S. Harlan, and L. Bettison-Varga (2004), Miocene extension and extensional folding in an anticlinal segment of the Black Mountains accommodation zone, Colorado River extensional corridor, southwestern United States, Tectonics, 23,, doi: /2002tc Introduction [2] In recent years, studies of extended terranes have shifted from the reasonably well-understood two-dimensional, cross-sectional geometry to three-dimensional perspectives that incorporate along-strike dimensions of normal-fault systems. The three-dimensional perspective provides important insight into the overall anatomy and origin of normal-fault systems in both their culminations in metamorphic core complexes [e.g., Davis and Lister, 1988; McCarthy et al., 1988; Spencer and Reynolds, 1989] and terminations in accommodation and transfer zones [e.g., Rosendahl, 1987; Morley et al., 1990; Faulds and Varga, 1998]. With this change in emphasis, folds are widely recognized as a fundamental component of extended regions, resulting not only from movement on listric normal faults [e.g., Hamblin, 1965; Groshong, 1989; Dula, 1991; Xiao and Suppe, 1992; Schlische, 1993; Groshong, 1994] but also from isostatic flexure of tectonically denuded terranes [e.g., Spencer, 1984; Wernicke and Axen, 1988], displacement gradients on individual normal faults 1of19

2 [Schlische, 1992, 1993; Janecke et al., 1998], and overlap of normal faults [Rosendahl, 1987; Larsen, 1988; Morley et al., 1990; Peacock and Sanderson, 1994; Faulds and Varga, 1998]. Extensional folds are especially abundant in belts of overlapping oppositely dipping normal faults, commonly referred to as accommodation zones (see discussion of terms given by Faulds and Varga [1998]). [3] Folds within accommodation zones are most commonly related to reversals in the dip direction of major normal faults and concomitant changes in the tilt direction of associated fault blocks and half grabens. Relatively narrow extended regions or rifts, such as those in East Africa [Rosendahl et al., 1986; Ebinger et al., 1987; Rosendahl, 1987; Ebinger, 1989; Morley, 1989; Morley et al., 1990] and the Gulf of Suez [Moustafa, 1976; Colletta et al., 1988; Coffield and Schamel, 1989; Moustafa and Abd-Allah, 1992], are characterized by along-strike reversals in the polarity of half-grabens and overlap between the bounding, oppositely dipping border faults. Individual anticlines or synclines are well documented in these regions of overlap [e.g., Rosendahl, 1987; Ebinger, 1989; Morley et al., 1990]. In contrast, broad extended terranes, such as the Basin and Range province, typically contain large domains in which fault blocks and half grabens are tilted predominantly in one direction [Stewart, 1980; Spencer and Reynolds, 1989; Stewart, 1998]. At the boundaries of these tilt domains, multiple folds develop between the overlapping oppositely dipping systems of normal faults and attendant opposing half grabens [Faulds and Varga, 1998; Faulds et al., 2002b]. [4] The geometry of folds within extensional accommodation zones is typically controlled by the relative dip direction and magnitude of overlap between opposing normal faults or systems of faults [Faulds and Varga, 1998]. Anticlines occur where normal faults (or systems of faults) of opposite polarity face one another. The opposing rollovers in the hanging walls of the oppositely dipping normal faults intersect to form an extensional anticline. In contrast, where the opposed normal faults face away from each other, synclines develop, possibly through a combination of reverse drag [Hamblin, 1965] and minor isostatically induced uplift along the margins of the intervening horst block; fold trends depend on the relative magnitude of overlap between the opposing faults (Figure 1). Where overlap is large, the trend approaches the strike direction of normal faults and dipping strata, whereas in cases of minimal overlap, the fold trend may be quite oblique to the regional strike (see Figure 6 of Faulds and Varga [1998]). [5] Elucidating the geometry and kinematic development of the folds has important implications for hydrocarbon exploration and for constraining the three-dimensional strain field in extended terranes. Many large oil fields on passive continental margins reside within accommodation zones [Etheridge et al., 1988; Morley et al., 1990; Nelson et al., 1992; Gawthorpe and Hurst, 1993]. Because folds within the zones may provide structural and stratigraphic closure, as well as influence the distribution of sedimentary facies, their geometry and kinematic development are relevant to exploration models. The folds and their associated minor structures can also help determine whether or not localized crowding occurs between oppositely tilted fault blocks of converging extensional allochthons or whether accommodation zones are purely extensional features [e.g., Faulds et al., 1990, 2002b]. [6] Despite their importance, few accommodation zones and their related folds have been studied in detail, largely due to lack of exposure in many areas. Though the anticlines within accommodation zones form local structural highs, they are commonly interbasinal highs within topographically low half-graben [Serra and Nelson, 1988]. Thus such areas in active rifts are typically covered by recent sediments, and the best information about their internal geology comes from seismic data [Rosendahl et al., 1986; Rosendahl, 1987; Colletta et al., 1988] not readily available outside of industry. The best exposed accommodation zones reside in ancient rifts where exhumed by later uplift and/or erosion [Faulds et al., 1990; Maler, 1990; Chapin and Cather, 1994; Faulds and Varga, 1998]. Accommodation zones are particularly well exposed in the Cenozoic Basin and Range province of the western United States [Stewart and Johannesen, 1979; Stewart and Roldán-Quintana, 1994; Stewart, 1998], where minimal vegetation in desert climates facilitates both detailed mapping and structural analysis and the abundance of volcanic strata allow for tight bracketing of the timing of deformation through 40 Ar/ 39 Ar geochronology [e.g., Faulds et al., 2002b]. The Black Mountains accommodation zone in southern Nevada and northwest Arizona is one of the best characterized. This accommodation zone runs for over 230 km and separates the 25,000 km 2 Whipple tilt domain, characterized by west tilted strata and east dipping normal faults, from the 15,000 km 2 Lake Mead tilt domain, characterized by east tilted strata and west dipping normal faults (Figure 2). [7] Much of the published detail about the Black Mountains accommodation zone and its internal structure have derived from detailed studies of its northern reaches between the Highland Range and the White Hills (Figure 2) where the zone is oriented approximately parallel to the east/northeast direction of extension ( transverse accommodation zone of Faulds and Varga [1998]). Faulds et al. [1990] first characterized a portion of this northern zone and made the important observation from detailed mapping that the transition between tilt domains is characterized by overlapping normal faults of opposite polarity and not by strike-slip transfer faulting. Although the general character [Faulds and Varga, 1998] and obliquely trending folds in the northern reaches of the Black Mountains accommodation zone have been described [Faulds et al., 1990, 1999; Price and Faulds, 1999; Faulds et al., 2002a, 2002b], the long northerly trending anticline that defines the easternmost part of the zone (Figure 2) has not been addressed in any detail. This north trending section of the accommodation zone is approximately parallel to strike ( strike-parallel accommodation zone of Faulds and Varga [1998]) in contrast to the transverse section to the north. This contrast in geometry highlights several remaining and important questions regarding the origin of the Black Mountains accommodation zone and accommodation zones in general. Is there a fundamental difference in internal structure 2of19

3 Figure 1. Block diagram showing interactions between two half-graben of opposite polarity. Opposed normal faults of the two tilt domains overlap without connecting transfer faults in a zone of accommodation. Inward facing opposed normal faults form anticlinal accommodation zones, whereas outward facing and opposed normal faults form synclinal accommodation zones. Note that the trend of these extensional folds is dependent upon the degree of overlap between opposing normal faults. Stippled regions within each half-graben represent synextensional sediment that pinches out against adjacent normal faults, the updip portion of underlying hanging walls and the intrabasinal high represented by the anticlinal accommodation zone. Inspired by Rosendahl [1987] and Faulds and Varga [1998]. between transverse and strike-parallel accommodation zone segments? Transverse accommodation zones appear to comprise interdigitating fault tips of opposed polarity with little strike-slip faulting. Is this also the case in strikeparallel zones? Are anticlines within the strike-parallel segments purely extensional in origin, as appears to be the case in the transverse segment to the north, or are they in part due to local compression? The degree of opposing fault overlap is large along the strike-parallel segment raising the possibility of a contractional component to the deformation. Is the timing of anticline growth similar on opposing limbs as it appears to be for some other anticlinal segments of the transverse zone? [8] In this paper we examine a particularly well-exposed portion of the regionally strike-parallel portion of the Black Mountains accommodation zone located northwest of Kingman, Arizona (Figure 2). Detailed (1:12,000- and 1:24,000- scale) geologic mapping, structural analysis, and 40 Ar/ 39 Ar geochronology were employed to document the timing of deformation and geometry, age, and origin of the anticline in this area. A major conclusion of our study is that the internal structure of regionally strike-parallel parts of accommodation zones is broadly similar to transverse segments, despite the contrasting geometry with respect to extension direction. Timing of anticlinal limb growth may, however, be diachronous and reflect differences in the age of propagation of opposed normal faults into an area. 2. Geologic Setting [9] The geology of the Black Mountains accommodation zone northwest of Kingman is shown in Figures 3 and 4. West tilted strata and east dipping normal faults of the Whipple domain and the east tilted strata and west dipping normal faults of the Lake Mead domain converge in this area along a complex zone of accommodation. As shown on Figure 3, the accommodation zone is relatively straight and north/northwest trending in the southern and central parts of the mapped area, defining a broad anticline. In the central part of the area, the accommodation zone turns and zig-zags 3of19

4 Figure 2. Regional map showing tilt domains related to Miocene extension in relation to the Black Mountains accommodation zone. Thick black line represents the approximate trace of the accommodation zone and major anticlinal and synclinal segments. Inset box shows study area as displayed in Figure 3. Accommodation zone trace based on Faulds et al. [1990], Faulds [1993a], Varga et al. [1996], Faulds and Varga [1998], Faulds et al. [2002b]. to the west/southwest. As discussed below, this general pattern mirrors that of the accommodation zone more regionally but on a smaller scale Rock Sequences [10] The stratigraphy in the area is, in many ways, typical of that seen throughout the mountain ranges of the Colorado River extensional corridor [Anderson, 1971; Nielson and Beratan, 1990; Faulds, 1993b; Nielson and Beratan, 1995]. Like in many of the adjacent ranges, the region northwest of Kingman comprises a dominantly volcanic, middle Tertiary section with subordinate clastic rock resting nonconformably upon an erosion surface cut into a basement dominated by Precambrian crystalline rock. The erosion surface largely records the preextensional stripping of pre-tertiary strata within the Kingman uplift [Lucchitta, 1966; Faulds et al., 2001]. Numerous volcanic domes and intrusives within the northern part of the area also suggest that it was a magmatic center during Miocene extension, a characteristic it shares with other areas along the Black Mountains accommodation zone and other accommodation and transfer zones of the Basin and Range [Faulds and Varga, 1998; Rowley, 1998]. [11] Figure 5 compares generalized stratigraphic sections on the two limbs of the anticline. Crystalline rocks exposed in several isolated outcrops in the eastern and southwestern parts of the area (Figure 3) form the basement for the Miocene volcanic section. These areas expose quartzofeldspathic granitic orthogneiss with abundant biotite and megacrystic granite containing large (several cm) phenocrysts of potassium feldspar, similar in appearance to regionally extensive 1.4 Ga anorogenic granite described in surrounding areas [Anderson, 1983; Anderson and Bender, 1989]. Miocene-age volcanic rocks either rest directly upon these crystalline rocks or locally upon a thin, 0 3 m thick arkosic sandstone. The arkosic sandstone resembles that 4of19

5 Figure 3. Generalized geologic map of the Black Mountains accommodation zone northwest of Kingman, Arizona. Anticline and syncline symbols show areas where tilt domains and related opposing normal faults overlap. Areas with no pattern are Quaternary to Tertiary alluvial deposits. After Varga [2001]. found in a similar stratigraphic position throughout the northern Colorado River extensional corridor [Nielson and Beratan, 1990; Faulds, 1993b]. [12] From base to top, the Miocene volcanic section in the area comprises (1) andesite lava flows and breccias with minor tuff, (2) a rhyolite sequence, and (3) a sequence dominated by olivine basalt lava flows. The section includes several regionally important ignimibrite sheets derived from outside the area. We present seven new 40 Ar/ 39 Ar age dates from the Miocene section in the region (see Tables 1 and 2) which provide important age constraints on the timing of extension in this area. The Miocene section represents an 3.5 m.y.-long period of igneous activity that transgressed the entire history of extensional tilting and accommodation zone formation, but which began significantly prior to extension. Abrupt lateral changes in thickness and facies 5of19

6 Figure 4. Cross sections A-A 0 and B-B 0 (see Figure 3 for location). Tb = upper olivine basalt sequence, Tbs = tuff of Bridge Spring, Tr = rhyolite volcanic sequence, Ta = lower andesite sequence, Tps = Peach Springs Tuff, Pc = Precambrian crystalline rock, Tri = Tertiary intrusive rock, QTa = Quaternary-Tertiary alluvial deposits. 6of19

7 Figure 5. Stratigraphic sequences across the Black Mountains accommodation zone in the study area. These diagrams depict the generalized stratigraphy in the (a) west and (b) east tilted domains and the approximate dip magnitudes only. The diagram does not show actual thicknesses of units. Patterns correlate to those used on Figure 3. With the exception of the Peach Springs Tuff, ages are those given in Table 1. characterize the Miocene volcanic section, which is up to 1.2 km in thickness in the map area of Figure 3, but for which the top is not exposed. Important characteristics of each of the three sequences are described below (summarized from Varga [2001]) Lower Andesite Sequence [13] The lower approximately one third of the section comprises pre-extensional, mafic to intermediate composition volcanic rocks and minor silicic tuffs collectively called the lower andesite sequence. The mafic to intermediate composition lava flows, flow breccias and minor, thin, rhyolite air fall tuff generally are similar to the lower part of the Patsy Mine volcanics of Anderson [1971] or to the Dixie Queen Mine volcanics [Faulds et al., 1995] present in areas to the north. Typical lava flows in this sequence contain 30 40% phenocrysts (plagioclase > Table 1. Summary of 40 Ar/ 39 Ar Age Data a Sample Number b Rock Type Lat./Long. Material Dated c Local Unit Dip Age d, Ma Age Basis e Tuff of Bridge Spring ignimbrite N W san 25 W 15.2 ± 0.1 plateau C 68.1% Upper Olivine Basalt Sequence basalt N W wr 10 W 13.8 ± 0.2 isochron 291 ± basalt N W wr W 15.1 ± 0.1 isochron 316 ± basalt N W wr 15 W 15.1 ±.2 wt.-average C 77.8% Rhyolite Volcanic Sequence obsidian N W bio 6 W ± 0.08 plateau C 96.4% obsidian N W bio 20 E ± 0.05 plateau C 80.2% Highway Tuff ignimbrite N N bio f 35 E ± 0.04 plateau C 54.5% a See Table 2 for detailed isotopic data. b Sample preparation conducted at College of Wooster and University of Iowa. Analyses by L. Snee and S. Harlan of the USGS Denver assisted by Nate Wilds. c Abbreviation are as follows: san, sanidine; bio, biotite; wr, whole rock. d Error is ±1 standard deviation. e The percent 39 Ar released and temperature steps of data included in plateau and weight-averaged ages; initial 40 Ar/ 36 Ar given for isochron ages. f Separated from pumice blocks. 7of19

8 Table Ar/ 39 Ar Data a T, C Radiogenic K-Derived 40 Ar b 39 Ar b 40 Ar R / 39 c Ar K 39 Ar K / 37 d Ar Ca Radiogenic Yield, % 39 Ar K,% Apparent Age and Error, e Ma , Tuff of Bridge Spring, Sanidine Total Gas Date: ± 0.11 Ma; Plateau Date: ± 0.09 Ma; J = ± 0.1%; wt mg ± ± ± ± f ± f ± f ± f ± f ± , Basalt, Whole Rock Total Gas Date: ± 0.03 Ma; No Plateau; Weight Average Date: 15.1 ± 0.2 Ma Isochron Date: ± 0.04 Ma ( 40 Ar/ 36 Ar) i = 294 ± 2 ( C); J = ± 0.1%; wt mg ± ± g ± g ± g ± g ± ± ± ± , Basalt, Whole Rock Total Gas Date: ± 0.04 Ma; No Plateau; Isochron Date: 13.8 ± 0.2 Ma ( 40 Ar/ 36 Ar) i = 291 ± 3 (All Steps); J = ± 0.1%; wt mg ± ± ± ± ± ± ± ± ± , Basalt, Whole Rock Total Gas Date: ± 0.05 Ma; No Plateau; Isochron Date: 15.1 ± 0.1 Ma ( 40 Ar/ 36 Ar) i = 316 ± 8 ( C); J = ± 0.1%; wt mg ± ± ± ± ± ± ± ± ± , Obsidian, Biotite Total Gas Date: ± 0.17 Ma; Plateau Date: ± 0.08 Ma; J = ± 0.1%; wt mg ± ± ± ± ± f ± f ± f ± f ± f ± f ± of19

9 Table 2. (continued) T, C Radiogenic K-Derived 40 Ar b 39 Ar b 40 Ar R / 39 c Ar K 39 Ar K / 37 d Ar Ca Radiogenic Yield, % 39 Ar K,% Apparent Age and Error, e Ma , Obsidian, Biotite Total Gas Date: ± 0.05 Ma; Plateau Date: ± 0.05 Ma; J = ± 0.1%; wt mg ± ± ± ± ± ± f ± f ± f ± f ± f ± , Highway Tuff, Biotite 1 Total Gas Date: ± 0.14 Ma; Plateau Date: ± 0.10 Ma; J = ± 0.1%; wt mg ± ± ± ± ± ± ± f ± f ± f ± f ± f ± , Highway Tuff, Biotite 2 Total Gas Date: ± 0.19 Ma; Plateau Date: ± 0.04 Ma; J = ± 0.1%; wt mg ± ± ± ± ± ± ± f ± f ± f ± 0.03 clinopyroxene > orthopyroxene olivine), with abundant petrographic evidence for magma mixing, such as mantled and sieved phenocrysts and other disequilibrium textures [e.g., Bacon, 1986]. The unit is approximately 300 m thick and is bracketed in age by the 18.5 Ma Peach Spring Tuff [Young and Brennan, 1974; Nielson et al., 1990] at or near its base and the overlying ± 0.04 Ma (better of two ages determined, see Table 1) Highway tuff Rhyolite Volcanic Sequence [14] Overlying the lower andesite sequence is an extensive section of rhyolite lava flows, obsidian flows and pyroclastic rocks generally similar to the Volcanics of Red Gap Mine [Faulds et al., 1995]. The lava flows (Figure 6a) are generally sparsely phyric containing 5 20% phenocrysts (plagioclase > biotite ± sanidine ± clinopyroxene ± quartz). Obsidian occurs at the base of individual rhyolite flows and as exogeneous domes. Much of the tuffaceous material in this unit contains lithic fragments (up to 2 m in diameter) derived from older local units and is probably locally derived. [15] The northern half of the map area represents the concentration of volcanic centers related to the rhyolite volcanic sequence. Evidence for this is exposed exogenous domes, numerous rhyolite dikes of composition similar to the felsic volcanic sequence, and the dramatic change in thickness of this unit. The sequence is over 300 m thick in the central part of the map area and thins to 1 2 m of pyroclastic material in areas to the south near Little Thorne Spring (compare cross sections A-A 0 and B-B 0 in Figure 4). Numerous rhyolite-composition dikes and several silicic stocks are present and probably related to the flows of the rhyolite volcanic sequence. The largest of these are intrusions in the northeastern part of the map area near the HB Basin fault and at Little Thorne Spring (Figure 3). The Little Thorne Spring intrusion may have fed the compositionally similar Cottonwood dike, a silicic dike exposed for nearly 5 km from just north of Little Thorne Spring to north of Cottonwood Spring (Figure 3). Both the Little Thorne Spring intrusion and Cottonwood dike are probably synextensional as discussed below. 9of19

10 [16] The age of the rhyolite volcanic sequence is bracketed by a ± 0.04 Ma age on biotite from the Highway tuff near the base of the section in eastern exposures and by a ± 0.08 Ma biotite age on a rhyolite flow interbedded with olivine basalt flows in the transition sequence with the upper olivine basalt sequence (see Table 1) Upper Olivine Basalt Sequence [17] Transitional with (Figure 6a), and overlying the rhyolite volcanic sequence is a thick (1 km minimum thickness as top of unit not exposed) section of dominantly 1 2 m thick, olivine-phyric (olivine plagioclase > pyroxene) basalt lava flows. Associated with these flows are minor basalt flow breccia, scoriaceous pyroclastic rock and minor sandstone and conglomerate This unit is best exposed in the western part of the area (Figure 3). Numerous dikes of mineralogy similar to basalts of this sequence cut the area shown in Figure 3 and may have served as feeders for the mafic flows within the unit. The basal, approximately one third of the section contains interbedded rhyolite lava and pyroclastic deposits indicative of the transitional nature between the olivine basalt and rhyolite sequences. The upper olivine basalt sequence is regionally equivalent to the Mount Davis Volcanics as defined by Anderson [1971]. [18] The upper olivine basalt sequence is constrained by a ± 0.08 Ma age on biotite from a rhyolite lava flow within the basal, transitional part of the section and a whole rock age of 15.1 ± 0.2 Ma on a basalt near the top of the exposed section. Two additional whole rock ages (13.8 ± 0.2 and 15.1 ± 0.1 Ma) on basalts of this section exposed to the west of the area shown in Figure 3 are given in Table Major Ash Flow Tuff Units Peach Springs Tuff [19] Pink- to salmon-colored ash flow tuff occurs near the base of the lower andesite sequence throughout the area. In the northern and eastern part of the region (Figure 3), the tuff consists of a single, 25 m thick cooling unit with variable amounts of non-welded ash flow and ashfall tuff at its base or overlying it. To the southwest, near the Little Thorne Spring intrusion (Figure 3), the tuff thickens and contains two cooling units. The tuff carries 30 50% phenocrysts dominated by sanidine with minor plagioclase, biotite and sphene; quartz is rare to absent. Sanidine commonly displays blue adularesence. We correlate this tuff with the regionally extensive, 18.5 Ma Peach Spring Tuff [Young and Brennan, Notes to Table 2. a Mineral separates and (or) basalt groundmass concentrates were derived from rock samples that were crushed, ground, and sieved to mesh size ( micrometers). Mineral concentrates were passed through magnetic separator and heavy liquids and then handpicked to greater than 99% purity. Biotite was treated with dilute HCl, sanidine in dilute HF. Phenocrysts and xenoliths were removed from groundmass concentrates using a magnetic separator or handpicking and final concentrates were washed in dilute HCl. All samples then were cleaned with reagent-grade acetone, alcohol, and deionized water and air-dried in an oven at 75 C. Between 34 to 138 mg of mineral and 230 to 270 mg of groundmass concentrate were wrapped in aluminum foil packages and encapsulated in silica vials along with neutron-fluence standards prior to irradiation. The standards for this experiment are hornblende MMhb-1 with percent K = 1.555, 40 Ar R = mole/gm, and K-Ar age = Ma [Samson and Alexander, 1987] and FCT sanidine with an internally calibrated age of Ma as measured against MMhb-1. For irradiation, an aluminum canister was loaded with six silica vials, each containing samples and standards similar to that described by Snee et al. [1988]. Standards were placed between every two samples as well as at the top and bottom of each silica vial. Samples were irradiated in one of three different irradiation packages in the TRIGA reactor at the U.S. Geological Survey in Denver, Colorado. Length of irradiation was either 20 or 30 hours at 1 megawatt. Each irradiation package was rotated at 1 rpm during irradiation. All samples and standards were analyzed in the Denver Argon Laboratory of the U.S. Geological Survey using a Mass Analyser Products 215 rare-gas mass spectrometer on a Faraday-cup collector. Each sample was heated in a double-vacuum low-blank resistance furnace (similar to that described by Staudacher et al. [1978]) for 20 min, in a series of 9 to 13 steps, to a maximum of 1450 C, and analyzed using the standard stepwise heating technique described by Snee [1982]. Each standard was degassed to release argon in a single step at 1250 C for MMhb-1, hornblende or at 1350 C for FCT sanidine. For every argon measurement, five isotopes of argon ( 40 Ar, 39 Ar, 38 Ar, 37 Ar, and 36 Ar) are measured. Detection limit at the time of these experiments was moles of argon. Standard techniques were employed to produce 40 Ar/ 39 Ar age spectra, apparent K/Ca diagrams, and isochron diagrams as described by Snee [2002]. b Abundance of radiogenic 40 Ar and K-derived 39 Ar is measured in volts and calculated to five decimal places. Voltage may be converted to moles using moles argon per volt signal. The 40 Ar R / 39 Ar K is calculated to three decimal places. All three are rounded to significant figures using analytical precision. c The 40 Ar R / 39 Ar K has been corrected for mass discrimination. Mass discrimination was determined by calculating the 40 Ar/ 36 Ar ratio of aliquots of atmospheric argon pipetted from a fixed pipette on the extraction line; the ratio during these experiment was between and 299.1, which was corrected to to account for mass discrimination. The 40 Ar R / 39 Ar K was corrected for all interfering isotopes of argon including atmospheric argon. 37 Ar and 39 Ar, which are produced during irradiation, are radioactive and their abundances were corrected for radioactive decay. Abundances of interfering isotopes from K and Ca were calculated from reactor production ratios determined by irradiating and analyzing pure CaF 2 and K 2 SO 4 ;thek 2 SO 4 was degassed in a vacuum furnace prior to irradiation to release extraneous argon. Corrections for Cl-derived 36 Ar were determined using the method of Roddick [1983]. Production ratios for this experiment were determined ( 40 Ar/ 39 Ar) K,( 38 Ar/ 39 Ar) K,( 37 Ar/ 39 Ar) K,( 36 Ar/ 37 Ar) Ca,( 39 Ar/ 37 Ar) Ca, and ( 38 Ar/ 37 Ar) Ca ; measured values are available upon request. d Apparent ages and associated errors were calculated from unrounded analytical data then rounded using associated analytical errors. Apparent ages of each fraction include the error in J value (0.1%), which was calculated from the reproducibility of splits of the argon from several standards. Apparent ages were calculated using decay constants of Steiger and Jäger [1977]. All apparent age errors are cited at 1 sigma. Uncertainties in the calculations for apparent age of individual fractions were calculated using equations of Dalrymple et al. [1981] and the critical value test of McIntyre [1963]. Isochron analysis followed the procedures of York [1969]. e To calculate apparent K/Ca ratios, divide the 39 Ar K / 37 Ar Ca by 2. The accuracy of apparent K/Ca ratios is dependent upon fast to thermal neutron ratios in the particular reactor. In the U.S. Geological Survey TRIGA reactor the correction factor has not been determined since Dalrymple et al. [1981]. Because reactor fuel in the USGS TRIGA has been changed since 1981, this ratio must be viewed as approximate but is internally consistent for each sample and reveals within-sample variability. f Fraction included in plateau date. Plateaus determined according to the method of Fleck et al. [1977]. g Fraction included in weight-averaged data. 10 of 19

11 Figure 6. (a) View to the south of east tilted units in the transition zone between the upper olivine basalt and rhyolite volcanic sequences. Note the interlayering of rhyolite pyroclastic units (Tr) and olivine basalt lava flows (Tb). (b) View to north of west dipping arkosic sandstone and conglomerate within the upper olivine basalt sequence of the western part of the region north of Little Thorne Spring (Figure 3). Note blocks of Peach Springs Tuff (pst) contained within channel fill. (c) View to the south of dip-fanning within a small half-graben in the northern part of the study area. Note upward decreasing, eastward dips within pyroclastic units of the rhyolite volcanic sequence. Encircled area shows seated person for scale. An age of Ma (Table 1) was obtained for an obsidian at the base of a rhyolite flow near the top of this sequence. (d) View to south showing dip-fanning between 15.2 Ma tuff of Bridge Spring (dipping here at about 25 ) and 15.1 Ma upper olivine basalt capping the sequence (dipping west at approximately 15 ). 1974; Glazner et al., 1986] because of its similarity in stratigraphic position and mineralogy Highway Tuff [20] The informally named Highway tuff defines the base of the rhyolite volcanic sequence in the eastern part of the area. This welded, crystal-rich (plagioclase biotite > clinopyroxene = sanidine) tuff with black fiamme rests upon lava flows of the lower andesite sequence. The tuff is up to 30 m thick. Two separates of biotite from the Highway tuff yields a plateau ages of ± 0.04 Ma and ± 0.1 Ma (Table 1); we prefer the former age due to its lower standard error. The tuff has no known correlative outside of the area shown on Figure 3. The presence of 1 m diameter blocks of older lava flows contained within a coignimbrite lag deposit near its base suggests a relatively nearby source area Tuff of Bridge Spring [21] A distinctive gray to cream-colored non-welded to moderately welded tuff is contained within the lower part of the upper olivine basalt sequence. The tuff varies in thickness between 0 3 m, and consists of 0.5 m of basal, nonwelded tuff with abundant scoriaceous, mafic volcanic lithic fragments overlain by moderately welded tuff with medium gray colored pumice. The welded tuff carries approximately 15% phenocrysts of principally sanidine and biotite. Sanidine provides a 15.2 ± 0.1 Ma plateau date (Table 1). The tuff is correlated to the Tuff of Bridge Spring of Anderson [1971]. The source of the Tuff of Bridge Spring is believed to be a caldera in the northern Eldorado Mountains [Gans et al., 1994]. 3. Relationship of the Igneous Sequences to Extension [22] The presence of datable volcanic sections within half-graben basins bounded by normal faults affords an 11 of 19

12 Figure 7. Map showing detailed geologic relationships near Little Thorne Spring (see Figure 3 for location of this map). Note that the Little Thorne Spring rhyolite stock (LTSI) cuts and is not appreciably offset by the Little Thorne Spring fault. Units explained in caption for Figure 4. excellent opportunity to track the temporal evolution of extension as stratal tilt magnitude is generally related to degree of extension [e.g., McClay and Ellis, 1987; Axen, 1988]. In the volcanic-dominated Colorado River extensional corridor, dating of tilt fans within half-graben has been a key factor in deciphering the onset of extension and changes in extensional rates over time [e.g., Faulds et al., 1995; Beratan, 1996; Faulds et al., 2002b]. Our 40 Ar/ 39 Ar isotopic data (Tables 1 and 2) and recognition of several regionally recognized ignimbrites allow temporal constraints to be placed on the timing of extension in the area as well as on the growth of extension-related folds. [23] Figure 5 summarizes the stratigraphy of the accommodation zone in relationship to relative stratal tilt and age data. In this area, the Miocene volcanic sequence spans a period between 18.5 Ma and <15.1 Ma. Mapping of extension-related dip fanning within these sequences suggests that east tilting of units in the eastern and northern parts of the area began somewhat earlier than did west tilting in the western part of the accommodation zone. Fanning of east dips within exposures along the northern part of the map area (Figure 6c) indicates that, at least locally, the east tilting and formation of the eastern limb of the anticline began during eruption of the rhyolite volcanic sequence. As much as 20 of tilting occurred after eruption of the 16.3 Ma Highway tuff and prior to deposition of the ± 0.05 Ma rhyolite flow (Table 1). Dramatic changes in thickness of the Highway tuff across individual fault blocks [Varga, 2001] suggest that this tuff accumulated on a surface of considerable relief and that tilting and formation of half-graben basins may have been approximately coeval with eruption of this unit. [24] In contrast, west tilting of strata along east dipping normal faults appears to have begun somewhat later than east tilting, during accumulation of the upper olivine basalt sequence. Fanning of dips shows that tilting began at about the time of deposition of the Tuff of Bridge Spring at 15.2 Ma (dips of about 30 ) and was still active at 15.1 Ma (basalt dips of 15, Table 1). In the western part of the area shown on Figure 3, 15.1 Ma basalt dips between whereas 13.8 Ma basalt dips only about 10 (Figure 6d and Table 1). Some silicic igneous activity appears to have continued during west tilting. In the southwestern part of the map area (Figure 7), the Little Thorne Spring intrusion is not appreciably offset by the Little Thorne Spring fault, a spoon-shaped normal fault that significantly offsets the upper andesite sequence. This stock is mineralogically equivalent to the Cottonwood dike which is exposed for a distance of about 5 km north of Little Thorne Spring. The Cottonwood dike cuts 15.2 Ma Tuff of 12 of 19

13 Bridge Spring [Varga, 2001] which was deposited at about the beginning of west tilting. [25] Coarse-grained sandstone, matrix-supported conglomerate and minor siltstone is preserved in channels within the basal part of the upper olivine basalt sequence [Varga, 2001]. This unit is 0 m to over 7 m thick and contains clasts up to 0.75 m in diameter of Proterozoic crystalline rocks, the Peach Springs Tuff and intermediate volcanic rock from the lower andesite sequence (Figure 6b). The presence of this clastic unit may signal the emergence of significant topography and exposure of older units. Topographically elevated footwalls of normal faults are likely sources for these coarse clastic sediments. 4. Structure of the Accommodation Zone [26] The most obvious structural aspect of the area is the moderately tilted Miocene sequence that dips in opposite directions defining a crude, open anticline in the central part of the area (Figure 3). The general form of the anticline is seen in cross section A-A 0 which runs northeast/southwest across the area (Figures 3 and 4). Local structural complexities, flow folds in volcanic rocks and volcanic constructional structures complicate bedding orientations, but a summary of 900 bedding readings across the entire region illustrates the bimodal nature of dips across the anticline (Figures 8a and 8b) and define a horizontal, N10 west trending fold axis with a vertical axial surface (Figure 8b). The interlimb angle of the structure is about 110, with limb dips of The amount of extension across this structure is 14% (constant bed length) as calculated at the level of the Peach Springs Tuff across the fold as shown in section A-A 0 (Figure 4). [27] An approximate inflection point on the western limb of the structure is located about 4 km from the fold hinge, suggesting a wavelength of about 16 km for this upright structure. This is about the wavelength for folds in the Highland Range [Faulds et al., 2002b] exposed along the transverse part of the Black Mountains accommodation zone (Figure 2). However, it should be stressed that smaller-scale folds exist locally wherever fault tips of opposite polarity overlap. [28] Normal faults (Figure 3) generally strike to the north and dip either to the east or west. Many of these faults are well exposed and contain well-developed striae and kinematic indicators such as secondary Riedel shears and obvious offset markers. Many faults are expressed in the field by 0.5 m-thick, tabular, silicified breccia zones that form erosionally resistant ridges. Striae generally have moderate to steep rakes within fault surfaces (Figure 8c). [29] Where west and east dipping normal faults intersect, east dipping faults typically cut and offset west dipping faults (see Figures 3 and 4). This relationship is consistent with the stratigraphic evidence discussed above that growthfault basins began to form at least 0.2 m.y. earlier in the east tilted domain than in the west tilted domain. In the central part of the map area, several originally west dipping normal faults lie in the hanging walls of younger, east dipping normal faults. The result of this geometry is that, locally, these relatively older faults have anomalously steep west-dips or are overturned such that their apparent offset in the field is reverse in motion. A notable exception to this relative timing is the west dipping Axial fault (Figures 3 and 4). Offset equivalents of this fault do not appear to the west, in the west tilted domain and it, thus, seems likely that east dipping normal faults do not cut it, or at least displace it significantly. As shown in cross section A-A 0, this fault may have acted in concert with formation of numerous, relatively small offset, east dipping normal faults in accommodating formation of the rollover in this area. [30] Generally, normal faults within both tilt domains lose displacement toward the accommodation zone where it turns to the west/southwest and is essentially normal to fault strike directions. A good example of this is the Cottonwood Spring fault zone (Figure 3). This fault zone has 1 km of offset in the southern part of the area, where it is interpreted to place the upper olivine basalts sequence against Proterozoic basement. The fault zone must lose displacement to the north where it bifurcates into several small-displacement faults (compare cross sections B-B 0 and A-A 0, Figure 4), each of which eventually terminate where overlapped by down-to-the-west normal faults. The Mud Spring fault (Figure 3) also clearly dies as traced to the north. In the central part of the area, this fault juxtaposes the olivine basalt and rhyolite volcanic sequences. Displacement on the Mud Spring fault decreases northward toward the accommodation zone where the rhyolite volcanic sequence is offset by only a few meters. More generally, where the faults of the two tilt domains interact in the accommodation zone, they have relatively small displacements. In this zone, the opposing normal faults tips commonly overlap, forming a series of small-relief anticlines and synclines (Figures 3 and 9). [31] Strike-slip connections (transfer zones) between the opposing fault systems are notably absent within the accommodation zone (Figure 9). Faults with strike-slip or oblique-slip offset do occur, however, within tilt domains, either as transfer faults that account for differences in the magnitude or geometry of offset across them, or as the edges of spoon-shaped normal faults. Figure 7 shows good examples of such faults in the southwestern part of the area. Faults oblique to the dominant north/northwest trend of normal faults in the region are also common where the accommodation zone is approximately northeast/southwest trending (Figure 3). Fault-slip data on these structures indicate that they are dominantly oblique-slip normal faults with only a minor component of horizontal movement. [32] The anticline extends from the southeast into the central part of Figure 3 as a relatively simple structure seen in cross section A-A 0 (Figure 4). In the terminology of Faulds and Varga [1998] this structure is a strike-parallel, anticlinal-antithetic accommodation zone, as its gross morphology is that of an elongate anticline between normal fault systems of opposite polarity. An important feature of this kind of structure is that the fold hinge is structurally low with respect to the flanking limbs in contrast to anticlines produced in contractional settings. No evidence was observed for shortening across this structure. Analysis of 13 of 19

14 Figure 8. Equal-area stereonets of structural data from the study area. (a) 900 poles to bedding. (b) Poles to bedding contoured using the Kamb [1959] method; contours are in intervals of 2 times the standard deviation of data from an assumed random distribution. Best fit fold axis is 0 /170. (c) Data for 59 measured normal fault surfaces (great circles) and striae (dots). Arrows show the motion of the hanging wall across each fault. (d) Extension axes [Marrett and Allmendinger, 1990] for faults shown in Figure 8c contoured using the Kamb method. Contour interval is 2 times the standard deviation of data from an assumed random distribution. These data suggest a direction of extension oriented at 5.7 / faults and their striae using the methods of Marrett and Allmendinger [1990] indicates that the faults were produced as the result of east/northeast, horizontal extension oriented approximately 084 (Figures 8c and 8d). [33] The rather simple geometry of the anticline in the southern part of the map area becomes more complex to the north, where the accommodation zone is approximately northeast/southwest trending. In this area, dips of strata are generally low and define a series of connected anticlines and synclines between faults of opposing dip (Figures 3 and 9). [34] Figure 10 is a much generalized block diagram depicting the structure of the Black Mountains accommo- 14 of 19

15 Figure 9. Simplified map of detailed geology within a small area of the transverse segment of the accommodation zone as shown in Figure 3. Note that faults of opposite polarity generally lose displacement and terminate where they overlap in accommodation zone. All of the rock units shown on this map are part of the rhyolite volcanic sequence. Units are: Trf = undifferentiated rhyolite flows, Trt = undifferentiated rhyolite tuff, Trfr = distinctive red-colored rhyolite flow; QTa = unconsolidated alluvium. Section corner shown is within T23N, R19W. dation zone in the region northwest of Kingman, Arizona. It shows the relationship between the geometry (inward dipping vs. outward dipping) of opposed normal faults and the amount of overlap between faults of opposite polarity. The strike-parallel, elongate anticline of the southern and central part of the area is a manifestation of the significant overlap between opposed and inward dipping normal fault systems of the Whipple and Lake Mead tilt domains. In contrast, the short anticline and syncline segments of the transverse portion of the accommodation zone reflect the interdigitating faults of the two tilt domains with relatively little overlap. As is apparent on this diagram, the more overlap that exists between faults of opposite polarity, the more the trend of the fold produced is parallel to strike direction of faults. Minimal overlap between fault domains produces gentle folds with axes nearly perpendicular to regional strike trends. [35] The Black Mountains accommodation zone in the area northwest of Kingman forms localized, interbasinal structural highs within structural troughs or half-graben (Figure 10). This geometry is typical of accommodation zones in general and commonly serves to control facies patterns of synextensional strata [Serra and Nelson, 1988; Gawthorpe and Hurst, 1993; Faulds and Varga, 1998; Faulds et al., 2002b]. In the area, this geometry may have controlled the distribution of synextensional arkosic sandstone and conglomerate (Figure 6b). This unit occurs within the upper olivine basalt sequence in the southern 1/3 of the map area shown on Figure 3 and appears to pinch out farther north toward the accommodation zone [Varga, 2001]. 5. Relationship of the Study Area to the Greater Black Mountains Accommodation Zone [36] The accommodation zone northwest of Kingman shown on Figure 3 is part of the larger, >230 km-long Black Mountains accommodation zone that forms the boundary between the west tilted strata of the Whipple tilt domain to the south and the east tilted strata of the Lake Mead tilt domain to the north [Faulds et al., 1990; Varga et al., 1996; Faulds and Varga, 1998]. As currently mapped (Figure 2), the boundary runs from the area of the Highland Range of southern Nevada [Faulds et al., 2002b] to the east through the Eldorado, northern Black Mountains and White 15 of 19

16 Figure 10. Schematic block diagram showing the general structural relationships across the Black Mountains accommodation zone in the area northwest of Kingman, Arizona. The figure was drawn to show a generalized fault pattern that would produce the accommodation zone pattern shown in Figure 2 but does not depict the precise pattern of actual faults as mapped on Figure 3. Note the strike-parallel segment of the accommodation zone is controlled by the considerable overlap of opposed and inward facing normal faults while the short anticlinal and synclinal segments of the transverse portion of the accommodation zone is caused by minimal fault overlap. Hills [Faulds, 1993a; Faulds et al., 1999; Price and Faulds, 1999], where it turns to the south and passes through the area of Figure 3, Union Pass and, finally, the Warm Springs area at the southern tip of the southern Black Mountains [Varga et al., 1996; Faulds and Varga, 1998]. [37] The circuitous route of the Black Mountains accommodation zone reflects the geometry of opposing normal faults of the Lake Mead and Whipple tilt domains. Between the Highland Range and White Hills, faults of the opposed domains have modest overlap and, consequently, this transverse segment of the accommodation zone is defined by a series of anticlines and synclines that reflect the geometry of overlap (inward versus outward facing) of the two fault systems. From the White Hills to the Warm Springs area, the Black Mountains accommodation zone forms a more or less north trending anticline reflecting the significant overlap between faults of the Whipple domain and the southern projections of west dipping normal faults (western Cerbat Mountains and southern Grand Wash fault systems) of the Lake Mead domain that cut the transition zone along the western side of the Colorado Plateau (Figure 2). [38] From a regional perspective, the Black Mountains accommodation zone between the White Hills and Warm Springs area forms a strike-parallel, antithetic-anticlinal accommodation zone [Faulds and Varga, 1998]. Detailed mapping such as that shown in Figure 3, however, illustrates that this part of the accommodation zone exhibits significant local complexity. The accommodation zone in this small area (Figures 3 and 10), reflects the geometry of the accommodation zone more regionally (Figure 2). For example, immediately north of the area of Figure 3, the accommodation zone migrates to the east, across Detrital Valley to join the anticline of the White Hills. To the south, the accommodation zone migrates to the west through the Union Pass area. Between Union Pass and the Warm Springs area, the zone is diffuse and comprises several subparallel, low-amplitude folds. [39] The timing of formation of the Black Mountains accommodation zone further reflects the temporal development of the normal fault systems within the Lake Mead and Whipple domains. In the area of Figure 3, down-to-the-west faulting began sometime in the interval between Ma and Ma while the down-to-the-east faulting ensued at least 0.2 m.y. later. The initiation of west dipping normal faults in this area is compatible with the similarly oriented faults in the White Hills to the north which formed about 16 Ma [Price and Faulds, 1999]. In contrast, east dipping normal faults in the Mt. Perkins area north of the area shown in Figure 3 began to form at about 15.7 Ma, about 0.5 m.y. earlier [Faulds et al., 1995] compared to those in the study area. To the south near Warm Springs, faulting related to both domains initiated earlier. East dipping 16 of 19