Seismic velocity structure of seaward-dipping reflectors on the South American continental margin

Seaward dipping reflectors (SDRs) are a key feature within the continent to ocean transition zone of volcanic passive margins. Here we conduct an automated pre-stack depth-migration imaging analysis of commercial seismic data from the volcanic margins of South America. The method used an isotropic, ray-based approach of iterative velocity model building based on the travel time inversion of residual pre-stack depth migration move-out. We find two distinct seismic velocity patterns within the SDRs. While both types show a general increase in velocity with depth consistent with expected compaction and alteration/metamorphic trends, those SDRs that lie within faulted half grabens also have high velocity zones at their down-dip ends. The velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile. The sills therefore result from late-stage melt delivery along the large landward-dipping faults that bound them. In contrast the more outboard SDRs show no velocity anomalies, are more uniform spatially and have unfaulted basal contacts. Our observations imply that the SDRs document a major change in rift architecture, with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised, dike-fed eruptions similar to seafloor spreading

Crustal structure of the Peruvian continental margin from wide-angle seismic studies

Active seismic investigations along the Pacific margin off Peru were carried out using ocean bottom hydrophones and seismometers. The structure and the P-wave velocities of the obliquely subducting oceanic Nazca Plate and overriding South American Plate from 8°S to 15°S were determined by modelling the wide-angle seismic data combined with the analysis of reflection seismic data. Three detailed cross-sections of the subduction zone of the Peruvian margin and one strike-line across the Lima Basin are presented here. The oceanic crust of the Nazca Plate, with a thin pelagic sediment cover, ranging from 0–200 m, has an average thickness of 6.4 km. At 8°S it thins to 4 km in the area of Trujillo Trough, a graben-like structure. Across the margin, the plate boundary can be traced to 25 km depth. As inferred from the velocity models, a frontal prism exists adjacent to the trench axis and is associated with the steep lower slope. Terrigeneous sediments are proposed to be transported downslope due to gravitational forces and comprise the frontal prism, characterized by low seismic P-wave velocities. The lower slope material accretes against a backstop structure, which is defined by higher seismic P-wave velocities, 3.5–6.0 km s−1. The large variations in surface slope along one transect may reflect basal removal of upper plate material, thus steepening the slope surface. Subduction processes along the Peruvian margin are dominated by tectonic erosion indicated by the large margin taper, the shape and bending of the subducting slab, laterally varying slope angles and the material properties of the overriding continental plate. The erosional mechanisms, frontal and basal erosion, result in the steepening of the slope and consequent slope failure

Structure of the Mediterranean Ridge accretionary complex from seismic velocity information

Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismic data were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending from the Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200–300-m-thin layer of Plio–Quaternary sediments with velocities of 1800–2200 m s−1 covers the whole Ridge. The Messinian evaporites (4000–4500 m s−1) occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In the intervening region however, the evaporites appear absent and the seismic velocities are generally lower. Arched reflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiary sediments shows laterally increasing velocities from 3300 m s−1 to 4600 m s−1. Assuming that the layer thinning is caused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4% and an associated fluid expulsion of 10 km3 km−1 along the trench axis. This corresponds to c. 60% of the initial fluid volume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal décollement of the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500–4600 m s−1 is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results from this study have widespread implications for the understanding of the regional geological history

The structure and evolution of the western Mediterranean Ridge

The Mediterranean Ridge is a unique accretionary complex, consisting of five key elements: the frontal slope, the upper slope, the crest of the Ridge, the Cleft area, and the Inner Plateau. The IMERSE data show that these correspond roughly to the locus of frontal accretion, of underplating, of a pre-Messinian wedge, of complex faulting and possible strike-slip tectonics, and of a backstop of Hellenic nappes covered by a Messinian forearc basin. The frontal portion of the complex is a post-Messinian accretionary wedge (composed of Messinian evaporites and overlying tightly folded Plio–Quaternary sediments), underlain by pre-Messinian sequences attached to the African Plate. The basal detachment at the front of the wedge occurs at the base of the evaporites. Moving further to the northeast (the upper slope), the basal detachment cuts to deeper levels leading to the development of duplex structures where pre-Messinian units are subcreted beneath the Messinian evaporites. Just behind the subcreted units, the evaporites thin and may be absent on the crest of the Ridge. This region we interpret as the site of a pre-Messinian accretionary wedge: we suggest that following the deposition of thick evaporites in the Messinian, the pre-Messinian accretionary tectonics continued as subsurface accretion (subcretion) beneath the evaporites. Although the crest of the Ridge is largely devoid of evaporites, local deep evaporite basins observed here formed as local closed basins on top of the pre-Messinian wedge. We infer that the Messinian sealevel was at about the level of the Ridge crest, that is 3000 m below present. Allowing for isostatic adjustment to the removal of the water load, this would imply a sealevel drop of at least 2000 m. The Cleft basins mark the northeast limit of the accretionary complex. Thick evaporite deposits to the northeast (beneath the Inner Plateau) may have been deposited in a Messinian forearc basin. The evaporites of the Inner Plateau are underlain by a thin pre-Messinian sequence and by crystalline basement of the Hellenic nappes. This basement forms the backstop to the accretionary complex

Seismische Untersuchungen zur Massenbilanz und Tektonik in aktiven Subduktionszonen, Kennwort SUBMASS Schlussbericht

SIGLEAvailable from TIB Hannover: F97B165 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Sonne Fahrt SO123 - MAMUT: Makran-Murray Traverse Geophysik plattentektonischer Extremfaelle Abschlussbericht

The main objective of the project was to derive a better understanding of the crustal structure and geodynamic evolution of the Makran accretionary wedge and Murray Ridge. The Makran Subduction Zone is characterized by a high input of sediments of about 7 km. Below the decollement, a sediment layer of about 3 km thickness is subducted beneath the accretionary wedge. Possibly a large portion of this sediment is added to the northern part of the wedge by underplating. In the landward part of the wedge, a sequence of changing high and low velocities is clearly visible. The subducting oceanic crust shows an unusual high velocity contrast between upper and lower crust. Pronounced wide-angle reflections support this feature. The interpretation of seismic data across Murray Ridge and Dalrymple Trough yield an undisturbed oceanic crustal structure north of the Dalrymple Trough. Here, a strong reflection between oceanic layer 2 and 3 is observed at large and small offsets. The transition from thick crust of the Murray Ridge to thin crust north of the Dalrymple Trough is interpreted as a transition from continental to oceanic lithosphere. South of the Murray Ridge the crust is particularly thin, with high seismic velocities. The bathymetric data of the Makran accretionary wedge show a NW-SE trending strike-slip fault, clearly cutting through the whole wedge. In combination with the merging plate boundaries of the Eurasian, Arabian and Indian Plates, the existence of a new microplate can be concluded in southeast Makran. (orig.)Available from TIB Hannover: F00B717+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung und Forschung (BMBF), Bonn (Germany)DEGerman

SO 96 - KODIAK-SEIS: Geophysikalisch-geochemische Untersuchungen des Aleutengrabens im Bereich der Kodiak-Inseln Abschlussbericht

The Alaska continental margin displays a strong structural diversity along strike. A high resolution bathymetry shows the pronounced variation in morphology. Sandbox analogue modelling suggests that various segments of the margin are in different stages of a cyclic deformational process. However, in addition to cyclic variation is a topography on the subducting plate that interrupts a steady state accretionary process. A gravity map was produced from ERS 1 altimeter data across the margin and adjacent deep sea areas. The map shows large scale geomorphic structures and indicates their horizontal extent. Crustal structure from the trench toward the volcanic arc was determined from refraction data. The inner shelf between Kodiak Island and the Kenai Peninsula displays unusally high velocities at the sea floor which are interpreted to show the uplift of this area from underplating. Beneath the zone of high near seafloor velocity is a series of arched reflections that define a large underlying low velocity body. This body corresponds to a pronounced gravity low. The detailed high precision velocity field derived from wide-angle and depth processing of the reflection line EDGE Alaska were used to quantify fluid escape from the accretionary prism. The depth processed image was balanced or restored to its original configuration which also constrained a tectonic interpretation. Three structural segments are recognised that indicate the tectonic processes involved in accretion. The trench is characterised by ductile deformation. Here fluid escape is blocked by the rapid trench axis sedimentation and pressure builds until fluid is vented along the deformation front. This venting is documented by direct observation of vent organism. Landward of the deformation front, faults extend to a detachment that separates the underthrust sediment from accreted sediment. Here the fluid escape is rapid across a zone of 8 km wide until the sediment is significantly dewatered. Farther landward in the classical imbricated part of the prism the dehydration has been largely accomplished and fluid escape is minimal. (orig.)Available from TIB Hannover: F97B1175 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Thermal structure and megathrust seismogenic potential of the Makran subduction zone

The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and recent, smaller earthquakes also suggest seismicity on the megathrust; however, its historical record is limited and hazard potential enigmatic. We have developed a 2-D thermal model of the subduction zone. The results are twofold: (1) The thick sediment cover on the incoming plate leads to high (~150°) plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a shallow depth, and (2) the shallow dip of the subducting plate leads to a wide potential seismogenic zone (up to ~350?km). Combining these results with along strike rupture scenarios indicates that Mw8.7–9.2 earthquakes are possible in the seaward Makran subduction zone. These results have important earthquake and tsunami hazard implications, particularly for the adjacent coastlines of Pakistan, Iran, Oman, and India, as the Makran has not been previously considered a likely candidate for a Mw?>?9 earthquake