ABSTRACT Intra-oceanic magmatic arcs are essentially submarine systems, so their subaqueous formation processes and evolution history are inherently enigmatic because their products are not easily accessed and require marine sampling, usually via dredging and scientific ocean drilling. Ocean Drilling Program (ODP) holes cored in the Izu-Bonin forearc (Leg 126) and International Ocean Discovery Program Site U1438 drilled in the rear-arc region, west of the Kyushu-Palau Ridge, allow comparison of lithostratigraphy, lithofacies, sediment compositions, and sediment accumulation patterns across a segment of the Izu-Bonin arc system. A short period of arc extension (ca. 32–31 Ma) created a forearc basin that was subsequently filled by volcaniclastic sediments starting at ca. 31–30 Ma. This was followed by arc rifting (27–26 Ma), leading to creation of the Shikoku back-arc basin, isolation of the Kyushu-Palau Ridge, and reestablishment of the arc axis. West of the Kyushu-Palau Ridge, Site U1438 shows high sediment accumulation rates with a slight pause during the forearc extension event. Rates ultimately waned (after 27–26 Ma) after a new phase of extension proceeded to rifting and ultimately seafloor spreading and isolation of the Kyushu-Palau Ridge. Similar lithofacies are present in both the forearc and rear-arc successions but in different proportions, with gravel-rich breccia-conglomerate units dominating the rear-arc at a time when gravel-bearing sandy units were deposited in the forearc, most linked to gravity-mass flows sourced to the north. Sandy volcaniclastic intervals deposited ca. 30–29 Ma have similar compositional modes across the arc: intermediate to mafic, brown to tachylytic glassy fragments with mainly microlitic and lathwork textures. With arc extension and rifting, these became more vitric-rich and felsic to intermediate in composition in the rear-arc, and locally in the forearc (ODP Site 787 only). More such forearc–rear-arc stratigraphic comparisons are needed to determine if any unique characteristics of forearc versus rear-arc successions would allow them to be discriminated in the rock record.

Abstract Sand and sandstone composition of volcanic origin may be clues to the provenance of the sediments and sedimentary rocks. Volcaniclastic provenance studies contribute significantly to unravelling the generation and provenance of sediment under investigation, which in the Aeolian archipelago comprises preserved units of outcrops dominated by lava flows intercalated with airfall tephras as source rocks. The focus of this paper is the study of the petrographical composition and textures of beach sands, which can then be used as a guide in the interpretation of provenance and origin of beach sand(stone)s rich in volcanic debris transported into deeper water. The composition of Aeolian beach deposits defines a single immature petrofacies with a large amount of unweathered glass and mafic minerals. Panarea island is dominated by dacites and new grain categories have been proposed to differentiate this provenance. Surface processes such as mechanical erosion (mass wasting and surface runoff) produce an overestimation of mafic components compared to the felsic components in the beach sand fraction.

Abstract The Continental Borderland adjacent to southern California and northern Baja California is an exceptionally wide (240 km) region of ridges, islands, and bathymetric basins as deep as 2 km. This continental margin area includes the Nicolas terrane, a relatively intact outer terrane to the west characterized by the presence of Cretaceous and Paleogene forearc sedimentary rocks, and the adjacent Catalina terrane, a highly extended inner terrane to the east characterized by metamorphic basement rocks exhumed during early Miocene oblique rifting. To better understand this continental rifting process, we used regional grids of multichannel seismic reflection data, and stratigraphic information from industry wells and seafloor samples to investigate the nature and tectonic development of the boundary between the Outer Borderland forearc Nicolas terrane and the Inner Borderland denuded, exhumed Catalina terrane. These data show that this terrane boundary is largely defined by an angular unconformity that overlies and helps form an eastward termination or wedge-out of Outer Borderland forearc strata of earliest Miocene through Cretaceous age. These forearc sedimentary rocks were removed along the boundary primarily as a result of early-to-middle Miocene uplift and erosional truncation associated with exhumation of the Catalina basement. The boundary is not predominantly fault controlled. Along the northern part of the boundary, the East Santa Cruz Basin fault system, previously postulated to control the Nicolas–Catalina terrane boundary, is a predominantly east-dipping, Miocene oblique-normal fault system that has been reactivated with blind to partly blind oblique-reverse displacement. It does not align with the terrane boundary, and given its geometry, slip history, and the presence of Nicolas forearc sedimentary rocks on both sides of this fault system, it is unlikely to have had a major influence on terrane boundary development. South of San Nicolas Basin, there is no simple, through-going fault system, and where faults are present, they are often discontinuous segments that strike oblique to the boundary. This implies that major displacements associated with translation of the Outer Borderland were not localized to the eastern Outer Borderland boundary itself, but rather were likely distributed farther east within the evolving Inner Borderland rift.

Abstract The Outer Continental Borderland offshore southern California consists of an arkose-bearing accretionary prism or truncated forearc basin with two subsequent discrete periods of calcalkaline volcanism and a third prolonged period of alkaline seamount volcanism. The volcanics exhibit progressively less continental influence/contamination with time. The earliest period consists of calcalkaline lavas dated at 30 million years ago (Ma) that erupted prior to the Pacific–Farallon spreading ridge reaching the California margin. These are moderately contaminated by continental rocks. The second more widespread period of calcalkaline activity, from 16 to 18 Ma, consists of lavas ranging from basaltic andesite to rhyolite that generally experienced less continental contamination than the 30 Ma volcanics. The 16 to 18 Ma lavas apparently formed as a triple junction migrated along the margin, although the offshore lavas are not midocean ridge basalt–like, as expected if generated within a slab window. The third, even younger period of volcanic activity produced Rodriguez Seamount on the Patton Escarpment; Northeast Bank within the Outer California Borderland; and San Juan, San Marcos, Little Joe, Davidson, Guide, Pioneer, and Gumdrop Seamounts on the Pacific Plate. These youngest lavas, previously described in the literature, have little to no signature of continental contamination, although lavas that built Rodriguez and Northeast Bank migrated through continental crustal materials. These results are largely based on samples either dredged or collected via remotely operated vehicle from the seafloor. The widespread occurrence of erratics in our California Borderland sample set indicates that their presence must be weighed when reconstructing the geology of the region.

Abstract New field observations, petrology, geochemistry, and 40 Ar/ 39 Ar geochronology are reported for the Scripps Dike, which crops out at the coast north of La Jolla, California. The northeast–southwest-trending and laterally discontinuous dike has a basaltic–trachyandesite bulk composition, with an emplacement age of 13.89 ± 0.13 Ma. Modeling of the dike composition indicates that it formed from 0.5 to 1.5% partial melting of a primitive mantle-type source, metasomatized by slab fluids, predominantly in the garnet stability field. The composition of the dike, including relatively high MgO (6.6 wt.%) and Sr/Y (~105), makes it akin to magnesian andesites in Baja California, Mexico, termed “bajaites.” Field evidence indicates that the current exposure of the dike is close to the original stalling depth, it was probably associated with explosive volcanism, and the dike flowed laterally. After accounting for alteration, the dike has an initial 87 Sr/ 86 Sr composition of 0.70390, with limited evidence for crustal contamination, consistent with derivation from a slab-fluid-metasomatized mantle source. The composition of the dike places it broadly in the range of Miocene California Continental Borderland (hereafter referred to as Borderland) volcanic rocks studied previously. A comparison of ages of volcanic rocks occurring along the Borderland margin reveals an approximately age-progressive trend to the southeast. This represents an opposite sense to the apparent age-progressive trend for Miocene to Recent volcanic rocks north of the Western Transverse Ranges. Possible models to explain the compositions and age relationships of Miocene to Recent volcanic rocks of the Borderland region include southeasterly migration of volcanism in response to Rivera Triple Junction movement and slab window formation, or the presence of a weak “hotspot” that has been active since at least the Miocene. Identification of the process(es) responsible for Borderland volcanism is currently limited by dissection and northwestward movement of Borderland rocks in response to northwest–southeast shearing of the Pacific–North American plate boundary, and by the quality and quantity of reported age-dates and paleomagnetic information. The formation processes of volcanism in the Borderland have ramifications for palinspastic reconstruction of the margin, as well as for the thermal and magmatic evolution of western California in response to a change in plate motion in a subduction to transform setting. The Scripps Dike provides evidence that regions of the mantle beneath the California Continental Borderland were metasomatized by slab fluids in a manner similar to portions of mantle beneath central Baja California, Mexico.

Abstract Study of shear zones and associated basins within an oblique rift can shed light on the development of a young transform continental margin. San Pedro Basin lies within the Inner Borderland Rift offshore southern California, where it is bisected by the San Pedro Basin Fault (SPBF). Based on seismic reflection and multibeam bathymetry data, we show that the SPBF attained continuity with the San Diego Trough Fault (SDTF) between 1 Ma and 800 ka, to form a 350-km-long shear zone. Prior to that time, the SDTF was linked to the Catalina Fault, forming a restraining bend that contributed to the uplift of Catalina Ridge. Seismically defined depositional sequences in San Pedro Basin record a multistage history of uplift and subsidence for the basin. Young, flat-lying sequences filling a sigmoidal depocenter indicate that subsidence has been occurring since about 1 Ma. This date is corroborated by a series of submarine lowstand depositional terraces surrounding Santa Catalina Island. A 5 Ma to 1 Ma progressively tilted sequence, onlapped by the flat-lying strata, is confined to the present basin. Folded sequences older than ca. 5 Ma extend beyond the present basin onto Catalina Ridge and are correlated to Mohnian and Luisian strata on Santa Catalina Island and Palos Verdes Peninsula. From these data, we interpret the growth history of San Pedro Basin to involve at least three successive, nested basins. The first, which we call the “San Pedro (SP) protobasin,” formed before 5 Ma and was of indeterminate size, including within its boundaries areas flanking the current basin that were subsequently uplifted, Catalina Ridge and Palos Verdes Anticlinorium. Between 5 Ma and 1 Ma, approximately, a second basin, nested within the first, formed as the two flanking structural highs initiated. Finally, a third basin, nested within the first two, began to form when the SPBF–SDTF link was established and rapid local subsidence began; this is the depocenter of the current San Pedro Basin, and its southwestern boundary is occupied by the trace of the SPBF. Our model of basin formation begins with the initial oblique Inner Borderland (IB) Rift, which formed during rotation and translation of crustal blocks away from the continental margin (about 20 Ma). The IB Rift was segmented due to preexisting structural configurations. Published reconstructions show that the SP protobasin was originally in a narrow zone flanked by active volcanoes. Continued extension widened and deepened the rift, while volcanism continued along the flanks of the rift until about 10 Ma. As the rift widened, nested basins formed within the original protobasins along the axis of the rift. These basins were later fragmented (after 5 Ma for the SP protobasin) by transpressive processes associated with the shift of the transform plate boundary to the southern San Andreas Fault. New nested basins also formed during this time as shear zones reorganized to shortcut restraining geometries.

Abstract Compared to their equivalents along passive margins, less is understood about the stratigraphic architecture of incised-valley fill along active margins. Using approximately 10 km of shallow-marine seismic data and five vibracores, we compare the coastal incised-valley fill of two small southern California mountainous streams within the slowly uplifting and semiarid Oceanside Littoral Cell to incised-valley fill models from other active and passive margins. Our seismic data images the upper 16 m of the valley fill and contains three seismic units. The top unit is composed of a discontinuous drape of high-amplitude discontinuous subparallel reflections between 0.2 to 2 m thick, assuming a seismic velocity of 1500 m/s. The second unit is a 5.5- to 8-m-thick unit of faint chaotic reflections discontinuously compartmentalized by higher-amplitude mounded reflections. The lowest unit recorded in the seismic data is composed of a series of faint horizontal continuous reflections. A prominent reflector at 10 m separates the middle chaotic seismic unit from the lowest seismic unit. The shallow vibracores sampled the upper two seismic units, revealing a moderately well-sorted fine to very fine sand overlain by two silty units. The upper silt unit is bioturbated while the lower silt unit is well-laminated with laminations of gypsum sand. We interpret the silty units to represent a well-flushed mudflat overlying an enclosed evaporative mudflat. The second of these two facies appears to be unique to southern California estuaries undergoing uplift as an equivalent facies is not found within the estuaries developed within subsiding basins of the southern California coast. The sandy unit is interpreted to represent sandy lagoon or sandflat deposits. No cores sampled the lowest seismic unit; however, based on previously published data from neighboring incised valleys, we interpret it to be an open-estuary/central basin deposit. The fill within these small incised valleys is similar to that found along passive margins. The architecture of the valley fill is not dominated by tectonics but by eustatic sea-level rise. It lacks the multiple progradational phases and large volumes of coarse clastics common to other incised-valley fills from active margins. We attribute this difference to the generally low rates of uplift along this portion of the coast.

Abstract The Arguello submarine canyon/channel system extends over 300 km from the continental shelf off Point Arguello and Point Conception in southern California westward onto the oceanic crust of the Pacific plate. In the northernmost reaches where the canyon system originates, all stages in the evolution of seafloor morphologic fluid flow features—from pockmarks to gullies to converging rills—are observed, similar to what has been described for the Ascension slope, north of Monterey Bay. These features appear to be active today and are linked to fluid leakage from the underlying hydrocarbon basin. The channel dissects a continental slope that exhibits features consistent with large-scale mass wasting. Upslope scarps may be the source of the morphological feature at the base of the slope previously referred to as the “Arguello submarine fan,” with topographic expressions (e.g., large channel meanders, ridges) that are more consistent with mass transport deposits than with deep-sea fan depositional lobes. The modern canyon crosscuts these deposits and parallels an older, meandering channel/canyon to the west. Modern seismicity along the shelf and slope may have, and potentially still can, trigger landslides on the slope. Seismicity associated with seamount volcanism, past subduction, and Borderland transrotational and extensional processes most likely played a role in stimulating mass wasting. The presence of abundant nearby petroleum suggests that gas venting and hydrate dissociation cannot be ruled out as a triggering mechanism for the slope destabilization occurring today. The canyon/channel continues due south on a path possibly determined by the structural grain of north–south-aligned abyssal hills underlying oceanic basement. At latitude 33°18′N, the channel makes a 90° turn (bend) to the west at the E–W-striking Arguello transform fault wall and develops into a meandering channel system that crosses over abyssal hill crustal fabric. The system ultimately straightens as it continues west before veering north, curving around a thickened crustal bulge at a corner offset in the Arguello fracture zone in complex basement structure, and then finally empties into an 800-m-deep basin depocenter.

Abstract Thorough and accurate models of modern sand sources to the Santa Monica Basin, offshore southern California, are needed to facilitate the interpretation of sediment supply to the Hueneme–Mugu Fan, the largest submarine fan within the basin. Bathymetry and near-seafloor seismic-reflection isopach mapping of basin fill indicate that dominant sources of sand are canyons/channels that enter the basin from the northwest, which are likely fed via longshore drift by the Santa Clara River and Calleguas Creek. Sand within Calleguas Creek varies in composition across its drainage basin, ranging from more quartzofeldspathic in the northeast to more volcaniclastic in the southwest, ultimately producing sand with a compositional fingerprint distinct from that of the Santa Clara River and Santa Monica Mountains. The abundance of volcanic material and lack of metamorphic grains in downstream Calleguas Creek sand stand in stark contrast to the Santa Clara River’s relatively abundant metamorphic lithic fragments. In addition, Calleguas Creek sand can be further differentiated from sand derived from both the Santa Clara River and Santa Monica Mountains because both of these other sources have much higher proportions of plagioclase. The composition of late Pleistocene (<60 ka) sandy turbidites at Ocean Drilling Program Site 1015 on the distal Hueneme–Mugu submarine fan validates Calleguas Creek’s contribution of sandy sediment to this site in the Santa Monica Basin: out of 14 samples, four samples show compositions similar to the Santa Clara River sand, whereas five are similar to Calleguas Creek sand, and six exhibit mixed compositions. There is no indication of input to this distal environment from the southern Santa Monica Mountains. Trends in sand composition within the Santa Monica Basin can be related to alternating and/or mixing of sediment sources, possibly related to sea-level change, as well as frequency of floods/storms, earthquakes, and other destabilizing processes affecting offshore shelf-to-slope regions.

Abstract The Santa Cruz Basin (SCB) is one of several fault-bounded basins within the California Continental Borderland that has drawn interest over the years for its role in the tectonic evolution of the region, but also because it contains a record of a variety of modes of sedimentary mass transport (i.e., open slope vs. canyon-confined systems). Here, we present a suite of new high-resolution marine geophysical data that demonstrate the extent and significance of the SCB submarine landslide complex in terms of late Miocene to present basin evolution and regional geohazard assessment. The new data reveal that submarine landslides cover an area of ~160 km 2 along the eastern flank of the Santa Rosa–Cortes Ridge and have emplaced a minimum of 9 to 16 km 3 of mass transport deposits along the floor of the SCB during the Quaternary. The failures occur along an onlapping wedge of Pliocene sediment that was uplifted and tilted during the later stages of basin development. The uplifted and steepened Pliocene strata were preconditioned for failure so that parts of the section failed episodically throughout the Quaternary—most likely during large earthquakes. Once failed, the material initially translated as a block glide along a defined failure surface. As transport continued several kilometers across a steep section of the lower slope, the material separated into distinctive proximal and distal components. The failed masses mobilized into debris flows that show evidence for dynamic separation into less and more mobile components that disturbed and eroded underlying stratigraphy in areas most proximal to the source area. The most highly mobilized components and those with the lowest viscosity and yield strength produced flows that blanket the underlying stratigraphy along the distal reaches of deposition. The estimated volumes of individual landslides within the complex (0.1–2.6 km 3 ), the runout distance measured from the headwalls (>20 km), and evidence for relatively high velocity during initial mobilization all suggest that slides in the SCB may have been tsunamigenic. Because many slopes in the California Continental Borderland are either sediment starved or have experienced sediment bypass during the Quaternary, we propose that uplift and rotation of Pliocene deposits are important preconditioning factors for slope failure that need to be systematically evaluated as potential tsunami initiators.

Abstract This article uses measurements from outcrops of the Point Loma Formation to define the hierarchical organization of a distributive submarine fan and spatial changes in its constituent parts. A four-tier hierarchy for lobes is documented: bed, element, complex, and system. Across each hierarchical tier, there is an increase in size, duration of deposition, number of cross-cutting relationships, number of superposed stratal units, and degree of compensational stacking. Lobe elements contain systematic axis-to-margin and longitudinal decreases in amalgamation ratio, erosion, net sand content, proportions of sand-rich facies, and maximum grain size, with the exception of shale clasts in fringe position. The cross-sectional aspect ratios of lobe elements in the Point Loma are ~1000, a similar value to those measured in other systems, although aspect ratios are slightly higher at the distal reaches of lobe elements than in proximal locations. The key longitudinal patterns in lobe complexes are decreases in proportion of sand-rich facies, maximum grain size, amalgamation ratio of elements, net sand content, and amount of interelement erosion and a longitudinal increase in the degree of the amount of compensational stacking. Lobe complexes stack laterally and progradationally to build a lobe system.

Abstract Sand-rich turbidite systems in structurally complex, fault-bounded deep-water basins often form prolific hydrocarbon reservoirs. Outcrop studies of ancient examples of these turbidite systems provide information on architecture, depositional controls, and sandstone distribution. Outcrops of the Miocene upper Modelo Formation in eastern Ventura basin, California, are interpreted as a longitudinal transect through a fault-controlled deep-water depositional system, from proximal structural terraces, through a submarine canyon, to the basin floor and toward the basin margin. This study area is divided into three regions based on geographic location, geologic features, and stratigraphic character and architectures. Region 1, interpreted as the feeder system, contains syndepositionally active normal faults (e.g., Devil Canyon fault) near the proximal basin margin; these faults are associated with abrupt changes in depositional environments, lithofacies associations, paleobathymetry, and stratigraphic thickness. An interpreted submarine canyon, with a low net-sandstone content infill, was an area of bypass. Isopach maps reveal that gross thickness is greatest near the canyon mouth. In Region 2, interpreted as the proximal basin floor, a fairway located immediately basinward of the canyon has the highest sandstone content in the basin. In Region 3, interpreted as the medial–distal basin floor, gradual changes in proportions of lithofacies associations are the result of subtle variations in the gradient of the basin floor near the distal basin margin, as interpreted from an isopach map of the upper Modelo Formation. In this area, channel complexes comprising structureless, amalgamated sandstone overlie thin- to medium-bedded, poorly sorted, clay-rich sandstones deposited at lobe fringes. This upward architectural pattern is interpreted as a product of an outwardly expanding depocenter through time.