The Ocean-Continent Transition at Rifted Continental Margins: What Is It, How Is It Formed, and How Do We Locate It? III
Presiding: G Manatschal, Centre de Geochimie de la Surface, Universite Louis Pasteur; I Norton, ExxonMobil Upstream Research Company
T51A-01 08:30h
Non-volcanic rifting: New processes and new materials
The study of non-volcanic rifting of continental lithosphere to form new oceans is encountering a number of new processes and new materials. It is now well established that upper mantle peridotite has been exposed at the seafloor during rifting. This exposure seems to have occurred by processes similar to the exposure of middle and lower crustal rocks in metamorphic core complexes and the exposure of upper mantle rocks, "oceanic megamullions," at slow spreading mid-ocean ridges. The serpentinization of this peridotite, both where it is exposed and where it underlies extending continental crust, creates a "slippery" surface for possible low-angle normal faults to operate. The serpentinized material may also behave in a diapiric fashion to create ridges and other seafloor topography. These new materials, beyond the "normal oceanic crust" and "normal continental crust" that we used to talk about, make the task of identifying an "ocean-continent" boundary increasingly difficult, and probably irrelevant. Rifting seems to create a continuum of oceanic and continental processes, whose details no doubt vary from margin to margin. Our effort needs to focus on understanding these processes. Non-volcanic rifted margins are potential laboratories for studying progressive rifting of continental crust. We see clear examples of tilted fault blocks that have been cut by a second generation of faults. The earlier generation of faults created larger tilted fault blocks with the bounding faults soling in the middle crust. The next generation of faults cut to the crust mantle boundary and bound smaller fault blocks. Unraveling the history of faulting and deformation in these environments awaits the availability of modern 3D seismic reflection data. These data will give us the capability to correlate stratigraphy from basin to basin around the ends of the tilted fault blocks, and hence to get relative dates on the formation of the individual faults. These data will give us 3D constraint on the shape of the faults and fault bounded blocks. Palinspastic restoration of these will help us understand the large scale deformation of the margin.
T51A-02 09:00h
Application of a New Model of Continental Lithosphere Breakup and Sea-floor Spreading Initiation to the Woodlark Basin Western Pacific
A new model of continental lithosphere thinning leading to continental breakup has been applied to the most recent segment of sea-floor spreading initiation in the Woodlark ocean basin in the western Pacific. Sea-floor spreading in the Woodlark ocean basin commenced approximately 8 Ma ago in the east and has propagated westwards with time reaching the Moresby sea-mount region at the present day. The most recent segment of westward sea-floor spreading propagation initiated at the beginning of the Brunhes epoch. To the west of the Moresby sea-mount, ahead of the propagating spreading tip, the extending continental crust is characterised by up to 3 km of sag-basin subsidence over a width of 150-200 km, but with little upper crustal faulting implying depth-dependent lithosphere stretching. Significant but local brittle deformation is associated with the late-stage (<0.5 Ma) Moresby fault system. Depth-dependent lithosphere thinning, in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust, has been observed at many non-volcanic and volcanic rifted continental margins including conjugate margin pairs. Our new model of continental lithosphere thinning leading to continental breakup and sea-floor spreading initiation assumes that lithosphere thinning occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere. Depth-uniform stretching of continental lithosphere cannot explain observed depth-dependent lithosphere stretching. Model formulation uses corner-flow predicted by kinematic iso-viscous stream-function or finite element solutions to advect continental lithosphere and asthenosphere material and their temperature fields. Corner-flow is defined by Vx, the divergence half-velocity, and Vz, the vertical upwelling velocity. Thinning of continental margin crust and lithosphere temperature field evolution is sensitive to the velocity ratio Vz/Vx. The new model of continental breakup and sea-floor spreading initiation successfully predicts the observed bathymetric profile of young ocean floor and rifted continental margin for the most recently initiated segment of Woodlark Basin sea-floor spreading to the east of Moresby sea-mount. The preferred model of Woodlark sea-floor spreading initiation and rifted margin formation requires that the upwelling divergent flow field propagated upwards from the base of the lithosphere from 8 Ma to 2 Ma leading to continental breakup and sea-floor spreading initiation at approximately 1 Ma.
T51A-03 09:15h
The Effects of Gravitational Instability on the Tectonic Evolution of Continental Orogens
Convergent continental orogens have been the research focus of numerous geological and geophysical surveys and dynamical modeling studies, but the mechanisms controlling the evolution of these systems are still under debate. The temporal and spatial distribution of tectonic processes in orogenic regions is complex, and the interplay between buoyancy forces and regional tectonics in some of these systems is still not well understood. In several cases the extensional collapse of mountain belts is associated with dramatic thinning of the mantle part of the lithosphere, more so than the crust. A key region for investigating these processes is the Carpathian-Pannonian system of eastern Europe, where rapid extension took place in the Pannonian Basin simultaneously with contractional deformation in the surrounding orogens. Previous studies show that gravitational instabilities may play a fundamental role in the tectonics of mountain ranges. In general, the lithosphere is colder and thereby denser than the underlying asthenosphere. Under some circumstances this may cause the lithosphere to sink into the underlying asthenosphere. We use dynamic numerical models to investigate how such gravitational instabilities may affect the evolution of continental orogens. We show how a crust initially thickened by localized convergence may promote lithospheric gravitational instabilities that cause the collapse of high topography and focused, depth-variable lithospheric thinning developing simultaneously with contractional deformation and lithospheric downwelling in the adjacent areas. We investigate the relative importance of buoyancy and regional tectonics in convergent continental orogens and show how density and viscosity (Newtonian or non-Newtonian) affect the evolution of a model system. We investigate the distribution and amplitude of lithospheric downwelling and examine how the instability develops adjacent to the corner of an initially rectangular region of thickened crust in a full 3D flow model. Observations of lithospheric thickness, Moho and surface topography, sediment distribution, and the observed spatial and temporal distribution of tectonics provide constraints on the numerical models that may lead to an improved understanding of the mechanisms that control the evolution of continental orogens.
T51A-04 09:30h
Seismic Behavior of OBS Data Recorded in the Southern Okinawa Trough
A network of four ocean bottom seismographs (OBSs) deployed in the southern Okinawa Trough for 4 weeks were studied the seismic behavior of the regional earthquake events. We combined the OBS data with available land data in Taiwan to relocate the hypocenters with different models and picked the result with the overall minimum RMS residual for analysis. The locations determined with OBS data incorporated improve the locating quality and hypocenters are shifted toward OBS network while their foci are deepened. It implies that offshore seismicity based on land network data alone may be mislocated with shorter epicentral distances and shallower depths. Factors such as azimuth, depth, distance and site effect seem to affect the frequency contents of OBS signatures. Signals propagated along the axis of the Okinawa Trough generally show lower frequencies. Distance and depth affect the frequency content but not so prominently as does due to the azimuth factor. Many seismic signals missed at OBS1 at the north side of the trough axis are possible due to the faulted walls and fractured crust associated with the rifting which induces a shadow zone to prevent seismic waves from arriving at the station. Some of the remarkable low frequency signals might be caused by the magma chambers beneath the trough such that the shear waves even disappeared in most seismograms observed at OBS2. The microearthquakes exhibit two types of focal mechanisms: the rifting and the subducting types. Focal mechanisms of deeper events from the subducting slab are thrusts with maximum principal axes orientated NW-SE, consistent with the direction of the convergent plate motion between the Eurasia and the Philippine Sea plates. Shallow events within the rifting trough possess the normal faulting mechanisms related to the back arc spreading.