Boulder, CO, USA – The December Geosphere, The Geological Society of America's e-journal, is now online. Topics include detailed data integration from multiple fields, including tectonics, oceanography, sedimentology, and paleontology, to study the southwestern U.S. climate 17 million years ago to 6 million years ago; sedimentation in a piggyback basin; Angel Lake orthogneiss in the East Humboldt Range, Nevada; and a study of the South Balkan extensional system within southern Bulgaria, Macedonia, eastern Albania, northern Greece, and northwestern Turkey.
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Interplay of oceanographic and paleoclimate events with tectonism during middle to late Miocene sedimentation across the southwestern USA
Charles E. Chapin, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
This paper uses sedimentation and erosion history as proxy for changes in the climate of the southwestern USA during the middle and late Miocene (ca. 17 Ma). The continental sedimentary record was compiled along a 2000-km transect from coastal California to the western Great Plains and correlated with the coeval marine Monterey Formation. Integration of data from the fields of plate tectonics, oceanography, sedimentology, paleoclimatology, and paleontology were used to interpret the causes and effects of major changes in the continental and marine sedimentary records. Two major changes in climate were interpreted: (1) a multiphase climate transition (ca. 17 Ma) caused mainly by changes in ocean circulation due to opening and closing of seaways connecting ocean basins and growth of the East Antarctic Ice Sheet—these climate changes were from warmer and wetter to cooler and drier; and (2) intensification of the North American monsoon caused by opening of the Gulf of California at ca. 6 Ma. A dramatic increase in summer thunderstorm activity resulted in integration of drainages, termination of closed-basin sedimentation, incision of uplifts, and exhumation of basin-fill sedimentary deposits. Such changes are usually interpreted as resulting from tectonic uplift; however, the complex interplay of tectonics and climate, rather than one or the other, was responsible for the Miocene changes in the southwestern United States.
Evolution and dynamics of the Cenozoic tectonics of the South Balkan extensional system
B.C. Burchfiel et al., Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 01239, USA. The work reported in this paper received NSF funding.
This paper discusses the tectonic evolution of the Southern Balkan extensional system from its beginning as an Andean margin from Late Cretaceous to early Eocene, through a transition from Andean to extension settings from middle Eocene to Oligocene time, to a regional extensional setting from the early Miocene to present. The regional tectonic setting begins as part of the Aegean extensional system but evolves into a system that is largely decoupled from the Aegean system as the North Anatolian fault develops from Late Miocene to Recent time. Late Cenozoic extension within the South Balkan extensional system is strongly diachronous, with north-south and east-west normal faulting that migrates in tandem from east to west, respectively, an expression of slab differential roll back along northern and southern parts of the Hellenic subduction zone. Extension in the Hellenic orogen was accompanied by late Cenozoic clockwise rotation of up to 50 degrees. The South Balkan extensional system is an example of the complex temporal and spatial relations between geometry, kinematics, and dynamics of lithospheric structure during the Cenozoic evolution of Mediterranean backarc extensional regions.
Source: Geological Society of America