|25 Jan. 2012
GSA Release No. 12-03
Director - GSA Communications & Marketing
GSA Bulletin Highlights:
New research posted ahead of print in January
Boulder, CO, USA – New GSA Bulletin postings discuss how subsurface data can be used to understand the form and origin of giant submarine landslides, give new clues to the tectonic history of the Eastern Cordillera, present an alternative theory on how the mountains along the Atlantic margin of northeastern Brazil formed long after the opening of the South Atlantic, integrate several kinds of geological dating for Upper Cretaceous rocks from the Pacific Coast of North America, and more.
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The initiation of submarine slope failure and the emplacement of mass transport complexes in salt-related minibasins: A 3D seismic reflection case study from the Santos Basin, offshore Brazil
Christopher Aiden-Lee Jackson, Department of Earth Science & Engineering, Imperial College, Prince Consort Road, London SW7 2BP, England, UK. Posted online 13 Jan. 2012; doi: 10.1130/B30554.1.
In this study, Jackson shows how subsurface data can be used to understand the form and origin of giant submarine landslides. He demonstrates that giant landslides can be triggered by the subsurface movement of salt. Giant blocks, which are several tens of meters in width, length, and height, can be contained in the deposits associated with submarine landsliding.
Discriminating rapid exhumation from syndepositional volcanism using detrital zircon double dating: Implications for the tectonic history of the Eastern Cordillera, Colombia
Joel E. Saylor et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, 78712, USA. Posted online 13 Jan. 2012; doi: 10.1130/B30534.1.
Uranium-lead (U-Pb) radiometric ages of zircon grains record their crystallization. Where volcanism is synchronous with deposition of sedimentary strata, zircon U-Pb ages approximate the age of their host strata. Zircon (U-Th)/He radiometric ages record the time at which they were cooled by being unearthed, often during mountain building. In zircons from sedimentary strata, these ages relate to the timing of mountain building in the sediment source region. Difficulty arises where volcanism occurs at the same time as rapid unearthing. Saylor et al. solve this problem by obtaining both U-Pb and (U-Th)/He ages from the same zircon grains. Zircon grains whose crystallization and cooling age are similar are of volcanic origin, while those with a large difference between these two ages were cooled as a result of exhumation during mountain building. The Colombian Andes resulted from an eastward-moving wave of mountain building that affected northern South America starting about 65 million years ago. Zircon grains from about 55-million-years-old sedimentary strata with about 55-million-year-old radiometric ages are of volcanic origin while the most rapid cooling due to mountain building occurred at about 35–40 million years ago. This suggests a change from a volcanism-dominated mountain range to one dominated by mountain building.
Episodic burial and exhumation in NE Brazil after opening of the South Atlantic
P. Japsen et al., Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark. Posted online 13 Jan. 2012; doi: 10.1130/B30515.1.
Mountains along passive continental margins such as southwestern Africa, southeastern Australia, and western India are commonly regarded as remnants from continental breakup. In contrast, Japsen et al. show that the mountains along the Atlantic margin of northeastern Brazil formed long after the opening of the South Atlantic. Their synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) Following Early Cretaceous breakup, about 110 million years ago, the margin underwent burial beneath a thick sedimentary cover. (2) Uplift and erosion which began around 80 million years ago led to almost complete removal of these deposits. (3) The resulting large-scale, low-relief Eocene erosion surface (peneplain) was deeply weathered and finally buried under a thick sedimentary cover about 25 million years ago (Early Miocene). (4) The formation of the present-day mountains began about 17 million years ago when uplift and erosion produced a new, lower-level peneplain by river incision below the uplifted and re-exposed, Eocene peneplain. Similar chronologies of uplift and erosion in Africa and the Andes suggest the controlling processes are global. Japsen et al. suggest that both vertical movements and lateral changes in plate motion have a common cause, which is lateral resistance to plate motion.
Integration of macrofossil biostratigraphy and magnetostratigraphy for the Pacific Coast Upper Cretaceous (Campanian–Maastrichtian) of North America and implications for correlation with the Western Interior and Tethys
Peter D. Ward et al., Department of Earth and Space Sciences, The University of Washington, Seattle, Washington 98195, USA. Posted online 13 Jan. 2012; doi: 10.1130/B30077.1.
This work, by Ward et al., integrates several kinds of geological dating for Upper Cretaceous (100 to 65 million years ago) rocks from the Pacific Coast of North America. The work greatly increases the resolution for dating fossils in these strata, and shows that many species of important fossils (ammonites) existed both along the Pacific Coast as well as in central North America in the Cretaceous period.
Evidence for middle Eocene and younger emergence in Central Panama: Implications for Isthmus closure
Camilo Montes et al., Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Ancón Republic of Panamá. Posted online 13 Jan. 2012; doi: 10.1130/B30528.1.
In a study by Montes et al., new geologic mapping and analytical data from central Panama greatly restrict the width and depth of the Central American Seaway, and challenge the widely accepted notion that closure of this seaway triggered northern hemisphere glaciation in late Pliocene times (about three million years ago). Geologic mapping revealed the presence of an angular unconformity—a geologic feature that separates strata of different ages, orientations, and affinities—along the southeastern flank of the San Blas Range, Panama. This angular unconformity separates nearly undeformed shallow marine strata above, from strongly folded and faulted rocks below, indicating a period of deformation and erosion followed by a period of sedimentation. Fossils above the angular unconformity date the time of deformation and erosion as prior to late Eocene times (about 37 million years ago). Similarly, analytical data from apatite and zircon crystals below the angular unconformity suggest that cooling related to deformation and erosion took place about 45 million years ago. Early Miocene (about 21 million years ago) fluvial strata in the Panama Canal Basin contain zircon crystals that match those found in the San Blas Range Range, further suggesting that the San Blas Range remained above sea level from about late Eocene to early Miocene times.
Neogene block-rotation in central Iran: Evidence from paleomagnetic data
Massimo Mattei et al. (Francesca Cifelli, corresponding), Dipartimento di Scienze Geologiche, Largo San Leonardo Murialdo 1, 00146 Roma, Università Roma TRE, Italy. Posted online 13 Jan. 2012; doi: 10.1130/G30479.1.
Central Iran is a mosaic of different tectonic blocks once separated by ocean basins that closed as a result of the convergence between the Arabia and Eurasia plates. Shortening related to the Arabia-Eurasia convergence in the Tertiary period has been taken up mainly in the Zagros, Alborz, and Kopeh Dag fold-and-thrust belts of Iran, whereas the intervening, fault-bounded crustal blocks of central Iran (Yazd, Tabas and Lut blocks) show little internal deformation. Central Iran is separated from the Alborz belt by northeast-southwest left-lateral strike-slip and thrust faults, whereas north–south right-lateral strike-slip faults define the boundary between the Tabas and Lut blocks within central Iran. Structural and seismological data from Mattei et al. suggest that northeast-southwest left-lateral and north–south right-lateral faults can accommodate the north/northeast-south/southwest Arabia Eurasia convergence if they are allowed to rotate clockwise and counterclockwise, respectively. Paleomagnetic results from Oligocene–Miocene sedimentary units confirm this model. In fact, counterclockwise rotations of 20°–35° have been measured in Central Iran, south of the Great-Kavir fault, characterized by the presence of north-south to north-northwest–south-southeast right-lateral strike-slip faults. These data show that part of the shortening related to Arabia-Eurasia convergence has been accommodated in Central Iran by vertical axis rotations of fault-bounded crustal blocks.