The timing and spatial extent of mid-Cenozoic ignimbrite fl are-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene fl are-up vol canism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identifi ed three informal syn extensional formations. The ca. 27.5 Ma Parajes formation is an ~1-km-thick succession composed primarily of welded to nonwelded silicic outfl ow ignimbrite sheets erupted from distant sources. The 27-24.5 Ma Témoris formation is interpreted as an ande sitic volcanic center composed of locally erupted mafi c to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fl uvial and debris fl ow deposits, and an upper section of thin distal silicic outfl ow ignimbrites. The 24.5-23 Ma Sierra Guaza pares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fl uvially or debris fl ow-reworked equivalents. These three formations record (1) the accumulation of outfl ow ignimbrite sheets, presumably erupted from cal deras mapped ~50-100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite fl are-up; (2) development of an andesitic volcanic fi eld in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fi ssure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite fl are-up. The main geologic structures identifi ed in the Guazapares Mining District region are NNW-trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Témoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Témoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations. We interpret extensional faulting and magmatism i...

The upper Oligocene to lower Miocene Peñas and Aranjuez formations are exposed in north-northwest-trending outcrop belts of the central Andean backthrust belt situated within the central Andean plateau along the boundary between the northern Altiplano and the Eastern Cordillera of Bolivia. Sedimentary lithofacies analyses indicate that these coarse-grained siliciclastic formations were deposited primarily in alluvial fan to braided fl uvial environments. An upsection change from principally fi ne-grained sandstone to cobble conglomerate is consistent with increased proximity to the sediment source with time. Paleocurrent analyses reveal that fl ow was predominantly directed toward the west-southwest away from Cordillera Real, the elevated core of the Eastern Cordillera. Provenance data from conglomerate clast compositions and sandstone petrofacies suggest derivation from recycled quartz-rich metasedimentary and sedimentary rocks from the Paleozoic section in the Eastern Cordillera. The paleofl ow orientations, sediment provenance, and increased proximity of the sediment source suggest that deposition of the Peñas and Aranjuez formations was related to surface uplift of the Eastern Cordillera relative to the Altiplano. Growth strata observed in the Aranjuez Formation further indicate that shortening was synchronous with deposition, probably in a hinterland basin. New 40 Ar/ 39 Ar ages from a lowermost exposed igneous unit and interbedded ash-fall tuff beds in the Aranjuez and Peñas formations show that synorogenic sedimentation and fold-thrust deformation in the frontal (west-southwestern) zone of the central Andean backthrust belt was concentrated during late Oligocene-early Miocene time. These age results are consistent with previous studies of east-derived sedimentation in the Altiplano and indicate regional uplift of the Eastern Cordillera at this time. Upsection trends in provenance data further suggest a progressively greater contribution from younger Paleozoic strata, possibly due to activation of new thrust faults during west-southwestward propagation of the backthrust belt toward the Altiplano. Such a progression of late Oligocene-early Miocene shortening along the Altiplano-Eastern Cordillera boundary likely refl ects signifi cant crustal thickening, potential isostatic uplift, and initial topographic expression of the eastern margin of the central Andean plateau.

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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 basinbounding 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.

Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the Sierra Madre Occidental (SMO) silicic large igneous province are probes of petrogenetic processes in the lower and upper crust. Existing petrogenetic and tectonomagmatic models diverge, having either emphasized significant crustal reworking of hydrated continental lithosphere in an arc above the retreating Farallon slab or significant input of juvenile mantle melts through a slab window into an actively stretching continental lithosphere. New isotopic data are remarkably uniform within and between erupted units across the spatial and temporal extent of the SMO, consistent with homogeneous melt production and evolution. Isotopic values are consistent with enriched mantle magmas (80%) that assimilated Proterozoic paragneisses (~20%) from the lower crust. 18Ozircon values are consistent with fractionation of mafic magma and not with assimilation of hydrothermally altered upper crust, suggesting that the silicic magmas evolved at depth. Isotopic data agree with previous interpretations where voluminous juvenile melts entered the lithosphere during the transition from a continental arc experiencing slab rollback (Late Eocene) to the arrival of a subducting slab window (Oligocene and Early Miocene) and failure of the upper plate leading to the opening of the Gulf of California (Late Miocene). An anomalously large heat flux and extension of the upper plate allow for the sustained fractionation of the voluminous SMO magmas and assimilation of the lower crust.