Tectonic, Volcanic, and Hydrological Controls on Rift Sedimentation
In this synthesis of current models for the initiation and evolution of continental rift zones, we discuss how the thermo-mechanical properties of the continental lithosphere produce topographic relief and control rift architecture. Our aim is to identify the key processes and parameters controlling the geometry and kinematics of continental rift basins, thereby providing a theoretical framework for fault arrays within rift basins. Case studies of the East African and Baikal rifts are used to illustrate rifting processes and the importance of lithospheric rheology in determining basin form.
On the basis of theory and observations, we suggest a rift classification scheme based on the lithospheric strength at the rime of rifting. At the length scale of individual rift basins, lithospheric rheology also plays an important role. Lengths of border faults in cratonic parts of the East African and Baikal rifts are 80-120 km, whereas border faults that form in younger lithosphere or collapsing orogens are on the order of 30-60 km, indicating that the dimensions of rift basins are also controlled by the mechanical properties of the lithosphere. Variations in lithospheric thickness between continental domains probably have corresponding variations in thickness of the mechanical lithosphere. Once created, these lateral variations in rheology clearly will concentrate stress along terrane boundaries, suggesting that terrane boundaries are susceptible to later tectonic episodes. These results indicate that we can develop predictive models of the regional morphology and along-axis segmentation of active and ancient rift basins based on a relatively small data base: short, narrow basins with narrow uplifted flanks will form in young, hot, weak, lithosphere and be characterized by < 15 km seismogenic layer thickness/effective elastic thickness; long, wide, deep basins with broad uplifted flanks will develop in old, cold, strong continental lithosphere with seismogenic layer thickness/effective elastic thickness a 30 km.
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Sedimentation in Continental Rifts
Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.