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 Offshore CO 2 sequestration in basaltic formations of the North Atlantic Igneous Province may allow permanent storage of large volumes of CO 2 through rapid carbonate mineralization. Characterizing the internal architecture of such reservoirs is key to assessing the storage potential. In this study, six photogrammetry models and three boreholes on the Faroe Islands have been used to characterize the internal lava sequence architectures as a direct analogue to potential offshore North Atlantic Igneous Province storage sites. The studied formations are dominated by c. 5 to 50 m thick simple and compound lava flows, with drill core observations documenting a transition from pāhoehoe moving towards ‘a’ā lava flow types interbedded with thin (<5 m thick) volcaniclastic rock units. The identification of flow margin breccias is potentially important as these units form excellent reservoirs in several other localities globally. Stacked, thick simple flows may present sealing units associated with dense flow interiors. Connected porous and permeable lava flow crusts present potential reservoirs; however, the degree of secondary mineralization and alteration can alter initially good reservoir units to impermeable barriers for fluid flow. Large-scale reservoir volumes may be present mainly within both vesicular, fractured pāhoehoe and brecciated flow margins of transitional simple lava flows.

Abstract Volcaniclastic rocks are commonly overlooked as reservoirs or seals in hydrocarbon plays because their compositions are variably unstable and reactive during burial diagenesis. This study investigated the petrography and petrophysical characteristics of 60 volcaniclastic and four siliciclastic samples from three Paleogene volcanic provinces – East Greenland, Faroe Islands and Ethiopia. The volcaniclastic samples have highly variable helium porosities (average 25.2%), but negligible total optical porosities (average 1.9%), implying reduced reservoir potential. The samples have, however, highly variable air permeabilities (average 11 mD), suggesting that they could make tight reservoirs. The permeabilities are related to either early calcite cements or the devitrification of volcanic glass. Mercury injection capillary pressure data were collected for a subset of 33 samples that at leakage/breakthrough saturations could, under near-surface conditions, hold oil column heights of between 4 and 1181 m (average 240 m). The best seals consistently have zeolite contents of >20 vol% owing to their small pore throat radii. Conversely, the worst seals are dominated by smectite and a conspicuous absence of zeolite minerals. The zeolite-rich volcaniclastic rocks could, therefore, make good shallow seals. These features also apply to CO 2 storage, but questions remain about the reactivity of the volcanic material and secondary minerals with injected CO 2 , but also the adsorbent properties of zeolites, particularly clinoptilolite, in the presence of CO 2 .

Abstract The Beishan Orogen contains the youngest suture in the Central Asian Orogenic Belt and is vital to understanding its terminal collision history and closure of the last vestiges of the Palaeo-Asian Ocean. Multidisciplinary evidence is provided for a close temporal and spatial linkage between the Liuyuan Complex back-arc ophiolite and the Ganquan Complex arc rocks. The Ganquan Complex mainly comprises felsic volcanic and volcaniclastic rocks and is divided into a lower and upper sequence, both of which exhibit arc-like geochemical signatures and have magmatic zircons characterized by mantle-like δ 18 O values. Their U–Pb zircon ages range between 295 and 286 Ma and between 283 and 281 Ma, with corresponding ε Hf values varying from −4.2 to +1.9 and from +1.3 to +7.6, respectively, indicating a more juvenile magma source over time. The increase of juvenile isotopic signatures in the Ganquan Complex rocks is attributed to arc-trench migration, which progressively removed the arc from influences of an old arc basement contaminant and led to back-arc spreading and formation of the Liuyuan ophiolite. Cessation of the multi-phase Ganquan arc system at c. 280 Ma coincided with the accretion of the continental Baidunzi Complex in the south.