Lewis J. Abrams

Subseafloor sedimentary life in the South Pacific Gyre

The low-productivity South Pacific Gyre (SPG) is Earths largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1–1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.

Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the southwestern Pacific Basin

The trace of the ridge-ridge-ridge triple junction that connected the Pacific, Farallon, and Phoenix plates during mid-Cretaceous time originates at the northeast corner of the Manihiki Plateau near the Tongareva atoll, for which the structure is named. The triple junction trace extends >3250 km south-southeast, to and beyond a magnetic anomaly 34 bight. It is identified by the intersection of nearly orthogonal abyssal hill fabrics, which mark the former intersections of the Pacific-Phoenix and Pacific-Farallon Ridges. A distinct trough is commonly present at the intersection. A volcanic episode from 125 to 120 Ma created the Manihiki Plateau with at least twice its present volume, and displaced the triple junction southeast from the Nova-Canton Trough to the newly formed Manihiki Plateau. Almost simultaneously, the plateau was rifted by the new triple junction system, and large fragments of the plateau were rafted away to the south and east. The Tongareva triple junction originated ca. 119 Ma, when carbonate sedimentation began atop the Manihiki Plateau. Subsequent spreading rates on the Pacific-Phoenix and Pacific-Farallon Ridges averaged 18–20 cm/yr until 84 Ma.

The Seismic Stratigraphy and Sedimentary History of the East Mariana and Pigafetta Basins of the Western Pacific

The successful completion of Leg 129, resulting in the first and only holes to penetrate igneous basement (not necessarily layer 2) in the East Mariana and Pigafetta basins, now allows the calibration of our regional multichannel seismic site surveys and the extrapolation of drilling results throughout these oldest Pacific basins. Our study indicates that mid-Cretaceous flows/sills overlie Jurassic/Lower Cretaceous sediments and oceanic crust throughout the East Mariana Basin and the southeast Pigafetta Basin. Jurassic-age oceanic crust and overlying Upper Jurassic-Lower Cretaceous sediments unquestionably exist at Site 801 and extend semicontinuously between Sites 801 and 800. Turbidite sequences of varying thicknesses and ages are ubiquitous features of both basins. Cretaceous turbidite sequences were derived from Magellan and Marcus-Wake seamounts and Seamount chains of Aptian age or younger. The seamounts/atolls of the Caroline Ridge more than 300 km to the south of Site 802 were the source of extensive Miocene-age volcanogenic turbidites which are restricted to the south central East Mariana Basin, while the carbonate caps that developed on Ita Mai Tai Guyot and other edifices of the Magellan chain were the source for the redeposited shallow-water carbonate sequences recovered along the eastern margin of the East Mariana Basin. The Ogasawara Fracture Zone, Magellan Seamounts, and associated flexural moat separate the Pigafetta Basin from the East Mariana Basin and influence the source and distribution of redeposited material. A distinct basinwide reflection that is correlated with the shallowest chert/porcellanite/clay and chert/chalk sequences in the Pigafetta Basin and East Mariana Basin, respectively, is a time-transgressive horizon (Eocene-Campanian) resulting from the passage of the Pacific Plate beneath the equatorial zone of high productivity.

Neotectonics of southern Puerto Rico and its offshore margin.

Puerto Rico is located within a zone of tectonic transition between mainly east-west, North America–Caribbean strike-slip motion to the west in Hispaniola and east-northeast–oriented underthrusting to the east beneath the Lesser Antilles island arc. Various models and tectonic mechanisms have been proposed for the Neogene to present-day deformation of southern Puerto Rico, its island margin, and the Muertos trench by previous workers that include normal, thrust, and strike-slip faulting accompanied by large-scale rotations. In this study, we present the results of a regional study integrating onland mapping of striated fault surfaces in rocks ranging in age from Oligocene to possibly as young as earliest Pliocene, and offshore mapping of faults deforming the uppermost sediments beneath the seafloor. The tectonic geomorphology and distribution of late Quaternary marine terraces and beach ridges in south-central Puerto Rico suggest either stability or slow late Quaternary uplift along the south-central part of the coast. In contrast, the coastline of southwestern Puerto Rico exhibits no late Quaternary coastal sediments and a pattern of long-term drowning of coastal features. Fault striation studies of three formations composing the Puerto Rico–Virgin Islands carbonate platform of south-central Puerto Rico (Juana Diaz Formation basal clastic unit, Juana Diaz Formation upper carbonate unit, Ponce Formation) indicate two distinct extensional phases affecting the youngest formation (Ponce Formation of middle Miocene–early Pliocene age). The first event, a north-northeast–directed extensional event is accommodated by normal faults striking mainly to the west-northwest. A second, southeast-directed extensional event crosscut and reactivated faults formed during the fi rst event and produced at least one northeast-trending Quaternary rift bounded by northeast-striking normal faults (Ponce basin). Offshore seismic profi ling by previous workers and reported in this study support the presence of late Holocene seafl oor-rupturing, northeast-striking normal faults that accommodate southeast extension of the southern margin of Puerto Rico. The post–early Pliocene extension direction is roughly perpendicular to the east-northeast–trending sections of the stable or slowly uplifting coastline along much of southern Puerto Rico. In addition to northeast-striking normal faults, offshore profi les confi rm the presence of late Holocene, seafl oor-rupturing left lateral strike-slip faults along the offshore extension of the Great Southern Puerto Rico fault zone. Where the Great Southern Puerto Rico fault zone curves to the northeast, the fault becomes less strike-slip and more normal in character and produces greater extensional and tilting effects in the linked Whiting half-graben. A neotectonic model for southern Puerto Rico to explain both directions of extension known from fault striation studies and the present tectonic geomorphology of the preserved Puerto Rico–Virgin Islands carbonate platform in south-central Puerto Rico involves late Miocene–early Pliocene oblique collision of the Bahama Platform with Hispaniola to the northwest of Puerto Rico and ounterclockwise rotation and extension of the area of southern Puerto Rico. A later crosscutting extensional event during the post–early Pliocene involves left-lateral transtension of the southern margin of Puerto Rico with most strike-slip motion concentrated along the Great Southern Puerto Rico fault zone.

Reconnaissance study of late quaternary faulting along cerro GoDen fault zone, western Puerto Rico

The Cerro Goden fault zone is associated with a curvilinear, continuous, and prominent topographic lineament in western Puerto Rico. The fault varies in strike from northwest to west. In its westernmost section, the fault is ~500 m south of an abrupt, curvilinear mountain front separating the 270- to 361-m-high La Cadena de San Francisco range from the Rio Anasco alluvial valley. The Quaternary fault of the Anasco Valley is in alignment with the bedrock fault mapped by D. McIntyre (1971) in the Central La Plata quadrangle sheet east of Anasco Valley. Previous workers have postulated that the Cerro Goden fault zone continues southeast from the Anasco Valley and merges with the Great Southern Puerto Rico fault zone of south-central Puerto Rico. West of the Anasco Valley, the fault continues offshore into the Mona Passage (Caribbean Sea) where it is characterized by offsets of seafl oor sediments estimated to be of late Quaternary age. Using both 1:18,500 scale air photographs taken in 1936 and 1:40,000 scale photographs taken by the U.S. Department of Agriculture in 1986, we identifi ed geomorphic features suggestive of Quaternary fault movement in the Anasco Valley, including aligned and defl ected drainages, apparently offset terrace risers, and mountain-facing scarps. Many of these features suggest right-lateral displacement. Mapping of Paleogene bedrock units in the uplifted La Cadena range adjacent to the Cerro Goden fault zone reveals the main tectonic events that have culminated in late Quaternary normal-oblique displacement across the Cerro Goden fault. Cretaceous to Eocene rocks of the La Cadena range exhibit large folds with wavelengths of several kms. The orientation of folds and analysis of fault striations within the folds indicate that the folds formed by northeast-southwest shortening in present-day geographic coordinates. The age of deformation is well constrained as late Eocene–early Oligocene by an angular unconformity separating folded, deep-marine middle Eocene rocks from transgressive, shallow-marine rocks of middle-upper Oligocene age. Rocks of middle Oligocene–early Pliocene age above unconformity are gently folded about the roughly last-west–trending Puerto Rico–Virgin Islands arch, which is well expressed in the geomorphology of western Puerto Rico. Arching appears ongoing because onshore and offshore late Quaternary oblique-slip faults closely parallel the complexly deformed crest of the arch and appear to be related to extensional strains focused in the crest of the arch. We estimate ~4 km of vertical throw on the Cerro Goden fault based on the position of the carbonate cap north of the fault in the La Cadena de San Francisco and its position south of the fault inferred from seismic refl ection data in Mayaguez Bay. Based on these observations, our interpretation of the kinematics and history of the Cerro Goden fault zone includes two major phases of motion: (1) Eocene northeast-southwest shortening possibly accompanied by leftlateral shearing as determined by previous workers on the Great Southern Puerto Rico fault zone; and (2) post–early Pliocene regional arching of Puerto Rico accompanied by normal offset and right-lateral shear along faults fl anking the crest of the arch. The second phase of deformation accompanied east-west opening of the Mona rift and is inferred to continue to the present day.

Toward an integrated understanding of Holocene fault activity in western Puerto Rico: Constraints from high-resolution seismic and sidescan sonar data

It has been postulated that the western boundary of the Puerto Rico–Virgin Islands microplate lies within the Mona Passage and extends onland into southwestern Puerto Rico. This region is seismically active, averaging one event of magnitude 2.0 or larger per day, and over 150 events of magnitude 3.0 or greater occurred during the past fi ve years. Moreover, there have been at least 13 historical events of intensity VI (MM) or greater in the past 500 years. We conducted a high-resolution seismic and sidescan sonar survey of the insular shelf of western and southern Puerto Rico during May 2000 in an effort to identify Holocene faults and to further assess the seismic hazard in the region. We focus on an ~175 km part of the surveyed area offshore of western Puerto Rico, extending from Punta Higuero to Boqueron Bay. This area was targeted as a likely place to image recent faults, because multi-channel seismic profi les offshore western Puerto Rico show numerous WNW-trending normal and strike-slip faults that offset Oligocene-Pliocene age carbonate rocks and underlying Cretaceous basement rocks. Analyses of these data identify three zones of active deformation within the survey area: (1) the Cerro Goden fault zone; (2) the Punta Algarrobo/ Mayaguez fault zone that lies offshore the city of Mayaguez; and; (3) the Punta Guanajibo/Punta Arenas fault zone. Two of the offshore fault zones, the Cerro Goden and Punta Algarrobo, show strong correlation with fault zones onland, Cerro Goden and Cordillera, respectively. Many of the mapped faults offshore appear to reactivate older WNW-trending basement structures and show evidence of some component of right-lateral motion that is consistent with geodetic measurements. The offshore deformation zones are also associated with headlands and linear NW-SE magnetization lows (serpentinite dikes?) mapped offshore. Elongate outcrops of serpentinite in western Puerto Rico are colinear with the fault zones we have mapped offshore,