Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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4 Passive Margins: Group 1
4 Passive Margins: Group 1
Pages 29-44
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From page 29... ...
However, the kinematic theory of plate tectonics describes the motions of rigid plates on a sphere, whereas rifts are a deforming continuum of continental lithosphere governed by an array of processes that are either unaddressed or not predicted by pa ate tectonics per se. In the last decade, researchers have used a diverse range of approaches to studies of the rifting process that have resulted in a number of major advances.
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From page 30... ...
The last ten years have witnessed changes in thinking about Basin and Range extensional tectonism as profound as those a century ago when inquiry into the origin of the province began, including the discovery of a class of fault systems as significant to extensional tectonics as-the discovery of overthrusts was to compressional tectonics. Deep Seismic Reflection Profiling on Extended Continental Lithosphere and Laboratory Measurements of the Strength of Lithospheric Materials Marine- and land-based reflection surveys in many parts of the world show that the lower continental crust is often highly reflective in rifts, and that the Moho tends to be flat beneath rifted areas.
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From page 31... ...
Given that the sinuosity of hinge zones on passive margins occurs at a variety of scales' the segmented character of the East African rifts represents a significant new starting point for understanding processes at all later stages of continental separation. Marine Geophysical Studies of the Deep Structure of Passive Margins in the Central and North Atiant~c Oceans :s Major technological advances in shipboard acquisition of multichannel seismic reflection and refraction profiles, complemented by potential field studies, drilling, dredging, and submersible observations, have revealed -strongly contrasting end-products of continental separation.
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From page 32... ...
Theoretical Modeling of Subsidence and Thermal History in Sedimentary Basins and the Lithosphere as a Continuum Concomitant with-the measurement of large extensional strains of the continental lithosphere in areas such as the Basin and Range and the starved passive margin in the Bay of Biscay, a number of workers have-turned their attention to theoretical studies of the relationship between heat and mass transfer during crustal stretching and the syn- and post-rift vertical motion history recorded in sedimentary basins. Basin stratigraphic descriptions based on-multichannel seismic data and exploration drilling in regions like the U.S.
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From page 33... ...
THE SINGLE MOST IMPORTANT SCIENTIFIC OBJECTIVE The formation of sedimentary basins by extensional processes in a variety of tectonic settings is a fundamental expression of deformation on a lithospheric scale. The thermal and mechanical properties of the lithosphere before , during, and after extension are the key factors that determine the physical shape of sedimentary basins and the stratigraphic pattern of sediments that infill them during and after rifting.
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From page 34... ...
Thus, the single most important scientific Objective is to understand how the thermal and mechanical evolution of rift systems at crust to lithosphere scales controls the variability of continental margins in space and time. The objective is unlikely to be achieved without accepting as a prerequisite that basin forming processes and their expression in the lithostratigraphic record are intimately related.
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From page 35... ...
These field and thermochronologic studies should be complemented by rock mechanics experiments aimed at constraining the properties and deformation mechanism of faulted rocks e In addition, several potentially active low-angle normal fault systems in the Basin and Range (e.g., Sevier Desert detachment and Panamint Valley fault zone) offer the potential for in situ stress measurements adjacent to low-angle normal faults at depths 3S
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From page 36... ...
Structural and Magmatic Segmentation of Rifts in the Context of the Mechanics of Rifting and Heterogeneities of the Lithosphere A fundamental property of rift systems is variability in structure and geometry between and within them. Some studies (e.g., East African rift)
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From page 37... ...
To this end the suite of observational measurement suggested includes: I Geologic field mapping and sampling to define segmentation and the regional geologic framework (erg e ~ fault geometries, stratigraphic relations, pre-rift structural fabric, etc.~.
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From page 38... ...
Consideration of these phenomena in an evolving rift system has been only a minor component of most descriptions of extension, yet it must be of critical importance. There is an urgent need to extend what has been learned from numerical model ing o f f ~ ow and melt migrat i on at spreading centers into the more complex intra-continental rift environment to develop physical models of magmatism that include parameters describing rift geometry, separation rates, initial thermal structure, etc.
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From page 39... ...
The Transition from Rift to Ridge Magmatism A precise known edge of spatial and temporal red ations between extension and magmatis~a ranging from the earl lest stages of intra-continental rifting to mature seafloor spreading will shed light on the issue of whether the magmatism is solely a passive response to the extension of the lithosphere (i.e., decompressive melting of the upper mantle) or whether there is "active" upwelling of hot mantle intrudes the overlying lithosphere and localizes subsequent extension.
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From page 40... ...
For example, to test existing models of magma genesis in rift systems, we must utilize methodologies that adequately handle the often weathered rock record. Improved laboratory measurements of Vp and Vs in a variety of igneous samples at a range of confining pressures and percentage partial melt are needed to improve our understanding of velocities identif fed in the crust and upper mantle by the indirect seismic techniques employed.
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From page 41... ...
This can be obtained via a combination of field mapping and drilihole sampling of partially exposed systems, meshed with detailed seismic reflection-refraction grids set out with the primary goal of-calibrating the seismic information with characteristics of each of the major classes of deposits. The geological characterization of geophysical signatures in various portions of rifts is an important first step in developing detailed subsidence history.
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From page 42... ...
Of particular importance for most models is determining the degree to which isostatic compensation for various features is local or regional, using various admittance techniques on gravity spectra. Uplift and Subsidence as an Independent Constraint on the Overall Structure of the Rift -While detailed mapping and seismic imaging of the upper crust can constrain its structural history, deeper crustal phenomena are more difficult to constrain directly with these methods.
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From page 43... ...
There is important feedback in cause and effect with respect to sedimentation and tectonism. Where sediment supply is abundant, rift valleys fill more rapidly and the footwalls of bounding normal faults are less readily deformed compared to areas where sediment is in short supply.
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