Of1989 01 Late Cenozoic Evolution Of The Southern Great Basin

Author : Adam Matthew Forte
Publisher :
ISBN 13 : 9781267970350
Total Pages : pages
Book Rating : 4.9/5 (73 download)

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Download or read book Late Cenozoic Evolution of the Greater Caucasus Mountains and Kura Foreland Basin written by Adam Matthew Forte and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The structure, morphology, and evolution of large orogenic systems have fundamentally informed our first-order understanding of how strain is accommodated within the Earth's crust. Yet, because of the inherently destructive nature of mountain building processes and the maturity of most well-studied orogens (e.g. the Alps, Himalayas, Andes), the early stages of orogeny are seldom preserved or difficult to interpret. As a result, our understanding of early orogenesis remains incomplete. However, because later stages of orogenic growth are heavily influenced by pre-existing structural architecture, characterizing processes that are fundamental in either the structural development or preservation of the structural record in young orogenic systems is essential. The Greater Caucasus (GC), which form the northern margin of the Arabia-Eurasia collision between the Black and Caspian Seas and represent the main locus of shortening within the central collision, are an active, young range (~5 million years old) and so represent a locality to investigate these early orogenic processes. In detail, the first order structures within the Greater Caucasus are poorly understood, with most previous work assuming that the majority of shortening is localized along the southern range-front of the range. The new structural and stratigraphic work presented here, indicates that the majority of post Pleistocene shortening is localized within the Kura Fold-Thrust Belt, a narrow belt of elevated topography ~20 km south of the GC range front in Azerbaijan and Georgia. In detail, initiation of the Kura Fold-Thrust Belt began between 2 and 1.5 Ma and has shortened at an average rate of ~8 mm/yr, equivalent to the current geodetically measured total shortening rate in this region. The Miocene to Pleistocene stratigraphic record of the Kura Basin, exposed in the Kura Fold-Thrust Belt, records the growth of the GC and elucidates earlier stages of their development. The stratal architecture, as defined in five measured sections within the fold-thrust belt, and provenance analysis from a suite of samples taken from within these measure sections, suggest that the southward propagation of deformation into the Kura Fold-Thrust Belt is the most recent stage of progressive southward movement of the active thrust front. The initial southward propagation formed the modern day range front of the GC and exposed a distinctive mafic to intermediate volcanic and volcaniclastic source along the southern range front. This package originally formed the dominant source for sediments deposited during the early Pliocene within the northern Kura Basin. As progressive movement on this new thrust system continued, the thrust sheet containing the volcanics was mostly eroded away, with only a small amount of this material left in the modern geology of the range. While the provenance record of these strata faithfully record the structural evolution of the Greater Caucasus, the distribution of facies and the paleogeography of the northern Kura Basin appears largely insensitive to the growth of the > 5 km high Greater Caucasus. The spatial and temporal distribution of facies is instead primarily controlled by rapid (103 - 105 years) and large (> 1 km) changes in the base-level of the Caspian Sea. The Caspian Sea became separated from the open ocean during the progressive consumption of the Tethys Ocean as part of the larger Alpine-Himalaya collision, of which the Caucasus are a part, making the Caspian an internally drained basin. Once this occurred, the base-level of the Caspian Sea became chaotic, dependent on the climatically mediated hydrologic balance, sediment flux into the basin, and dynamic topography in the form of subsidence. In addition to exerting a primary influence on the distribution of facies within the Kura Basin, the large and rapid changes in the base-level of the Caspian significantly impact the geomorphology of the Greater Caucasus. Most rivers within the GC deviate significantly from equilibrium profiles, suggesting that as a whole the topography of the GC are not in a steady state. This likely results from waves of incision and aggradation within the river systems formed in response to fluctuations in Caspian base-level. An additional important factor in the non-steady state, or transient, nature of the GC is the apparent collision between the Greater and Lesser Caucasus Mountains. As evidenced from both the neotectonic geology within the Kura Fold-Thrust Belt and the older structural evolution of the main GC derived from the provenance history, the Greater Caucasus have been progressively expanding primarily via propagation of thrusts southward. This suggests that in terms of a bi-vergent orogenic wedge, the southern side of the range represents the pro-wedge. Recently within the tectonic history of the Greater Caucasus, the central portion of the southward propagating pro-wedge encountered an obstruction, namely the Lesser Caucasus Mountains. This effectively increases the basal friction within the pro-wedge, forcing the mountain range to begin deforming along the northern margin of the range in order to continue accreting material into the range to offset material lost through erosion. This has fundamentally altered the structural geometry of the range, with a nascent fold-thrust belt forming within the northern, central portion of the GC and also reinitiating structures within the GC hinterland. The results from this work highlight two key processes that are common during the early stages of continental collision. The first is the progressive closure of ocean basins and the formation of internally drained, segmented lake basins with chaotic base-levels. These changes in base-level can form the first-order control for the distribution of facies, obscuring direct links between stratigraphic architecture and orogenic evolution. The second is the result of two smaller mountain ranges colliding. This is likely a process common during terrane accretion as part of larger collisional events. The collision of these two ranges can effectively change the structural properties within the collision zone and can dramatically alter the structural evolution of one or both of the orogenic systems. Together these two processes have the potential to dramatically shape both the first-order evolution of an orogenic system, but also the degree to which this evolution can be interpreted from the stratigraphic record.