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Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes
James M. McLelland, Department of Geology, Colgate University, Hamilton, New York 13346, USA, et al. Pages 1299-1317.
Keywords: anorthosite, AMCG, geochronology, zircon, SHRIMP, Adirondacks.
A large portion of the Adirondack Mountains in upstate New York is underlain by a distinctive igneous rock known as anorthosite. All of the 46 High Peaks are situated within a large (70 × 50 mile) body of this rock referred to as the Marcy Massif. Precise dating of intrusion of the massif has been achieved by submitting sparse populations of the mineral zircon to U-Pb analysis by sensitive high-resolution ion microprobe (SHRIMP). Results for 13 samples indicate that the average age of anorthosite intrusion is 1154 ± 4 Ma. Similar dating of 15 rocks representative of orthopyroxene-bearing granitic rocks associated with the anorthosite yields an average age of 1158 ± 5 Ma. This result is within error of the emplacement age for the anorthosite and documents that the anorthosite and granitic rocks represent a coeval association of a type known as an AMCG suite. A variety of field and laboratory observations indicate that the anorthosite and granitic rocks did not evolve from a common parent. Instead, they represent a bimodal suite in which the anorthosites were derived from the mantle, while the granitic rocks are the melting products of deep continental crust. The heat responsible for the crustal melting was derived from the anorthosites and related gabbros. This upwelling of igneous rocks followed a collision of continental plates in the region during the interval 1200-1160 Ma.
Rock-type identification in a simulated underground environment using second-derivative thermal infrared reflectance spectra
Jilu Feng, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada, et al. Pages 1318-1326.
Keywords: rock cores, spectroscopy, thermal, underground mining.
New technology could provide geologists with a remote-controlled "set of eyes" that will dramatically improve the safety, quality, speed, and efficiency of underground mining operations. The study uses remote sensing and infrared technology to determine rock types from samples collected in mines. Major mining companies see this research as another step in ultimately designing completely tele-operated and automated mines. Geologists currently have to go into the mines and visually inspect the samples, making judgments on rock types and mineral content.
The Geology of the Darien, Panama, and the late Miocene-Pliocene collision of the Panama arc with northwestern South America
Anthony G. Coates, Smithsonian Tropical Research Institute, Republic of Panama, et al. Pages 1327-1344.
Keywords: Neogene, stratigraphy, paleobathymetry, Darien, Panama, Central American Isthmus.
This article provides an account of the geological history, and the first integrated geologic map, of the Darien region of Panama through detailed lithostratigraphic, biostratigraphic, and paleobathymetric analyses of several Neogene sections. This region is the last unstudied portion of the Central American isthmus and is key to understanding the timing of the various stages in the collision of Central America with South America during the Miocene. This collision resulted in the emergence of the Central American isthmus at the end of the Miocene, from which followed the Great American Biotic Interchange, a profound reorganization of ocean realms and circulation, global climate change, and major evolutionary and ecological changes for species in the sea.
Controls on sequence stratigraphy of a middle Miocene-Holocene, current-swept, passive margin: Offshore Canterbury Basin, New Zealand
Hongbo Lu, Department of Geological Sciences and University of Texas Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas, Austin, Texas 78712-0254, USA, and Craig S. Fulthorpe, University of Texas Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, Austin, Texas 78759-8500, USA. Pages 1345-1366.
Keywords: continental shelf, continental slope, multichannel seismic, sequence stratigraphy, sediment drift, New Zealand.
The offshore Canterbury Basin, eastern South Island, New Zealand, exemplifies sequence development on a prograding, passive continental margin strongly influenced by submarine currents. Multichannel seismic reflection profiles, tied to boreholes, reveal that the middle Miocene (about 16 million years ago) to Recent margin is constructed from nineteen unconformity-bounded, sedimentary units (sequences). Correlation of sequence boundary ages with the oxygen isotopic record, a proxy for global climate change, suggests that cycles of global sea-level have controlled the timing of sequence boundaries. In contrast, the three-dimensional geometries of the preserved sequences are locally controlled by ocean currents, which built multiple, large, mounded sediment drifts.
Evolution of a terminal Neoproterozoic carbonate ramp system (Buah Formation, Sultanate of Oman): Effects of basement paleotopography
Andrea Cozzi, Institute of Geology, Department of Earth Sciences, Zürich, Switzerland, et al. Pages 1367-1384.
Keywords: carbonate ramp, carbon isotopes, correlation, basin analysis, Late Neoproterozoic, Oman.
The spectacular Terminal Proterozoic outcrops of north and central Oman revealed that, approaching the Precambrian-Cambrian boundary, carbonate rocks of the Buah Formation were deposited in a shallow sea in a distally steepened carbonate ramp depositional setting. Storms and tides shaped the ramp facies belts, which were ultimately controlled in their areal distribution, on a regional scale, by the presence of long lived slowly subsiding basement highs. These facies belts prograded laterally for hundreds of kilometers, as demonstrated by lithologic and chemostratigraphic correlation.
The Piermont allochthon revisited and redefined at its type locality
Nicholas E. Timms, Department of Earth Sciences, James Cook University, Townsville, Queensland, Australia. Pages 1392-1407.
Keywords: New Hampshire, Acadian tectonics, Piermont allochthon, synmetamorphic detachment, metaconglomerate.
New geological field mapping of the Orford to Piermont area of northwestern New Hampshire indicates that there are two deformed rock masses with distinctly different deformation histories. These two rock masses have been juxtaposed by subhorizontal ductile faulting during a complex deformation history that involved the formation of the Appalachian Mountains in what is known as the Acadian Orogeny. Rocks of the upper unit contain pebble conglomerate and were rafted over complexly deformed rocks containing intrusions below. The intensity of deformation in pebble clasts increases toward the lower boundary of the upper unit and probably occurred during ductile faulting.
Tectonics of Pliocene removal of lithosphere of the Sierra Nevada, California
Craig H. Jones, Department of Geological Sciences and CIRES, University of Colorado, Boulder, Colorado 80309-0399, USA, et al. Pages 1408-1422.
Keywords: Sierra Nevada, Cenozoic, tectonics, delamination, orogeny.
The dominant physical features of California-the Coast Ranges, the Sierra Nevada, and the desert of the Great Basin-might all be the product of a single event, as reported in this paper. The uplift of the Sierra by over 1000m (3300 ft) is confirmed to be only 3-5 million years old; this coincides with the development of faults east of the Sierra in areas like Reno and Owens Valley and the emergence of the Coast Ranges. Previously workers have suggested the southern Sierra rose as dense material dripped off the bottom, rather like a submarine rises as it drops ballast. In the new work, this dripping is thought to extend all the way north to north of Lake Tahoe. This then causes faulting on the east side of the Sierra, effectively driving the Sierra and Great Valley westward away from North America and into the Pacific Plate. As the Sierra/Great Valley plate collides with the stiff Pacific Plate, the Coast Ranges rise up on a number of thrust faults, such as those rupturing in 1983 at Coalinga and in 2003 near San Simeon. This work also indicates that the San Andreas Fault should be slowing as the Eastern California Shear Zone slips at a higher rate due to the changes on the east side of the Sierra Nevada. The development of such prominent physical features from the removal of dense material in the upper mantle illustrates a different style of tectonics from those driven directly by the motion of rigid plates.
The Cottage Grove fault system (Illinois Basin): Late Paleozoic Transpression along a Precambrian Crustal Boundary
Amanda B. Duchek, Department of Geology and Illinois State Geological Survey, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA, et al. Pages 1465-1484.
Keywords: seismic reflection, strike slip, Illinois basin, folding, Precambrian, transpression.
Continental strike-slip faults remain one of the most enigmatic structures for geologists because they can be difficult to recognize in the field, frequently have complex interrelationships with dip-slip faults, and are difficult to kinematically reduce to two dimensions. This study presents a relatively detailed interpretation of a classic intra-plate strike-slip system in the southern Illinois Basin, the Cottage Grove fault system, by using an integrated data set of industry seismic reflection profiles, bore-hole logs, outcrop information, and underground coal mine mapping. The authors show that the entire Phanerozoic section beneath the fault is deformed into contractional features (e.g., flower structures), which are governed by three-dimensional complexities in the shape of the fault surface. It is also shown that the Cottage Grove fault system apparently marks a fundamental crustal boundary on the basis of magnetic intensity data and contemporary seismicity patterns.
Crustal structure, fossil subduction, and the tectonic evolution of the Newfoundland Appalachians: Evidence from a reprocessed seismic reflection survey
Arie J. van der Velden, Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada, et al. Pages 1485-1498.
Keywords: Newfoundland, Appalachian orogeny, crustal structure, seismic reflection profiles, Lithoprobe, subduction zones.
Van der Velden et al. (2004) present seismic images of the crust of Newfoundland to depths of 60 km. The seismic data were acquired by Lithoprobe, Canada's national geoscience project (www.lithoprobe.ca). Two interpretive cross sections are presented, which cross the island in the center (Corner Brook to Belleoram) and in the south (Stephenville to Burgeo). The seismic images reveal structures related to the Appalachian orogen, the 500 to 350 million year old mountain building event that in Newfoundland records the accretion of two Pan-African microcontinents to the eastern margin of North America through closure of the Iapetus Ocean. Northwest-dipping seismic reflections in the lower crust and upper mantle are interpreted as a fossil subduction zone that accommodated convergence between the North American margin and Ganderia, one of the accreted microcontinents. The Meelpaeg allochthon, a sheet of partially molten rocks, is interpreted to have been extruded from the middle crust in southeastern Newfoundland toward the surface in central Newfoundland. The Baie Verte-Cabot fault can be followed from the surface along a near-vertical zone of reflection truncations to the lower crust. The crust is about 35 km thick beneath Newfoundland, and the seismic crust-mantle boundary may have been established through partial melting of lower crust.
The Lackawanna synclinorium, Pennsylvania: A salt-collapse structure, partially modified by thin-skinned folding
Michael J. Harrison, Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA, et al. Pages 1499-1514.
Keywords: central Appalachians, salt structures, Appalachian Plateau, Valley and Ridge province, fold-and-thrust belts, Alleghanian orogeny. The Lackawanna synclinorium (Northern Anthracite basin) in northeastern Pennsylvania is a large structural basin containing abundant coal and the cities of Scranton and Wilkes-Barre. The 110-km-long basin formed primarily by the removal of thick (up to 700 m) subsurface salt deposits and tectonic activity associated with the formation of the central Appalachian Mountains. The synclinorium is the largest known salt-collapse structure in the Appalachians.
Tectonic setting of the Jurassic Smartville and Slate Creek complexes, northern Sierra Nevada, California
Howard W. Day, Department of Geology, University of California, Davis, California 95616, USA, and M.E. Bickford, Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA. Pages 1515-1528.
Keywords: California, ophiolite, tectonics, geochronology, Smartville complex, Slate Creek complex.
Jurassic volcanic rocks (about 206 to 144 million years old) in California and Oregon are found west of Jurassic and older rocks that are interpreted to be the edge of the North American continent at that time. Two competing hypotheses have been advocated for the origin of the volcanic rocks. One argues that the rocks represent a volcanic arc (chain of volcanic islands similar to Japan) that traveled perhaps thousands of kilometers across the ocean to the west on a different plate, which ultimately collided with North America. The other argues that the volcanic arc developed near or on the edge of the North American continent and was incorporated into the continent by deformation related to changes in plate motions during the Jurassic. We report new and revised U/Pb (zircon) ages for Early, Middle, and Late Jurassic rocks in the volcanic terranes of the western Sierra Nevada, California. The new ages confirm the Jurassic ages of several volcanic complexes in the region and reveal that the magmas associated with them are widely contaminated by inherited Precambrian zircon as old as 2.5 billion years. The isotopic signatures of the rocks suggest they were derived from the Earth's mantle, but the contamination could only have come from continental sources. The ages of the inherited zircon are consistent with a North American origin and the most plausible inference is that these volcanic rocks originated at or near the North American margin.
Paleoenvironments of the earliest stone tool makers, Gona, Ethiopia
Jay Quade, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 1529-1544.
Keywords: Gona, Ethiopia, carbon isotopes, early hominids, Oldowan, first stone tools, Awash, paleoenvironment.
The badlands of east-central Ethiopia have yielded some of the most spectacular human fossil finds in the world, fossils spanning most of the past 5 million years. An area in the center of this fossil-rich region, called Gona, is especially important not only for its human fossils but also because it contains the oldest known stone tools in the global archeological record. The focus of our research is on placing these toolmaking early humans at Gona in a firm paleoenvironmental context. Our results reveal that these early humans, perhaps the earliest representatives of the genus Homo, inhabited the mixed grasslands and forest on floodplains of a perennial river that flowed through the area. The first appearance of stone tools at 2.58 Ma is stratigraphically abrupt and occurs just above a major disconformity in the record representing a ~200,000 year time gap. This suggests that older stone tools may turn up in the 2.6-2.9 Ma time period.
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