Extension and magmatism in the Cerocahui basin, northern Sierra Madre Occidental, western Chihuahua, Mexico

The Sierra Madre Occidental of northwestern Mexico is the biggest silicic large igneous province of the Cenozoic, yet very little is known about its geology due to difficulties of access to much of this region. This study presents geologic maps and two new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages from the Cerocahui basin, a previously unmapped and undated ~25 km-long by ~12 km-wide half-graben along the western edge of the relatively unextended core of the northern Sierra Madre Occidental silicic large igneous province. Five stratigraphic units are defined in the study area: (1) undated welded to non-welded silicic ignimbrites that underlie the rocks of the Cerocahui basin, likely correlative to Oligocene-age ignimbrites to the east and west; (2) the ca. 27.5–26 Ma Bahuichivo volcanics, comprising mafic-intermediate lavas and subvolcanic intrusions in the Cerocahui basin; (3) alluvial fan deposits and interbedded distal non-welded silicic ignimbrites of the Cerocahui clastic unit; (4) basalt lavas erupted into the Cerocahui basin following alluvial deposition; and (5) silicic hypabyssal intrusions emplaced along the eastern margin of the basin and to a lesser degree within the basin deposits. The main geologic structures in the Cerocahui basin and surrounding region are NNW-trending normal faults, with the basin bounded on the east by the syndepositional W-dipping Bahuichivo–Bachamichi and Pañales faults. Evidence of syndepositional extension in the half-graben (e.g. fanning dips, unconformities, coarsening of clastic deposits toward basin-bounding faults) indicates that normal faulting was active during deposition in the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas), and may have been active earlier based on regional correlations. The rocks in the Cerocahui basin and adjacent areas record: (1) the eruption of silicic outflow ignimbrite sheets, likely erupted from caldera sources to the east during the early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up, mostly prior to synextensional deposition in the Cerocahui basin (pre-27.5 Ma); (2) synextensional late Oligocene mafic-intermediate composition magmatism and alluvial fan sedimentation (ca. 27.5–24.5 Ma), which occurred during the lull between the Early Oligocene and early Miocene pulses of the ignimbrite flare-up; and (3) post-extensional emplacement of silicic hypabyssal intrusions along pre-existing normal faults, likely during the early Miocene pulse of the ignimbrite flare-up (younger than ca. 24.5 Ma). The timing of extensional faulting and magmatism in the Cerocahui basin and surrounding area generally coincides with previous models of regional-scale middle Eocene to early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental controlled by post-late Eocene (ca. 40 Ma) rollback/fallback of the subducted Farallon slab.


Introduction
The Sierra Madre Occidental of western Mexico is the third biggest (up to 400,000 km 3 ) and best-preserved silicic large igneous province in Earth's history and is the largest of the Cenozoic (Figure 1; Bryan 2007;Ferrari et al. 2007;Bryan and Ernst 2008;. Despite its size and preservation, very little is known about the geology of the Sierra Madre Occidental due to difficulties of access to much of this region. A large part of the Sierra Madre Occidental remains unmapped and undated (>90%; Swanson et al. 2006), with previous work primarily restricted to the southern region of the igneous province and to the major highways that transverse the northern and central regions (e.g. McDowell and Keizer 1977;McDowell 1984, 1985;Wark et al. 1990; Aguirre-Díaz and McDowell above a subducting slab that progressively fell back and steepened following early Cenozoic low-angle subduction, as suggested by a general westward sweep of volcanism with time (e.g. Best and Christiansen 1991;Dickinson 2006Dickinson , 2013Ferrari et al. 2007;Henry et al. 2010;McQuarrie and Oskin 2010). In western Mexico, Eocene to Miocene slab fallback and arc extension associated with it ultimately led to Miocene rifting in the Gulf of California (Ferrari et al. 2007. Previous studies suggest that silicic large igneous provinces may initiate as magmatic events in continental regions undergoing broad lithospheric extension, prior to the rupture of continental lithosphere (Bryan et al. 2002;Bryan 2007;Best et al. 2013;. In addition, the generation of large silicic magma volumes (e.g. Hildreth 1981) and some very large-magnitude explosive silicic eruptions (Aguirre-Díaz and Labarthe-Hernández 2003; Costa et al. 2011) may be favoured by crustal extension. Therefore, studying the relationships between the timing of extensional deformation and magmatism is an important consideration toward understanding the development of the Sierra Madre Occidental.
Here we present new geologic mapping, stratigraphy, and geochronology in a previously unrecognized extensional basin in the northern Sierra Madre Occidental (Figure 1).
We refer to this basin as the 'Cerocahui basin' ; Plate 1, see http://dx.doi.org/10. 1080/ 00206814.2014.941022), and show that it is an extensive and thick section of red beds and mafic-intermediate composition volcanic rocks that accumulated between two major pulses of silicic magmatism during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental. This basin lies in the western part of the (previously defined) unextended core of the northern Sierra Madre Occidental ( Figure 1) and the rock exposure and topographic relief in this poorly known region make it an excellent locality for examining the relationships between extensional basin development and silicic large igneous province magmatism in the Sierra Madre Occidental. With these data, we correlate magmatic and extensional events over a broader region than initially described by Murray et al. (2013) in the adjacent Guazapares Mining District to the west (Figure 2), and show that the timing of magmatism and synextensional deposition in this region of the northern Sierra Madre Occidental, which includes both the Cerocahui basin and Guazapares Mining District, supports the interpretation that silicic flare-up magmatism migrated southwestward with time across northwestern Mexico and suggests that the eastern limit of the Gulf Extensional Province extends further into the unextended core than previously recognized.   Ferrari et al. 2002;. The location of the study area and the adjacent Guazapares Mining District region, described by Murray et al. (2013), is indicated by black box (Figure 2) on the western edge of the unextended core. TMVB, Trans-Mexican Volcanic Belt.  Figure 5) and the locations of major faults are shown. The red box indicates the area of the geologic map in Figure 3; detailed maps of the Guazapares Mining District region (green box) are presented in Murray et al. (2013). The location of the main roads and the 'Chepe' Copper Canyon railroad that transect the study area are also shown.

Figure 3
A A'   Table 2) Areas inferred from aerial photography

Regional geology
The Sierra Madre Occidental ( Figure 1) consists primarily of late Eocene to early Miocene ignimbrites that cover an area of~400,000 km 2 with an average thickness of 1 km (McDowell and Keizer 1977;McDowell and Clabaugh 1979;Aguirre-Díaz and Labarthe-Hernández 2003;. There were at least two main pulses of silicic ignimbrite volcanism during the mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental, one during the late Eocene-early Oligocene (ca. 36-28 Ma) that occurred throughout the Sierra Madre Occidental and another during the early Miocene (ca. 24-20 Ma) that is generally restricted to the southern part of the igneous province (Ferrari et al. 2002(Ferrari et al. , 2007McDowell and McIntosh 2012). During the final stages of and after each silicic ignimbrite pulse, basaltic andesite lavas, commonly referred to as the Southern Cordillera basaltic andesite province (SCORBA), were intermittently erupted across all of the northern Sierra Madre Occidental (Cameron et al. 1989;Ferrari et al. 2007).
Following the Laramide orogeny (late Eocene) in western North America, the age distribution of volcanic rocks in the southwestern US and the Sierra Madre Occidental suggests that ignimbrite flare-up magmatism generally migrated southwestward over time (e.g. Coney and Reynolds 1977;Damon et al. 1981;Best and Christiansen 1991;Christiansen and Yates 1992;Dickinson 2002Dickinson , 2006Dickinson , 2013Ferrari et al. 2007;Henry et al. 2010;McQuarrie and Oskin 2010;McDowell 2012;Busby 2013). This post-Laramide age trend is likely related to removal of the flat to low-angle subducted Farallon plate from the base of the North American plate by either steepening (slab rollback) and/or possible detachment of the deeper part of the subducted slab (e.g. Dickinson and Snyder 1978;Best and Christiansen 1991;Ferrari et al. 2007;Henry et al. 2010;McQuarrie and Oskin 2010;Best et al. 2013;Busby 2013;Dickinson 2013    1984; Ferrari et al. 2007), post-dated (e.g. McDowell and Clabaugh 1979;Wark et al. 1990;McDowell and Mauger 1994;Gans 1997;McDowell et al. 1997;Grijalva-Noriega and Roldán-Quintana 1998;Gans et al. 2003), or begun during (e.g. Aguirre-Díaz and McDowell 1993;Luhr et al. 2001;Murray et al. 2013) the late Eocene-early Oligocene pulse of the ignimbrite flare-up. The eruptions of the Southern Cordillera basaltic andesite (SCORBA) following the early Oligocene ignimbrite pulse have been interpreted as magmatism recording the initiation of regional-scale crustal extension in the northern Sierra Madre Occidental (e.g. Cameron et al. 1989;Cochemé and Demant 1991;Gans 1997;McDowell et al. 1997;González León et al. 2000;Ferrari et al. 2007). The central core of the Sierra Madre Occidental is relatively unextended compared to the surrounding late Oligocene-to Miocene-age extensional belts of the southern Basin and Range to the east and the Gulf Extensional Province to the west (Figure 1; Nieto-Samaniego et al. 1999;Henry and Aranda-Gomez 2000). Recent studies in the southwestern Sierra Madre Occidental suggest that synvolcanic extension in the Gulf Extensional Province started to develop by the late Oligocene (ca. 30-20 Ma) in an initially wide rift zone, followed by more focused extension in a narrow rift zone around ca. 20-18 Ma that ultimately led to the opening of the Gulf of California .

Guazapares Mining District
The recent study of the Guazapares Mining District region near Témoris (Figure 2) to the west of the Cerocahui basin (Murray et al. 2013) provides a stratigraphic and structural context for the Cerocahui basin. Three informal formations are recognized in the Guazapares Mining District region ( Figure 2). The oldest, the Parajes formation, consists primarily of welded silicic outflow ignimbrite sheets erupted ca. 27.5 Ma, although this formation is likely older based on a possible stratigraphic correlation with the ca. 29.8 Ma Divisadero tuff of Swanson et al. (2006). These ignimbrites erupted near the end of the early Oligocene pulse of the ignimbrite flare-up, presumably from calderas of similar age that lie largely to the east of both the Guazapares Mining District (Murray et al. 2013) and the Cerocahui basin, described herein. The ca. 27-24.5 Ma Témoris formation (Figure 2), which unconformably overlies the Parajes formation, records local, fault-controlled mafic to intermediate composition magmatism and subsequent distal silicic ignimbrite volcanism, synchronous with extension. Given the similar age and composition, the Témoris formation may be related to the Southern Cordillera basaltic andesite (SCORBA) province erupted in other parts of northern Sierra Madre Occidental following the early Oligocene ignimbrite pulse (Murray et al. 2013). The ca. 24.5-23 Ma Sierra Guazapares formation (Figure 2), which overlies the Témoris formation in angular unconformity, records local silicic magmatism, including vent-to-proximal-facies ignimbrite deposits, lavas, and hypabyssal rocks; these were erupted from and intruded into fault-controlled fissure vents within the Guazapares Mining District region. The Sierra Guazapares formation may record the onset of the early Miocene pulse of the mid-Cenozoic ignimbrite flare-up (Murray et al. 2013), although this represents much smaller-volume magmatism than the rocks erupted during this pulse in the southern Sierra Madre Occidental (e.g. Ferrari et al. 2007).
The main geologic structures in the Guazapares Mining District region are NNW-trending normal faults that bound a series of closely spaced half-graben basins ( Figure 2). Growth strata and angular unconformities between each formation indicate that these half-graben basins began to form by the time the upper part of the Parajes formation was erupted (ca. 27.5 Ma) and continued to develop during deposition of the Témoris and Sierra Guazapares formations. In addition, several of these extensional structures controlled the localization of andesitic and silicic volcanic vents and shallow-level intrusions of the Témoris and Sierra Guazapares formations (Murray et al. 2013).

The Cerocahui basin
The term 'Cerocahui basin' describes the approximately 12 km-wide basin with a mapped length of approximately 25 km (Figures 2 and 3; Plate 1), although it could extend further north and south. It is named for the village of Cerocahui in the southeastern part of the basin, located 12 km south of the town of Bahuichivo, a stop on the famous 'Chepe' Copper Canyon train (Figures 2 and 3; Plate 1). The Cerocahui basin lies on in the western edge of the largely unextended core of the northern Sierra Madre Occidental ( Figure 1). Previous work in the Cerocahui basin region has been restricted to regional 1:50,000 and 1:250,000 geologic mapping (Minjárez Sosa et al. 2002;Ramírez Tello and Garcia Peralta 2004) and unpublished mining company reports, which recognized a red bed sequence in the region but did not describe the geologic setting of these rocks. The geologic mapping for this study was primarily centred on the village of Cerocahui and around Bahuichivo, and on the two main roads that transect the basin on the north and the east (Figures 2 and 3; Plate 1). Much of the central and southwestern sections of the basin are inaccessible due to lack of roads or hazards related to drug cultivation in the region; however, the geology located in these areas (noted on Figure 3) is interpreted from aerial imagery and based on known geologic relationships from the more accessible areas along the major roads and around population centres ( Figure 2). NNW-trending normal faults are the primary geologic structures in the Cerocahui basin and the adjacent region (Figures 2-4; Plate 1). The W-dipping Bahuichivo-Bachamichi fault forms the eastern boundary of the Cerocahui basin. Immediately east (<1 km) and trending parallel to the Bahuichivo-Bachamichi fault is the W-dipping Pañales fault, which has a much thinner section of volcanic and sedimentary rocks on its hanging wall and lacks these rocks on its footwall; this fault thus represents a minor strand of the main basin-bounding fault (Figures 3 and 4A; Plate 1). Several W-dipping normal faults offset the deposits within the Cerocahui basin, including the Cerocahui, El Rodeo, and Irigoyen faults, but these have less offset than the eastern basin-bounding fault (Figures 3 and 4; Plate 1). Extension on these faults has gently to moderately tilted the strata of the Cerocahui basin to the north-to-northeast (~5-30°). The western boundary of the Cerocahui basin is inferred to be roughly in the location of the NW-trending, E-dipping Piedra Bola fault (Figures 2 and 3; Plate 1). This fault is younger than and offsets the lowermost deposits of the Cerocahui basin that extended westward onto the footwall block prior to fault motion (Figures 3 and 4A; Plate 1). Additionally, the basin fill on the hanging wall (east side) of the Piedra Bola fault dips away from the fault rather than toward it; therefore, the Piedra Bola fault is not considered a basin-bounding normal fault.

Silicic outflow ignimbrites of pre-basinal origin (Toi & Tp)
A section of tabular, largely welded silicic outflow ignimbrites is exposed in the footwall of the W-dipping Bahuichivo-Bachamichi and Pañales faults on the east side of the Cerocahui basin (map unit Toi) and in the footwall of the E-dipping Piedra Bola fault on the west side of the basin (Tp) (Figures 2 and 3). These two map units are tentatively correlated, although the ignimbrites east of the basin have not been studied in the same detail as those located west of the basin (Parajes Formation [Tp]) by Murray et al. (2013). At least seven distinct ignimbrites have been identified in the Parajes formation (Tp) on the west side of the Cerocahui basin; each ignimbrite ranges  Figure 5. Rock units of the Cerocahui basin and substrate, depicting the characteristics and depositional relationships between the prebasinal silicic outflow ignimbrites, Bahuichivo volcanics, Cerocahui clastic unit, basalt lavas, and silicic hypabyssal intrusions. Unit symbols are the same as in Figure 3. The ages in bold are from this study ( Figure 10; Table 2), and the approximate ages in italics are from Murray et al. (2013) and are based on inferred stratigraphic correlations in the Guazapares Mining District to the west (see Figure  12; text for discussion). from~20 to~210 m thick, has a densely welded to partially welded lower section that passes upward into a less-welded to non-welded top, and has normal coarse-tail grading of lithic fragments and inverse coarse-tail grading of pumice (Murray et al. 2013).
Because the silicic outflow ignimbrite section bounds the Cerocahui basin on both sides, and forms a widespread sheet where it has been mapped to the west and east (Figures 3 and 4), it is inferred to underlie the deposits of the Cerocahui basin. This interpretation is supported by an angular unconformity between gently dipping (~10°E) mafic-intermediate lavas (Tbl), fluvial sandstone, and an undifferentiated silicic ignimbrite (Tci) interpreted as part of the lowermost deposits of the Cerocahui basin (Bahuichivo volcanics, described below) and the underlying moderately dipping (~25°E) Parajes formation (Tp) on the western side (footwall) of the Piedra Bola fault near Irigoyen (Figures 3 and 4A; Plate 1). In the Guazapares Mining District region immediately west of the Cerocahui basin, normal faulting began during deposition of the youngest units in the Parajes formation (Tp) silicic outflow ignimbrite section after 27.6 ± 0.3 Ma (Murray et al. 2013). Map data in the silicic outflow ignimbrite section east of the Cerocahui basin are insufficient to determine whether any of the units are synextensional.
The silicic outflow ignimbrites on the east side of the Cerocahui basin are interpreted as the medial facies of outflow ignimbrite sheets, based on their sheet-like geometry and the presence of cliff-forming welded sections, similar to the ca. 27.5 Ma Parajes formation (Tp) to the west of the basin (Murray et al. 2013). Their sheet-like geometry is also similar to that of ignimbrites in the unextended core of the Sierra Madre Occidental, although that region also contains caldera-filling ignimbrites (e.g. Swanson et al. 2006). The silicic outflow ignimbrites on the east side of the Cerocahui basin (Toi) have not been dated directly, but they are clearly older than the basal basin-filling sedimentary deposits, and an intrusion of the Bahuichivo volcanics (Tbv) crosscuts one of the ignimbrites ( Figure 6A). The ignimbrites generally have <15% phenocrysts and lack potassium feldspar (e.g. Swanson et al. 2006;Murray et al. 2013). Crystal-rich ignimbrites are rare and may prove correlatable by future workers; one such ignimbrite is located~3.5 km southeast of Bahuichivo (Figure 3), with 30-35% phenocrysts of plagioclase, biotite (to 3 mm), quartz, hornblende, and 10% andesitic volcanic lithic lapilli.
Within the Cerocahui basin, the Bahuichivo volcanics are dominantly lavas, with individual flows up to~20 m thick that dip moderately eastward (to~20°E) toward the eastern basin-bounding faults (Figures 3 and 4). The base of the Bahuichivo volcanics is not exposed in the Cerocahui basin; the unit is the thickest (>500 m thick) in the southwestern mapped part of the basin and likely extends westward to the Piedra Bola fault based on aerial imagery. In the northwestern mapped part of the basin near Irigoyen (Figure, 3), lavas of the Bahuichivo volcanics (Tbl) are interstratified with alluvial red bed sandstones (Tcs) identical to those throughout the basin described below (Figures 3 and 4A), and locally wet sediment-lava intermixing (peperite) is present ( Figure 6A). Also interbedded with the Bahuichivo volcanics in this area is the Irigoyen ignimbrite (Tciy), a light grey, crystal-poor, nonwelded silicic ignimbrite with faint compaction foliation of slightly flattened white to tan pumice fragments. Based on this evidence, the lavas of the Bahuichivo volcanics are considered part of the Cerocahui basin fill.
The Bahuichivo volcanics are inferred to have erupted from fault-controlled volcanic centres along the eastern half-graben basin margin. Subvolcanic intrusions occur in the Bahuichivo area, where dikes and intrusions emplaced along small-offset NW-trending structures in the fault-block between the Pañales and Bahuichivo-Bachamichi faults complexly crosscut related lava flows (Tbv), as well as sandstones and conglomerates inferred to be related to the basin fill (Tcc and Tcs, described below) and pre-basinal silicic outflow ignimbrites (Toi) described above (Figures 3, 6B, and 6C; Plate 1). A mafic-intermediate dike that parallels the Irigoyen fault on the western side of the basin (Figures 3 and 6D) may have been an additional vent for the volcanic rocks (Tbl) located in this area. The localization of these shallow intrusions on NWtrending structures that trend parallel to the basin-bounding normal faults suggests that these structures provided a conduit for mafic to intermediate magmatism, and that extensional deformation occurred prior to and during emplacement of the Bahuichivo volcanics.  (Table 1). This unit consists of volcaniclastic sandstones, conglomerates, and breccias, with interbedded non-welded silicic ignimbrites and fluvially reworked tuffs (Figures 5,7,and 8), deposited in angular unconformity over the mafic-intermediate lavas of the Bahuichivo volcanics (Figures 3 and 4). All of the deposits of this stratigraphic unit thicken and coarsen eastward toward the basin-bounding normal faults, with conglomerates and breccias (lithofacies Gm and Gc; Table 1) restricted to the area adjacent to the Bahuichivo-Bachamichi fault (Figures 3, 4, and 8A-8D). The bulk of the deposits in the Cerocahui clastic unit consist of medium to very thickly bedded, moderately to very poorly sorted, medium-to-very coarse-grained volcaniclastic sandstones and conglomeratic sandstones (lithofacies Sm; Table 1; Figures 5, 7, and 8B). The conglomeratic sandstones are composed of <30% gravel-sized (>2 mm, to 0.5 m diameter) subrounded to subangular clasts derived from amygdaloidal mafic-intermediate lavas, silicic flow-banded lavas, and silicic welded to non-welded ignimbrites. Intercalated with the conglomeratic sandstones on the eastern margin of the basin are medium to very thickly bedded matrix-supported granule to boulder (to 1 m diameter) angular-to-subrounded conglomerates and breccias (lithofacies Gc and Gm; Table 1), which have similar clast compositions to the conglomeratic sandstones (Figures 7  and 8B-8D). The rocks of this unit contain sedimentary structures indicative of fluvial deposition, including channels that indicate southwestward-directed palaeoflow, cutand-fill structures, trough and low-angle cross-stratification (lithofacies Sx; Table 1), and normal to inverse graded bedding (Figures 7, 8B, and 8D-8G).
Silicic non-welded ignimbrites and fluvially reworked tuff (tuffaceous sandstones and conglomerates, lithofacies Vr; Table 1) are interbedded within the Cerocahui clastic unit, with four distinct and informally named ignimbrites recognized: the El Rodeo, Cerro Colorado, El Volcán, and Cerocahui ignimbrites (Figures 3, 4, 7 Figure 8G   biotite, and hornblende, trace lithic fragments, and 25% yellow and salmon-coloured pumice fragments. The Cerro Colorado ignimbrite (Tcic) crops out in the low-lying areas near Cerocahui and caps Cerro Colorado to the south of the village (Figures 3, 8A, and 8H). The Cerro Colorado ignimbrite is at least 70 m thick (Figure 7), has a light tan groundmass near the base that transitions to light grey at the top, 5% phenocrysts of plagioclase (to 1.5 mm) and biotite (<1 mm), trace white long-tube pumice fragments (to 2 cm), and trace mafic-intermediate volcanic lithic fragments (to 5 mm). The base of the Cerro Colorado ignimbrite locally consists of a reworked pumice lapilli-tuff deposit (lithofacies Vr; Table 1) that infills a~4 m-deep channel cut into underlying sandstone ( Figure 8F-8G). The El Volcán ignimbrite (Tciv) forms part of a prominent~80 m cliff in the middle part of the ridge north of Cerocahui and extends westward for 6.5 km, pinching out north of El Rodeo (Figures 3, 7, 8A, and 8I; Plate 1). The western deposits of this ignimbrite located at Cerro El Volcán have limited reworking and no interbedded sedimentary deposits and consist of several thin (<5 m thick) non-welded primary outflow sheets with a tan to white groundmass, 10-20% phenocrysts (<1 mm) of plagioclase, biotite, and trace clinopyroxene, hornblende, and quartz, <5% lithic fragments (<1 mm), and 15-30% white to yellow long-tube pumice fragments (to 10 mm). At the location of the measured stratigraphic section northwest of Cerocahui, closer to the eastern basin margin, the El Volcán ignimbrite is predominantly fluvially reworked and interbedded with conglomerates and sandstones (Figure 7; 292-332 m above the section base). The Cerocahui ignimbrite (Tcih) is a~27 m-thick unit that crops out on the ridge north of Cerocahui (Figure 3, 7, and 8A). This ignimbrite is nonwelded with a white groundmass, 10-15% phenocrysts of plagioclase, biotite, quartz, and hornblende, 5-10% lithic fragments (~1 mm), and 15% yellow pumice fragments. The base and top of the Cerocahui ignimbrite consist of reworked lapilli-tuff (tuffaceous sandstone and conglomerate) that is similar in composition to the primary ignimbrite, but is more stratified and better sorted than the rest of the ignimbrite deposit (Figure 7).
Based on stratigraphic relations and sedimentary lithofacies (Table 1), the Cerocahui clastic unit likely represents deposition in alluvial fan systems (e.g . Miall 1985;Kelly and Olsen 1993;Blair and McPherson 1994;Collinson 1996;Murray et al. 2010). The interpretation of these sedimentary rocks as alluvial deposits is supported by the presence of clast-to-matrix-supported conglomerates, breccias (lithofacies Gc and Gm; Table 1), and conglomeratic sandstones (lithofacies Sm; Table 1) interpreted as sediment-gravity flow deposits, stratified to cross-stratified sandstones (lithofacies Sh and Sx; Table 1) interpreted as fluvial deposits, and deposits that infill channels cut into underlying strata (Figures 7 and  8B-8G). The silicic ignimbrites interstratified with the volcaniclastic rocks were likely erupted from distal sources and deposited in the basin, based on their nonwelded nature and high proportion of interstratified fluvially reworked tuff (Figure 7).
Deposition of the Cerocahui clastic unit likely occurred during extensional deformation of the eastern basin-bounding normal faults. Evidence of synextensional deposition includes the increased thickness and coarseness of the Cerocahui clastic unit toward the basin-bounding fault, with these deposits either ending at the Bahuichivo-Bachamichi fault or thinning onto the faultblock between the Bahuichivo-Bachamichi and Pañales faults (Figures 3 and 4; Plate 1). Evidence of possible growth strata is indicated by an upsection decrease in bedding dip from~18°E to 6°E observed within the Cerocahui clastic unit along the Irigoyen-Bahuichivo road, as well as an upsection dip decrease from 13°E to 5°N in the vicinity of the measured stratigraphic section near Cerocahui (Figures 3 and 4; Plate 1). The more northerly bedding dips in the Cerocahui area may represent the slightly tilted original orientation of the alluvial fan deposits, or they may reflect greater fault activity and subsidence in the northern part of the basin, either on the basin-bounding faults or on unmapped intra-basinal faults, which tilted the rocks in that direction. In addition, angular unconformities between and within the Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas (described below) are observed within the basin (Figures 3, 4, 8A, and 8I; Plate 1). These unconformities and upsection changes in bedding dip angle can be explained either by the crustal flexural subsidence related to sediment loading or by the syndepositional tilting related to normal fault motion on the eastern half-graben margin (i.e. growth strata). The latter explanation of synextensional deposition is preferred, given the limited, thinner exposures of Cerocahui basin deposits east of the Bahuichivo-Bachamichi fault and the sub-parallel NWalignment of Bahuichivo volcanic intrusions to the halfgraben bound fault system. Syndepositional extension of the El Rodeo fault within the basin is suggested by the restriction of the El Rodeo ignimbrite (Tcir) to the hanging wall of this fault, gently dipping (<5°E) Cerocahui clastic unit rocks deposited in angular unconformity above moderately tilted (15°E) Cerocahui clastic unit sandstone on the hanging wall of the fault, and greater vertical fault offset of the El Volcán ignimbrite (Tciv;~125 m-vertical offset) compared to the overlying basalt lavas (described below;~100 m-vertical offset) (Figures 4B and 8I; Plate 1). Additional syndepositional to postdepositional offset of the Bahuichivo-Bachamichi and El Rodeo faults resulted in the development of drag synclines on the hanging wall adjacent to these faults (Figures 3 and  4B; Plate 1).

Basalt lavas: uppermost Cerocahui basin fill
Conformably overlying the Cerocahui clastic unit is a flat-lying to gently dipping (<5°) basalt lava unit (Tm; Figures 3, 4, 7, and 8A; Plate 1). The basalt lavas are widespread and appear to cap all of the ridges within the study area (Figure 3; Plate 1). This unit is composed of several lavas that have flow-banded interiors and vesicular flow tops, with individual flows to~10 m thick (Figure 7). These basalt lavas are grey with a microlitic to glassy groundmass, and contain trace phenocrysts of plagioclase, olivine, and clinopyroxene. The entire stratigraphic unit has an estimated thickness of over 300 m, with the greatest thickness in the northwestern part of the basin (Figure 3; Plate 1). The vents for the basalt lava flows have not been identified within the study area.
The basalts appear to have been erupted just prior to the end of extensional deformation in the basin, because the stratigraphic unit is vertically offset across the El Rodeo fault and the Bahuichivo-Bachamichi fault north of Bahuichivo, and is not present on the footwall of the Pañales fault ( Figure 4A). The general flat-lying orientation of the basalt lavas and the slight amount of offset of this unit across the Bahuichivo-Bachamichi fault (<50 m vertical offset) suggest that extensional deformation in the basin was limited following eruption of this unit.

Silicic hypabyssal intrusions: post-basinal magmatism
The youngest lithologic unit in the study area is composed of silicic hypabyssal intrusions/plugs (Tri) that were emplaced following volcaniclastic and volcanic deposition in the Cerocahui basin (Figures 3, 5, 8A, and 9). These subvertically flow-banded intrusions are located along the southern mapped section of the basin-bounding Bahuichivo-Bachamichi fault ( Figure 9A) and within the Cerocahui clastic unit along the Cerocahui fault where this fault that cuts the Cerocahui clastic unit diverges into two branches in the village of Cerocahui (Figures 3 and 8A). Although there are no direct crosscutting relationships, the silicic hypabyssal intrusions are inferred to be younger than the basalt lavas; this relative age relationship is based on the undeformed nature of the silicic intrusions and that they are emplaced along the southern projection of the basin-bounding fault near Cerocahui, which offsets the basalt lavas to the north near Bahuichivo (Figure 3; Plate 1), as well as along faults within the basin that offset older deposits of the Cerocahui clastic unit. Increased tilting of the Cerocahui clastic unit occurred adjacent to the margins of the intrusions during emplacement (Plate 1). The silicic hypabyssal intrusion near Cerocahui has a perimeter of flow-banded blocks ( Figure 9B), suggesting brecciation during emplacement. The rocks of this intrusion are white, flow-banded, with 5-25% euhedral phenocrysts of plagioclase and biotite. Extrusive rocks associated with these silicic intrusions have not been identified in the Cerocahui basin area.

Depositional age constraints Methodology & age interpretations
New U-Pb zircon ages were obtained from two silicic ignimbrites within the Cerocahui basin, providing constraints on the age of these previously undated deposits. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb analyses were performed at the Laboratorio de Estudios Isotópicos, Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), on zircons separated from the two silicic ignimbrite samples ( Figure 10; Table 2), using the analytical methods and age calculations detailed in Murray et al. (2013). Concordia plots, probability density distribution  Table 2, with mean 206 Pb/ 238 U ages of the youngest zircon population (interpreted emplacement age) for each sample being listed. For each sample, probability density distribution plots with age calculations (left) using the deconvolution method in Isoplot 3.70 (Ludwig 2008) and Tera-Wasserburg concordia plots (right) are shown. MSWD, mean square of weighted deviates of initial zircon ages. Note that the vertical and horizontal scales of the probability density distribution plots differ between each sample. Details on the experiments are given in Supplemental Table 1. Notes: LA-ICP-MS, laser ablation inductively coupled plasma mass spectrometry. Ages in italics represent the inherited zircon age population in a given sample (crystals that predate crystallization and eruption of a host magma). The youngest age population of each sample is interpreted as the phenocryst crystallization age and the maximum eruption age. Details of each analysis are given in Supplemental Table 1. North American Datum 1927 (NAD27) datum is used for latitude and longitude. Map unit labels correspond to Figure 3. The relative stratigraphic position of ages is shown in Figure 5. Locations of the samples are shown in Figures 3 and 7 and Plate 1. n, number of zircons used for age calculation. *Mean 206 Pb/ 238 U age calculated using the deconvolution method in Isoplot 3.70 (Ludwig 2008). and histogram plots, mean age, and age-error calculations were performed using Isoplot v. 3.70 (Ludwig 2008).
In the Sierra Madre Occidental, which has a long-lived 15-20 million year history of continuous magmatism, it is common to observe mixed-age populations due to zircon inheritance signatures. This is problematic for dating the younger (early Miocene) rocks, which often contain 'antecrysts', zircons that formed during earlier phases of related magmatism within the igneous province but are not directly crystallized from the host magma (e.g. Ferrari et al. 2013;Murray et al. 2013). Where mixed-age populations are suggested (e.g. mean 206 Pb/ 238 U ages that yield MSWD [mean square of weighted deviates] values much greater than unity, probability density function curves that are positively skewed and asymmetric, and/or have broad, bimodal, or polymodal peaks), the deconvolution method, based on the mixture modelling method of Sambridge and Compston (1994), was implemented in Isoplot. In our analyses, we interpret the oldest zircon age population calculated using this method that is less than ca. 38 Ma in a sample represents the crystallization age of antecrysts incorporated into the host magma and that the youngest zircon age population indicates the age of phenocryst crystallization (after Ferrari et al. 2013), which we interpret as the maximum possible eruption age of the rock. Age results are presented in the following and summarized in Figure  10 and Table 2, with the locations of the samples shown in Figure 3 and Plate 1; detailed analytical data are given in Supplemental

Results
Sample BM080719-7 is from the Irigoyen ignimbrite (Tciy) at the northwestern basin margin near Irigoyen (Figures 3,  4A, and 5). This ignimbrite (described above) is interstratified with lavas of the Bahuichivo volcanics (Tbl) and the lowermost sandstones (Tcc) of the Cerocahui basin (Figures 3 & 4A; Plate 1). U-Pb data for this sample reveal the presence of several xenocrysts with Proterozoic (ca. 1.7 Ga; n = 1), Palaeozoic (ca. 481 and 318 Ma; n = 2), Late Cretaceous (ca. 104-75 Ma; n = 4), and early Eocene (ca. 48 Ma; n = 2) ages (Supplemental Table 1). From the analysis of five nonxenocrystic zircons ( Figure 10; Table 2; Supplemental Table  1), two main zircon age populations are recognized in sample BM080719-7 consisting of an older grouping having a mean age of 31.3 ± 2 Ma and a younger grouping with a mean age of 28.1 ± 0.8 Ma ( Figure 10; Table 2). The zircons of the older age population are likely antecrysts, while the zircons from the younger age population are interpreted as phenocrysts. The phenocryst age overlaps within uncertainty with ages of the pre-basinal silicic outflow ignimbrites to the west (e.g. 27.6 ± 0.3 Ma Puerto Blanco ignimbrite of the Parajes formation; Murray et al. 2013). Given the stratigraphic constraints and the large age uncertainties with sample BM080719-7 due to the limited number of analysed nonxenocrystic zircons, the Irigoyen ignimbrite (Tciy) is likely equivalent-age or slightly younger than the underlying Puerto Blanco ignimbrite, with an eruption age (based on uncertainties) between ca. 27.9 and 27.3 Ma. This age suggests that initial eruption of the Bahuichivo volcanics within the Cerocahui basin occurred ca. 27.5 Ma.
Sample BM080718-1 is from the base of the Cerro Colorado ignimbrite (Tcic; described above) near Cerocahui, which is interbedded in the lower section of the Cerocahui clastic unit stratigraphically above the Bahuichivo volcanics (Figures 3, 4B, 5, and 7). Unlike the previous sample, xenocrysts were not found in this sample. From the analysis of 22 zircons (Table 2; Supplemental Table 1), two age populations are recognized in sample BM080718-1, consisting of an older group with a mean age of 28.2 ± 0.4 Ma and a younger group that has a mean age of 26.0 ± 0.3 Ma ( Figure 10; Table 2). Similar to the sample above, the older zircon age population is likely antecrystic, and the population of younger zircons is interpreted as phenocrysts. This phenocryst age overlaps within uncertainty with the age of the Témoris Formation in the Guazapares Mining District region, which is bracketed at ca. 27-24.5 Ma by U-Pb zircon ages of the underlying and overlying formations (Parajes and Sierra Guazapares formations, respectively) and interbedded silicic ignimbrites (Murray et al. 2013). In addition, this data provides a minimum age for the eruption of the underlying Bahuichivo volcanics at ca. 26 Ma.

Cerocahui basin evolution
The new geologic mapping, stratigraphy, and geochronology presented in this study show that the rocks of the Cerocahui basin region record late Oligocene (ca. 27.5 Ma to likely older than 24.5 Ma) synextensional volcanism and volcaniclastic alluvial deposition during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental. The developmental history of the Cerocahui basin includes ( Figure 11): (1) deposition of welded silicic outflow ignimbrite sheets; (2) synextensional magmatism and deposition of the Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas in the Cerocahui basin during a lull in silicic ignimbrite flare-up volcanism; and (3) emplacement of silicic hypabyssal intrusions along pre-existing extensional faults in the Cerocahui basin.
The silicic outflow ignimbrite sheets that underlie the Cerocahui basin are similar to late Oligocene outflow ignimbrite sheets in adjacent regions of the northern Sierra Madre Occidental, which erupted during the end of the early Oligocene pulse of the ignimbrite flare-up (Swanson et al. 2006;Murray et al. 2013). Similar to the ignimbrites of the Parajes formation in the Guazapares Mining District region (Murray et al. 2013), the degree of welding and flow thicknesses of the pre-basinal ignimbrites suggest that these rocks also possibly erupted from calderas within 50-100 km of the Cerocahui basin region that temporally overlap with the end of late Oligocene ignimbrite flare-up volcanism to the east, although more geochronologic data are needed to confirm this interpretation. There is no direct evidence of extensional deformation in the region of the Cerocahui basin during deposition of the silicic outflow ignimbrite sheets ( Figure 11A), such as occurred during deposition of the upper (post-ca. 27.5 Ma) part of the ignimbrite section in the Guazapares Mining District region (Murray et al. 2013). However, given that the oldest age within the Cerocahui basin is from a thin non-welded ignimbrite interbedded with the Bahuichivo volcanics that overlie the Parajes formation near the Piedra Bola fault (28.1 ± 0.8 Ma), and this is the same age (within uncertainty) of the timing of the onset of extension to the west, extension in the Cerocahui basin region may have also begun during deposition of the youngest silicic outflow ignimbrites.  Depositional relationships, growth strata, and subvolcanic intrusions that are likely fault-localized suggest that the Bahuichivo volcanics and Cerocahui clastic unit represent the synextensional growth of mafic-intermediate volcanic centres and volcaniclastic alluvial deposition in the Cerocahui basin during the late Oligocene ( Figure 11B-11E). The alluvial fan deposits of the Cerocahui clastic unit likely formed a bajada along the eastern margin of the Cerocahui basin adjacent to the basin-bounding fault and prograded into the subsiding half-graben from the east and accumulated over the Bahuichivo volcanics ( Figure 11C-11D).
The stratigraphic trend in conglomerate-breccia clast compositions in the Cerocahui clastic unit shows an upsection decrease in mafic-intermediate volcanic fragments and an upsection increase in welded and non-welded ignimbrite clasts, with fragments of silicic lava restricted to the lowest rocks of the section (Figure 7). The flow-banded silicic lava clasts suggest erosion of silicic volcanoes or plugs in the vicinity of the Cerocahui basin, as maficintermediate volcanic fragments are intermingled with the silicic lava clasts in the alluvial deposits; further study is needed to determine the source of these silicic lava clasts and its relative timing to the eruption of the Bahuichivo volcanics. The upsection trends in clast composition appear to record inverse stratigraphy related to unroofing of the active half-graben footwall block (Figures 7 and 11C-11D), with erosion of the Bahuichivo volcanics ( Figure 11C) followed by erosion of the silicic outflow ignimbrite sheets ( Figure 11D). The rocks on the footwall of the Bahuichivo-Bachamichi fault consist of silicic outflow ignimbrites, the Bahuichivo volcanics, and limited conglomerate and ignimbrite deposits of the Cerocahui clastic unit, whereas rocks on the footwall of the Pañales fault to the east are restricted to prebasinal silicic outflow ignimbrites; sedimentary and volcanic deposits related to the Cerocahui basin strata described above are not identified immediately east of this fault ( Figure 3; Plate 1). This absence of Cerocahui basin fill supports the interpretation that extensional footwall uplift led to erosion of the Bahuichivo volcanics first, and then the underlying silicic outflow ignimbrites, with their erosional products deposited in the adjacent half-graben basin to the west ( Figure 11C-11D).
Following deposition of the Cerocahui clastic unit, basalt lavas were erupted and ponded within the Cerocahui basin ( Figure 11E). As noted above, these lavas are offset by the basin-bounding fault system, as well as by normal faults within the basin (Figures 3 and  4; Plate 1), suggesting synextensional volcanism. Younger silicic hypabyssal intrusions intruded along normal faults in the basin, suggesting that these pre-existing structures were utilized as pathways for magma accent and emplacement ( Figure 11E).

Regional correlations
Based on similar lithology, timing of synextensional deposition, and proximity, the three stratigraphic subdivisions within the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas) are broadly correlative with the ca. 27-24.5 Ma Témoris formation in the Guazapares Mining District region (e.g. Murray et al. 2013) (Figure 12). Similar to the stratigraphy of the Cerocahui basin, the Témoris formation to the west is dominated by synextensional mafic-intermediate volcanic rocks and fault-localized intrusive equivalents, volcaniclastic alluvial fan deposits, and an upper section of interbedded alluvial deposits and distal silicic ignimbrites deposited above these mafic-intermediate lavas.
However, there are much greater proportions of sandstones, conglomerates, and breccias in the Cerocahui basin than there are in the Témoris formation ( Figure 12). In addition, the basalt lavas that cap the Cerocahui clastic unit are not present in the upper part of the Témoris formation to the west, although there basalt lavas are interbedded within the lower part of the formation (Murray et al. 2013) (Figure 12).
The sizes of half-graben basins in the Cerocahui and Guazapares Mining District regions also differ. Normal faults are more diffuse in the Guazapares Mining District region, with several closely spaced half-graben basins that are generally smaller (~1 to 4 km wide, 100 to >600 m deep) than the~12 km-wide, >1200 m-deep Cerocahui basin ( Figure 2). Perhaps this half-graben size difference is related to the geographic position of the Cerocahui basin immediately adjacent to the region defined as the unextended core of the Sierra Madre Occidental to the east, with the Cerocahui basin and Guazapares Mining District regions likely representing the eastern limit of the Gulf Extensional Province. The structures of the Cerocahui basin and the Guazapares Mining District region may represent the transition at the edge of the Gulf Extensional Province from the unextended core to the east, into the region of highly extended core-complexes to the west in Sonora.
The silicic hypabyssal intrusions in the Cerocahui basin are not dated directly, but they are tentatively correlated with the ca. 24.5-23 Ma Sierra Guazapares formation of the Guazapares Mining District region (e.g. Murray et al. 2013), which records the onset of local silicic flareup-related magmatism~20 km to the west during the onset of the early Miocene pulse of the ignimbrite flare-up. The Sierra Guazapares formation includes fault-localized fissure magmatism with silicic hypabyssal intrusions emplaced along pre-existing faults (Murray et al. 2013), similar to the fault-controlled silicic intrusions in the Cerocahui basin ( Figure 11E). However, as noted above, in the Cerocahui basin, these intrusions do not pass upward into ignimbrites or lavas as they do in the Sierra Guazapares formation; it is not known whether this is an artefact of preservation (i.e. the top of the section is eroded), or whether silicic volcanism was minimal in the Cerocahui region.
The late Oligocene timing of volcanism and synextensional deposition in the Cerocahui basin is generally consistent with regional data patterns suggesting a post-ca. 40 Ma southwestward migration of arc-front magmatism across the Sierra Madre Occidental (e.g. Damon et al. 1981;Gans 1997;Gans et al. 2003;Ferrari et al. 2007;Henry et al. 2010). The ca. 27.5-26 Ma Bahuichivo volcanics post-date late Eocene to early Oligocene volcanism to the northeast of the study area, and are older than coeval with late Oligocene to early Miocene volcanism to the west in Sonora (Ferrari et al. 2007;Murray et al. 2013 and references therein). The late Oligocene age of extension of the Cerocahui basin and Guazapares Mining District region is roughly coeval with the onset of extension in the end Oligoceneearly Miocene fault-bound grabens and core complexes located farther west in Sonora, although extension in this study area ended earlier than it did in the west (Gans 1997;McDowell et al. 1997;Wong et al. 2010;Murray et al. 2013 and references therein).

Conclusions
The rocks in the Cerocahui basin and adjacent Guazapares Mining District region record late Oligocene to early Miocene magmatism and synextensional deposition in the northern Sierra Madre Occidental. The oldest rocks in this region are silicic outflow ignimbrite sheets that erupted during the end of the early Oligocene pulse of the ignimbrite flare-up from sources likely to the east, representing medial outflow facies that were mostly deposited prior to development of the Cerocahui half-graben basin. These ignimbrites are likely correlative with the ca. 27.5 Ma Parajes formation immediately to the west in the Guazapares Mining District region, which suggests synextensional deposition of the youngest ignimbrites of the formation, and to ignimbrite sections described to the east by Swanson et al. (2006). The overlying synextensional deposits of the Cerocahui basin include: (1) the basal basin fill, consisting of the ca. 27.5-26 Ma Bahuichivo volcanics, mafic-intermediate lavas erupted from fault-localized synextensional volcanic centres primarily on the eastern half-graben margin; (2) the Cerocahui clastic unit, consisting largely of a bajada of alluvial fan-fluvial systems with minor interbedded distal Sierra Guazapares formation: vent facies: rhyolite lavas and plugs, rhyolitic cross-bedded ignimbrites, co-ignimbrite lag breccias, dome collapse breccia  ignimbrites that prograded into the half-graben basin from the active eastern fault margin, and (3) a >300 m-thick section of basalt lavas ponded within, and restricted to, the Cerocahui basin. The mafic-intermediate volcanic and alluvial deposits of the Cerocahui basin are likely equivalent to the ca. 27-24.5 Ma Témoris formation in the Guazapares Mining District region and represent a period of the Southern Cordillera basaltic andesite (SCORBA) magmatism erupted after the early Oligocene ignimbrite pulse. Following deposition in the Cerocahui basin, silicic hypabyssal intrusions were emplaced along normal faults in the Cerocahui basin. These silicic intrusions are likely related to the ca. 24.5-23 Ma Sierra Guazapares formation in the Guazapares Mining District region, which were emplaced during the initiation of the early Miocene pulse of the ignimbrite flare-up. The late Oligocene to early Miocene timing of magmatism and synextensional deposition in the Cerocahui basin and Guazapares Mining District regions generally supports the regional interpretation that ignimbrite flare-up magmatism and crustal extension migrated southwestward with time.