Drowned carbonate platforms in the Bismarck Sea, Papua New Guinea

Extinct volcanic islands in the Bismarck volcanic arc are fringed by well-developed coral reefs. Drowned platforms offshore from these islands provide evidence for subsidence in the central section of the arc, north of the Finisterre Terrane–Australia collision. Bathymetric and backscatter data collected onboard the R/V Kilo Moana in 2004 reveal regularly spaced (~200 m interval) drowned platforms at depths as much as 1,100 m below sea level. However, the adjacent mainland coast has well documented raised terraces indicating long-term uplift. Local subsidence may be due to cessation of magmatic activity and cooling, flexural loading by the uplifting Finisterre Range, loading by nearby active volcanic islands, and/or sediment loading on the seafloor north of the Finisterre Range. We present some simple models in order to test whether flexural loading can account for local subsidence. We find that volcanic and sedimentary loading can explain the inferred relative subsidence

A left-lateral strike-slip fault seaward of the Oregon convergent margin

We have mapped a recently active left-lateral strike-slip fault (the Wecoma fault) on the floor of Cascadia Basin west of the Oregon convergent margin, using SeaMARC I sidescan sonar, Seabeam bathymetry and multichannel seismic and magnetic data. The fault intersects the base of the continental slope at 45°10’N and extends northwest (293°) for at least 18.5 km. The fault’s western terminus was not identified and the eastern end of the fault splays apart and disrupts the lower continental slope. The fault extends to the base of the 3.5-km-thick sedimentary section and overlies a basement discontinuity that may be related to movement along the Wecoma fault. Prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left-lateral separation, allowing the general history of fault motion to be evaluated. The fault’s average slip rate since 10-24 ka is inferred to be 5-12 mm/yr, based on the age of an offset submarine channel. Surficial structural relationships, in conjunction with the maximum inferred slip rate, indicate that fault movement initiated at least 210 ka and that the fault has been active during the Holocene

Geophysical investigations of the Reykjanes Ridge and Kolbeinsey Ridge seafloor spreading centers

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995.Includes bibliographical references (leaves 77-86).Microfiche.ix, 86 leaves, bound ill. (some col.) 29 cmI used a suite of marine geophysical tools to study the structure and tectonics of the slow-spreading Kolbeinsey, Reykjanes and northern Mid-Atlantic Ridges. Including Iceland, these ridges constitute a continuous spreading center system more than 2055 km long, and vary in their structural expression and obliquity to the spreading direction. The northern Mid-Atlantic Ridge (MAR) and Reykjanes Ridge between 55°50'N and 63°00'N exhibit systematic along-strike variation in axial valley depth, axial boundary fault throw, relief along the neovolcanic axis, and degree of inter-segment structural discontinuity. The orthogonal northern MAR is separated from the Reykjanes Ridge by the Bight transform fault (56°47'N), a right-stepping linear fault 15 km wide. The volcanic axis of Reykjanes Ridge contains individual volcanic systems 4-45km long (fourth-order segments), superimposed on intermediate-wavelength (13-65km) axial topographic highs that constitute third- or second-order spreading segments. The modem Kolbeinsey Ridge axis contains three first-order segments oriented orthogonally to the spreading direction. These segments are separated by large right-stepping nontransform offsets, the Spar (69.0°N) and Eggvin (70.4°N) discontinuities. The northern KR segment is a robust volcanic edifice 125km long and more than 1000m high. Shallow crust extends east from the northern KR axis to Jan Mayen Island, and I suggest the Jan Mayen hotspot is located beneath the northern KR rather than near Jan Mayen Island. A tectonic reconstruction based on aeromagnetic data indicates that the axial structure of the KR changed from continuous to segmented after anomaly 4. The subsequent structural evolution of the ridge involved ridge propagation, along-strike migration of axial discontinuities, asymmetric spreading, and lateral migration of segment axes that occurred via ultrafast propagation or synchronous ridge jumps. Two of the three original discontinuities still exist and contain active north-directed propagators. An ephemeral, catalytic change in plate motion is inferred to have triggered the axial reorientation at anomaly 4 time, which initiated the subsequent phase of ridge propagation and migration of nontransform offsets

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A 5km swath-width SeaMARC I sidescan sonar survey, conducted over the zone of overlap between the southern rift zone of Axial Volcano and the northern tip of the Vance spreading segment on the Juan de Fuca Ridge (between 45°24'N and 45°50'N latitude), was analyzed to locate the present position of the Juan de Fuca spreading axis, and to determine the tectonic and volcanic structure of the seafloor. Sidescan data were processed in concert with the ship's Loran-C navigation to construct navigated, orthorectified mosaics of the sidescan imagery. In order to navigate the sidescan swaths, a simple numerical model was developed to describe the tracking behavior of the towed sidescan vehicle. Successive positions and orientations of the sidescan towfish were estimated, and were used to assign latitude/longitude values to individual sidescan pixels. Navigated sidescan pixels were mapped by computer onto an absolute (latitude/longitude) reference grid, and the resulting sidescan mosaic was compared directly to existing high-resolution SeaBeam bathymetry in order to discriminate the effects of large- and small-scale roughness on the observed backscatter distribution. The Juan de Fuca spreading axis between 45°25'N and 45°39'N is located within the axial valley of the Vance segment. Relative age relationships, based on crosscutting and superposition principles, indicate that the most recent volcanism within the axial valley has occurred along the valley's central ridge, and that the most recent resolvable extension within the axial valley has been concentrated between the central ridge and west valley wall. The Vance segment terminates at 45°39'N, and is not associated with a transform fault. The south rift zone of Axial volcano is a constructional volcanic feature that is not faulted, and a discrete axis of spreading over the south flank of Axial volcano is not resolvable in the sidescan imagery; however, the spreading locus north of 45°39'N is constrained to a zone between 130°06'W and 129°54'W. The lack of a well-defined spreading axis north of 45°39'N indicates that the physical manifestation of the divergent plate boundary has been modified or masked by hotspot volcanic processes associated with Axial volcano such that a definitive locus of spreading is not expressed in the surface morphology

Tectonic and volcanic structures of the southern flank of Axial Volcano, Juan de Fuca Ridge : results from a SeaMARC I sidescan sonar survey

A 5km swath-width SeaMARC I sidescan sonar survey, conducted over the zone of overlap between the southern rift zone of Axial Volcano and the northern tip of the Vance spreading segment on the Juan de Fuca Ridge (between 45°24'N and 45°50'N latitude), was analyzed to locate the present position of the Juan de Fuca spreading axis, and to determine the tectonic and volcanic structure of the seafloor. Sidescan data were processed in concert with the ship's Loran-C navigation to construct navigated, orthorectified mosaics of the sidescan imagery. In order to navigate the sidescan swaths, a simple numerical model was developed to describe the tracking behavior of the towed sidescan vehicle. Successive positions and orientations of the sidescan towfish were estimated, and were used to assign latitude/longitude values to individual sidescan pixels. Navigated sidescan pixels were mapped by computer onto an absolute (latitude/longitude) reference grid, and the resulting sidescan mosaic was compared directly to existing high-resolution SeaBeam bathymetry in order to discriminate the effects of large- and small-scale roughness on the observed backscatter distribution. The Juan de Fuca spreading axis between 45°25'N and 45°39'N is located within the axial valley of the Vance segment. Relative age relationships, based on crosscutting and superposition principles, indicate that the most recent volcanism within the axial valley has occurred along the valley's central ridge, and that the most recent resolvable extension within the axial valley has been concentrated between the central ridge and west valley wall. The Vance segment terminates at 45°39'N, and is not associated with a transform fault. The south rift zone of Axial volcano is a constructional volcanic feature that is not faulted, and a discrete axis of spreading over the south flank of Axial volcano is not resolvable in the sidescan imagery; however, the spreading locus north of 45°39'N is constrained to a zone between 130°06'W and 129°54'W. The lack of a well-defined spreading axis north of 45°39'N indicates that the physical manifestation of the divergent plate boundary has been modified or masked by hotspot volcanic processes associated with Axial volcano such that a definitive locus of spreading is not expressed in the surface morphology

AppelgateTBruce1989.pdf

A 5km swath-width SeaMARC I sidescan sonar survey, conducted over the zone of overlap between the southern rift zone of Axial Volcano and the northern tip of the Vance spreading segment on the Juan de Fuca Ridge (between 45°24'N and 45°50'N latitude), was analyzed to locate the present position of the Juan de Fuca spreading axis, and to determine the tectonic and volcanic structure of the seafloor. Sidescan data were processed in concert with the ship's Loran-C navigation to construct navigated, orthorectified mosaics of the sidescan imagery. In order to navigate the sidescan swaths, a simple numerical model was developed to describe the tracking behavior of the towed sidescan vehicle. Successive positions and orientations of the sidescan towfish were estimated, and were used to assign latitude/longitude values to individual sidescan pixels. Navigated sidescan pixels were mapped by computer onto an absolute (latitude/longitude) reference grid, and the resulting sidescan mosaic was compared directly to existing high-resolution SeaBeam bathymetry in order to discriminate the effects of large- and small-scale roughness on the observed backscatter distribution. The Juan de Fuca spreading axis between 45°25'N and 45°39'N is located within the axial valley of the Vance segment. Relative age relationships, based on crosscutting and superposition principles, indicate that the most recent volcanism within the axial valley has occurred along the valley's central ridge, and that the most recent resolvable extension within the axial valley has been concentrated between the central ridge and west valley wall. The Vance segment terminates at 45°39'N, and is not associated with a transform fault. The south rift zone of Axial volcano is a constructional volcanic feature that is not faulted, and a discrete axis of spreading over the south flank of Axial volcano is not resolvable in the sidescan imagery; however, the spreading locus north of 45°39'N is constrained to a zone between 130°06'W and 129°54'W. The lack of a well-defined spreading axis north of 45°39'N indicates that the physical manifestation of the divergent plate boundary has been modified or masked by hotspot volcanic processes associated with Axial volcano such that a definitive locus of spreading is not expressed in the surface morphology

The morphology and distribution of submerged reefs in the Maui-Nui Complex, Hawaii: new insights into their evolution since the Early Pleistocene

Reef drowning and backstepping have long been recognised as reef responses to sea-level rise on subsiding margins. During the Late Pleistocene (~500–14 ka) Hawaiian reefs grew in response to rapid subsidence and 120 m 100 kyr sea-level cycles, with recent work on the submerged drowned reefs around the big island of Hawaii, and in other locations from the last deglacial, providing insight into reef development under these conditions. In contrast, reefs of the Early Pleistocene (~1.8–0.8 Ma) remain largely unexplored despite developing in response to significantly different 60–70 m 41 kyr sea-level cycles. The Maui-Nui Complex (MNC — forming the islands of Maui, Molokai, Lanai and Kahoolawe), provides a natural laboratory to study reef evolution throughout this time period as recent data indicate the reefs grew from 1.1 to 0.5 Ma. We use new high resolution bathymetric and backscatter data as well as sub-bottom profiling seismic data and field observations from ROV and submersible dives to make a detailed analysis of reef morphology and structure around the MNC.We focus specifically on the south-central region of the complex that provides the best reef exposure and find that the morphology of the reefs varies both regionally and temporally within this region. Barrier and pinnacle features dominate the steeper margins in the north of the study area whilst broad backstepping of the reefs is observed in the south. Within the Au'au channel in the central region between the islands, closely spaced reef and karst morphology indicates repeated subaerial exposure.We propose that this variation in the morphology and structure of the reefs within the MNC has been controlled by three main factors; the subsidence rate of the complex, the amplitude and period of eustatic sea-level cycles, and the slope and continuity of the basement substrate. We provide a model of reef development within the MNC over the last 1.2 Ma highlighting the effect that the interaction of these factors had on reef morphology