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 Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.