Donald M. Hussong

Origin and emplacement of Mariana forearc seamounts

Large seamounts occur on the outer half of the Mariana forearc. They represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material. The degree of metamorphism in this material depends on the amount of water available and the pressure-temperature regime of the forearc wedge. The major source for the water involved in the metamorphism is most likely the descending slab. Theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc. Zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the Mariana forearc. Thermal models indicate that higher grade greenschist facies should occur farther from the trench. Seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench. Uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the Mariana forearc.

Nature and distribution of deformation across the Banda Arc–Australian collision zone at Timor

Recently acquired seismic-reflection and SeaMARC II (side-scan and swath bathymetry) profiles near Timor show that the Banda Arc–Australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system. Deformation is occurring, at present, most intensely at the foot of the inner slope of the Timor Trough. This deformation front is discontinuously advancing southward as new thrust slices develop within the subducted Australian margin strata. In contrast, present deformation is apparently negligible in the Savu Basin, the complex fore-arc basin north of Timor. A possible significant exception is a postulated right-lateral, northeast-trending fault zone offsetting the outer-arc high between Savu and Roti. Although back-arc thrusting has been documented north of the volcanic arc, this component of convergence is minor compared with the scale of ongoing deformation in the Timor Trough. The detailed nature of these surveys has also led to the recognition of along-strike variations in deformation in the Timor Trough and in the Savu Basin. These variations may be related to the variable degree of involvement of the Australian continental margin along the arc.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

Seismic studies on the Ontong Java Plateau, 1970☆

Abstract Marine seismic studies were conducted over the Ontong Java Plateau in the south-western Pacific Ocean during 1970 using two ships. Seismic refraction surveys revealed that crustal thickness of the plateau ranges from 35 to 42 km. A high-velocity basal crustal layer was detected in the northern part of the plateau but not in the southern part. Seismic reflection profiling showed the sediments to be about 1.5 km thick. In the sedimentary column were numerous intrusions with very little magnetic expression. These intrusions are probably not of igneous origin. There is a large discrepancy between observed gravity values and theoretical values calculated from the crustal structure.

Submarine-fan development in the southern Chile Trench: A dynamic interplay of tectonics and sedimentation

Submarine fans are well developed in the southern Chile Trench, from 33°S to 41°S latitude. SeaMARC-II side-scan sonar and seismic reflection records image steep erosional escarpments, as much as 400 m in relief, extending seaward across the trench basin from the mouths of submarine canyons. The scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough. Fanlobes are depositional and constructional up-gradient (south) from the canyon mouths. They are composed of aggraded channel/levee complexes, smooth and conformable sediment drapes, and crescentic levees rimming the headwalls of erosional scarps. Fanlobes are carved and dissected by erosional processes down-gradient (north) from the canyon mouths. They exhibit amalgamated lag pavements, composite sediment lobes, longitudinal furrows, braided channels, and canyon-mouth bars. Thick, massive-to-laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes, whereas fine-grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes. The texture and composition of the sediment supply, the onshore climate, and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans. Stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled, high-amplitude seismic facies: reflective lenticular bodies, truncation and scour surfaces, planar-amalgamation, and sigmoidaccretion structures. The severe erosional dissection of down-gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin. Extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum. The trench fans may have been deposited on progressively steeper trench gradients, because the buoyant Chile Rise migrates northward as it subducts beneath the Andean continental margin. Fan distributary channels, axial channels, slump scars, and erosional gullies are largely localized along structural features. Normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high-velocity mainstreams of turbidity currents. Orthogonal fault sets within the deposits of the trench basin, which parallel the spreading and transform structures of the extinct Pacific-Farallon Rise and the Chile Rise, are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin. Uplifted thrust ridges, generally restricted to a narrow zone along the base of the deformation front, are dissected by distributary channels, and channel courses are locally deflected seaward of these propagating structures. Transform-oriented basement ridges, associated with strike displacements of the axial channel and vertical faults in the trench basin, may accommodate renewed strike-slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin.

Structural styles of an accretionary wedge south of the island of Sumba, Indonesia, revealed by SeaMARC II side scan sonar

The accretionary wedge south of Sumba Island, Indonesia, is in the early stages of continent-island arc collision. Australian continental shelf sediments are being actively accreted to the Sunda arc at the Timor trough. We have correlated SeaMARC II side-scan images and bathymetry with 3.5-kHz and seismic-reflection profiles to construct a structural interpretation of this zone. Deformation is concentrated on the lower slope of the accretionary wedge, within 15 to 25 km of the thrust front, above which the strain rate appears to decrease abruptly. Three distinct structural styles are developed in the survey area. The characteristics of the easternmost region are (1) deformation in a fold-and-thrust geometry; (2) presence of mud volcanoes and mud ridges parallel to the thrust front, indicating elevated fluid pressure; and (3) the development of a long decollement extending 10 to 15 km from the thrust front. In contrast, the central region shows a more homogeneous thickening, resulting mainly from closely spaced faulting. Seaward-verging thrust faults and conjugate strike-slip faults are the main structures in this segment of the accretionary wedge. Mud volcanoes and a very long decollement are absent in the central region. We present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front. Short, thick thrust sheets may favor uniform shortening, whereas long thin sheets may be unstable to buckling and deform by folding. High fluid pressure may facilitate propagation of the decollement. In contrast to the accretion occurring in the other two regions, the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge.

Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala

Drill cores from a transect of the Mid-America Trench off Guatemala were obtained at three sites on the oceanic Cocos plate, and at four sites on the continental Caribbean plate. An ocean sub-bottom seismometer was successfully emplaced in the deepest hole in the trench landward slope where it was left to record data after departure of the drill ship. Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation even at holes drilled against the trench9s landward slope. In contrast, the section cored on the trench9s landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence, broken by hiatuses but in a normal stratigraphic succession that is capped by Pliocene to Quaternary hemipelagic slope deposits. Seismic records show that the section overlies probable igneous oceanic crust from which it is separated by a few hundred metres. That thickness of undrilled section is insufficient to accommodate the potential offscraped volume of oceanic sediment carried into the trench during Neogene plate convergence. At the estimated 10 cm/yr rate of convergence, much of the oceanic sediment must have been subducted rather than tectonically accreted to the Guatemalan margin. Current models for convergent margin tectonics do not satisfactorily explain the surprising occurrence of Cretaceous to Miocene mudstone at the base of this trench slope. The recovery of gas hydrates prevented drilling to some landward-dipping reflections presumed to be imbricate thrust slices at two sites near the middle of the trench landward slope.

Tholeiitic Basalt Ridge in the Peru Trench

A prominent ridge occurs in the axis of the Peru Trench between 7°25′ to 7°50′ and 8°30′ to 9°30′ S. It rises as much as 900 m above the turbidite fill in the trench axis at 9° 20′ and consists of relatively fresh tholeiitic basalt which is overlain by Pleistocene trench turbidites. The tholeiitic basalt was derived from oceanic layer 2 which has been faulted and uplifted relative to the trench floor as the Nazca lithospheric plate descended into the Peru Trench. A K-Ar date of 8.7 m.y. was obtained on the basalt. This date allows three possible interpretations: (1) that the oceanic crust along the eastern edge of the Nazca plate is younger than indicated by magnetic anomalies; (2) that midplate volcanism has occurred on the older oceanic crust of the Nazca plate; or (3) that the basalt is more weathered and hence older than our data suggest. The second interpretation is preferred within the framework of the existing data.

New estimates of rapid sea-floor spreading rates and the identification of young magnetic anomalies on the East Pacific rise, 6° and 11° S

Abstract Detailed surveys of the crest of the East Pacific Rise at 6° and 11°S form the basis for estimates of sea-floor spreading rates of 8.2 and 8.3 cm/yr, respectively. These estimates are significantly higher than previous ones of 6.5 to 7.5 cm/yr. The high quality of the magnetic profile at 6°S allows identification of shorter magnetic events such as the two Olduvai events of Cox. DSDP drill-hole data indicate that the present spreading rates have persisted throughout the Cenozoic.

Compressional faulting of the oceanic crust prior to subduction in the Peru-Chile Trench

An oceanic crustal section has been derived for the Nazca plate and across the Peru-Chile Trench at lat 12°S. Although based on unreversed seismic refraction data obtained by the Airgun-Sonobuoy-Precision-Echo-Recorder (ASPER) technique, 12 crustal determinations along the profile provide sufficient data density to permit good correlation of velocity structure between the stations. A thin, high-velocity oceanic crust characterizes the region. A major apparent offset in the crustal layers is interpreted as a low-angle thrust fault dipping east at an average of 6° from the ocean floor 300 km west of the trench axis. This feature, together with other indications of thrust faulting and crustal foreshortening, is taken as evidence that the upper lithosphere, at least to the depth of the Moho, is undergoing compression prior to subduction in the trench.