ABSTRACT The volcanic stratigraphy of the central Gulf of California margin of the Baja California peninsula preserves a valuable record of the transition from subduction of the Farallon plate (24–12 Ma) to oblique rifting (<12 Ma). Although strike-slip faults (as well as normal faults) are common in oblique rifts and are abundant on the new (younger than 6 Ma) seafloor in the Gulf of California, none has been previously reported in the onshore central Baja California margin. This study focused on a previously unmapped region in the central Baja California margin near Mulegé, where we identified a strike-slip fault, termed the Potrero fault, and described the regional magmatic and structural context for this fault. We did this by using geologic mapping of volcanic-volcaniclastic lithofacies, supported by petrography, geochemistry, and 40Ar/39Ar geochronology. The Potrero fault is a vertical fault that strikes N10°W, with dextral-oblique (down-to-the-east) slip. This fault juxtaposes older rocks on the west with younger rocks on the east. The older rocks on the west side of the Potrero fault are assigned to the Middle Comondú Group, which is early Miocene in age. They consist largely of a >800 m red bed sequence of coarse-grained andesitic volcanic debris-flow deposits (proximal facies) that transition westward into fluvial conglomerates and sandstones (distal facies). The proximal facies has interstratified coarse-grained trachyandesite block-and-ash-flow tuffs with a 40Ar/39Ar age of 18.72 ± 0.24 Ma. This section is cut by mafic- to intermediate-composition dikes, with lesser plugs, that have 40Ar/39Ar ages of 16.88 ± 0.30 Ma to 14.85 ± 0.05 Ma. This early Miocene assemblage is truncated by an angular unconformity and overlain by Pliocene high-Sr/Y trachyandesite lavas, with an 40Ar/39Ar age of 4.02 ± 0.04 Ma. The younger rocks on the east side of the Potrero fault are assigned to the Upper Comondú Group, which is middle to late Miocene in age. This unit is dominated by small lava shields, with diameters of 2–9 km and thicknesses up to 300 m. The lava shields have basaltic andesite, basaltic trachyandesite, andesite, high-Sr/Y trachyandesite, and dacite compositions, with 40Ar/39Ar ages of 13.39 ± 0.03 Ma to 10.74 ± 0.08 Ma (four samples). At two localities in the map area, the Upper Comondú Group lava shields rest in angular unconformity on the Middle Comondú Group red beds and dikes, and the eruptive equivalents of the dikes are missing along this unconformity. We correlated this unconformity with the unconformity at the top of the Middle Comondú Group on the west side of the Potrero fault to estimate a vertical component of slip of at least 800 m down-to-the-east across the Potrero fault. The lateral component of slip is not known, because the regions to the north and south are unmapped, so piercing points cannot be identified. The Middle Comondú Group in the Mulegé–La Trinidad area forms part of a regionally extensive, early Miocene lithostratigraphic unit, hundreds of meters thick, that outcrops for a distance of 500 km along the central to southern Gulf of California margin. It thickens and coarsens eastward, through what is now the Concepción Peninsula, where it also contains early Miocene dikes and hypabyssal intrusions and is similarly capped by an angular unconformity, with eruptive equivalents of the dikes and intrusions missing along the unconformity. We propose that the laterally extensive Middle Comondú Group was deposited in a rift basin, bounded by a west-dipping normal fault system that lay to the east of what is now the Concepción Peninsula, in the present-day offshore Gulf of California. We infer that the thick, coarse-grained volcanic andesitic debris-flow deposits of the Middle Comondú Group were shed from large andesite arc stratovolcanoes (Comondú arc) that also lay to the east in the present-day Gulf of California. We interpret the volumetrically minor block-and-ash-flow tuffs, dikes, and hypabyssal intrusions of the Middle Comondú Group to record minor magmatism in a forearc position. We also suggest that the angular unconformity at the top of the Middle Comondú Group records thermal uplift that occurred as the arc axis swept westward (trenchward) into the region, at ca. 14 Ma, due to continued slab rollback that began in the Oligocene under western Mexico. The Upper Comondú Group lavas in the Mulegé–La Trinidad area form part of a Middle to Upper Miocene lithostratigraphic unit, hundreds of meters thick, which outcrops for a distance of 700 km along the central to southern Gulf of California margin. This unit consists largely of andesite and basaltic andesite lavas erupted from stratovolcanoes in the axis of the Comondú arc. The Upper Comondú Group lavas thicken dramatically eastward toward the Bahía Concepción fault, a down-to-the-west normal fault that bounds the Concepción Peninsula on its west side. We thus infer that this fault became active in middle Miocene time (14 Ma). This fault records westward encroachment of normal faulting concurrent with the westward sweep of the arc axis, from the Gulf of California into Baja California. The Pliocene high-Sr/Y basaltic trachyandesite lavas that form the structurally highest part of the Mulegé–La Trinidad area form an erosional escarpment that does not extend to the Potrero fault, so the lavas cannot be used to determine whether the Potrero fault was active before, during, or after their eruption. The Pliocene high-Sr/Y basaltic trachyandesite lavas are a previously unidentified part of the regional postsubduction suite of “bajaites,” attributed to slab window magmatism.

Abstract Extensional tectonics in the Gulf of California region occurred during middle- to early-late Miocene time. One of the best examples is found at Bahía Concepciόn, which is the largest fault-bound bay on the peninsular gulf coast of Baja California. The extensional basin confined to Bahía Concepciόn developed within an accommodation zone related to the late Miocene east-west extension in the broader Gulf of California region. This contribution is focused on the large, central part of the gulf region along the peninsular axis, sometimes referred to as the Baja California Central Domain ( BCCD ). The mainly volcanic Comondú Group is extensively exposed throughout the BCCD , but on a local basis it occurs as huge andesite blocks associated with stratified units that characteristically dip in opposite directions. This is a direct result of a main east-west extensional episode that generated normal faults on the surface and listric detachment faults at depth. Half grabens are the most common result of such extensional episodes in the BCCD . This event has produced subsidence, creating depocenters for mostly nearshore marine basins. The oldest marine sedimentary units present in the BCCD are the late Miocene–early Pliocene Tirabuzón Formation. Less typically, on Peninsula Concepciόn and at Punta San Antonio, a late Miocene extensional episode has resulted in up-thrown granodiorite basement along bounding faults. Extension on the Bahía Concepciόn zone was responsible for development of a half-graben structure first fully flooded in late Pliocene time. The tectono-sedimentary evolution of the adjacent Santa Rosalía and Bahía Concepción areas is recorded by three stratigraphic stages: (1) pre-rift strata represented by the Comondú Group; (2) a syn-rift stage containing syntectonic siliciclastic deposits; and (3) post-rift strata, represented by the late Miocene to Pliocene flat-lying to low-angle marine sedimentary units. The three stratigraphic stages are interpreted from outcrops in this region.

Abstract Geodetic baseline measurements using the Global Positioning System (GPS) were acquired in November 1985 across the Gulf of California between Loreto (Bahia Concepción) and Cabo San Lucas in Baja California and Mazatlan on the mainland of Mexico. This experiment had two main objectives. The first was initiation of a research effort, hopefully spanning a decade or more, aimed at determining spreading rates in the Gulf and, with the eventual expansion of the network to northern Baja California, understanding the complex pattern of strain distribution along this part of the Pacific-North American plate boundary. The second objective, the focus of this paper, was assessment of deployment strategies, analytical techniques, and the inherent precision of GPS-based geodetic measurements in the region. We review several approaches for calibration of path delays due to variable tropospheric water vapor content, and investigate the effect of varying the number and geometry of fixed GPS tracking sites (the fiducial network). Day-to-day repeatability using optimum tropospheric calibration methods and four fiducial stations in the U.S. is better than 2.0 cm in horizontal baseline components for baselines up to 650 km in length spanning the Gulf of California. In the absence of any slowly varying systematic error sources, this performance should permit detection of spreading across the southern Gulf of California in less than 5 years, and estimation of spreading rates to better than 0.3 cm/yr in less than 10 years, assuming short-term (geodetic) and long-term (geologic) rates are comparable. Similar precision should be achievable throughout the region as the network is expanded.