In the western Bisbee Basin of southern Arizona, detailed mapping and sequence analysis of the Glance Conglomerate along the largest basin-bounding fault, the Sawmill Canyon fault zone, reveals interbedded clastic, volcanic, and volcaniclastic lithofacies and their relationship to intrabasinal faulting, unconformities, and basin-bounding faults. The basin fill is dominated by small polygenetic, multivent volcanic complexes ranging in composition from rhyolite to andesite typical of continental arc volcanism. Syndepositional basin-bounding faults, the Sawmill Canyon and Gringo Gulch fault zones, controlled subsidence within the basin and plumbed small batches of magma to the surface. Small intrabasinal faults show stratigraphically limited offsets that alternate between normal and reverse separation. Eight unconformable surfaces occur within the basin. Five are asymmetrical, with one very steep wall and one gradually sloping wall. They show extreme vertical relief (460–910 m) with very high paleoslope gradients (40°–71°) that dip away from the master fault. We interpret these as uplifted fault scarps or paleoslide scars. The other three unconformities are symmetrical, V-shaped surfaces that have less steep walls, with vertical relief of 200–600 m and paleoslope gradients of 20°–25°. We interpret the symmetrical surfaces to be walls of deep paleocanyons cut during basin uplift events or following large ignimbrite eruptions. Analysis of the unconformably bound stratigraphic sequences shows deposition to be related to subsidence along large basin-bounding faults modified by intrabasinal, high-angle, syndepositional normal and reverse faults. Erosion of the sequence-bounding unconformities took place during uplift associated with basin inversion. Alternation of uplift and subsidence and the juxtaposition of intrabasinal reverse and normal faults is typical of strike-slip basins. We interpret the Glance Conglomerate in the Santa Rita Mountains as the fill of an intra-arc strike-slip basin where strike-slip deformation was concentrated along the thermally weakened arc axis. We suggest a model for the Bisbee Basin of a strain-partitioned, obliquely convergent continental arc with backarc extension-transtension.

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacific–North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacific margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fluvial, debris-flow, and lacustrine sediments. “Dry” eruptions of welded ignimbrite were replaced by “wet” eruptions of nonwelded, easily reworked ignimbrite and phreatoplinian fall. This Late Jurassic transition from hyperarid to more temperate climatic conditions may thus form a superregional “time line” that ties the Cordilleran plate margin to events in the interior of the continent.

Abstract Paleomagnetic data have been obtained from heterogeneous, shallow-water, miogeoclinal carbonate rocks of the Pogonip Group (Early Ordovician) in the Desert Range of southern Nevada, the Egan Range of east-central Nevada, and the southern House Range of western Utah. These rocks locally contain abundant replacive chert that preserves relict textures from the host limestones as well as clearly detrital grains (e.g., blue-luminescing feldspars). Stylolites are abundant and are interpreted as late diagenetic features, as they cut late cements and truncate bedding lamination. Differential compaction along stylolites wrapping around the chert masses has resulted in macroscopic deformation, as evidenced by tilting of bedding of over 25° about chert masses in some cases. We have used the differential compaction fabrics in these rocks to test for the age of acquisition of a generally well-grouped and well-defined characteristic magnetization. All three carbonate sections give a low-inclination, southerly to southeasterly magnetization residing in magnetite (e.g., Decl. = 152°, Incl. = -21°, α 95 = 3°, kl = -61, k2 = -21, N = 48 independent samples, site 12; Pogonip Group, Sawmill Canyon, Egan Range). The magnetization is interpreted to be secondary and acquired after local compaction because directions of magnetizations from different samples are not dispersed by the compaction deformation. The uniform reversed polarity in addition to the direction of the magnetization, moreover, is interpreted to suggest a late Paleozoic age of remagnetization. In the Desert Range, the remagnetization had been previously attributed to a viscous partial thermoremanent magnetization (VPTRM) from deep burial. Based on several observations, we now argue for a chemical origin from late diagenetic magnetite, such as is now well-documented in the Appalachians and mid-continent. The cherts are almost nonmagnetic, as would be expected from their impermeability if the magnetite were precipitated from late fluids. Abundant authigenic alkali feldspar in the Desert Range is also consistent with late metasomatism. Finally, in the Egan Range, the remagnetization extends through a section exceeding 3 km in thickness, into rocks as young as Mississippian, which were never buried as deeply and thus not heated to the same degree as lower Paleozoic strata. These results underscore the utility of integrating observations based on paleomagnetic data with carbonate textures. “Micro”-field tests can constrain both the timing of magnetization acquisition and of diagenetic events. The micro-fold tests discussed apply to features that are not formed by tectonic deformation. The availability of field tests from early formed textures in sedimentary rocks is especially important given the recent recognition of widespread remagnetization in ancient rocks.