||20 October 2006
GSA Release No. 06-46
November / December Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - Topics include: eruptive history of ancient Mt. Mazama beneath Oregon's Crater Lake; new evidence from Antarctica and South Africa of the extent of the Middle-Late Permian mass extinction on land; and dynamics of great subduction earthquakes at the Chilean convergent margin.
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- Timing of anatexis in metapelites from the Adirondack lowlands and southern highlands: A manifestation of the Shawinigan orogeny and subsequent anorthosite-mangerite-charnockite-granite magmatism
Matthew J. Heumann and M.E. Bickford (corresponding author), Syracuse University, Earth Sciences, Syracuse, NY 13244-1070, USA, et al. Pages 1283-1298.
- Keywords: Adirondacks, zircons, anatexis, monazites, Grenville orogenic cycle, Shawinigan orogeny.
- Anatectic melting of metapelites (former shales) in the Adirondack Mountains of northern New York occurred about 1170 million years ago according to research by Heumann et al. The study showed that primary anatectic melting, which occurred at temperatures near 800 °C, and deformation of these strongly metamorphosed rocks occurred during the 1170-million-year-ago Shawinigan orogeny and subsequent emplacement of igneous rocks of the anorthosite-mangerite-charnockite-granite suite, rather than during the later, approximately 1040-million-year-ago Ottawan orogeny, as had been thought previously. An implication is that the Shawinigan orogeny was more widespread and more thermally intense than previously thought. Heumann et al. used the sensitive high-resolution ion microprobe (SHRIMP) instrument at the Geological Survey of Canada in Ottawa, as well as an electron microprobe micro-analyzer at the University of Massachusetts, to date both zircon (ZrSiO4) and monazite (cerium phosphate) minerals to unravel the melting history.
- Subsurface magnetostratigraphy of Pleistocene sediments from the Po Plain (Italy): Constraints on rates of sedimentation and rock uplift
Giancarlo Scardia, UniversitÓ di Milano-Bicocca, Dipartimento di Scienze Geologiche e Geotecnologie, Milano 20126, Italy; et al. Pages 1299-1312.
- Keywords: Pleistocene, magnetostratigraphy, Po Plain, Alps, rock uplift, climate variability.
- In the subsurface of the Po Plain, northern Italy, marine sediments are preserved, testifying the presence of a broad sea in the past. During a drilling program in the Po Plain, the youngest marine sediments were dated back to the early Pleistocene, and they were found to lie above the present sea level. In this paper, Scardia et al. analyze and try to explain the vertical uplift of these sediments, suggesting a relationship with the onset of the Pleistocene Ice Age. According to their conclusions, the cumulative strong erosion produced by the glaciers along the middle-late Pleistocene has progressively removed huge amounts of rock from the Alps inducing an isostatic uplift of the mountains and involving the proximal sector of the Po Plain. The uplift rates are estimated in the order of about 90-120 m in the middle-late Pleistocene.
- Small-volume basaltic volcanoes: Eruptive products and processes, and posteruptive geomorphic evolution in Crater Flat (Pleistocene), southern Nevada
Greg A. Valentine, Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM 87545, USA; et al. Pages 1313-1330.
- Keywords: scoria cone, basalt, lava flow, polycyclic, monogenetic, Strombolian, violent Strombolian.
- Small scoria-cone volcanoes are the most abundant continental volcanic landform, and they present hazards to cities such as Mexico City and Auckland (New Zealand) that are built within scoria cone volcanic fields. The proposed Yucca Mountain radioactive waste repository also resides within such a volcanic field. The processes associated with such volcanoes therefore are of interest both from the perspectives of understanding basaltic magmatism and of risk assessment. Valentine and co-authors present detailed descriptions and interpretations of a complex range of processes associated with eruption of small scoria cone volcanoes in southern Nevada. They show that, contrary to previous interpretations, the pyroclastic and lava products of these volcanoes all vented from the main scoria cones rather than multiple scattered vents at each volcano. Cone growth was interrupted by failure of cone segments and rafting of the segments on top of lava flows. Emplacement of lava flows also caused local reversals in the slopes around the growing cones, which in turn resulted in migration of lava boccas around cone bases and development of multiple lava fields. Eruption and emplacement processes produced distinct types of volcanic surfaces, which in turn have experienced different geomorphic evolution. This suggests that such volcanic surfaces, if they can be dated, provide a test bed for understanding the effects of initial conditions on geomorphic evolution in arid settings. This work helps to constrain the range of processes that might be associated with small basaltic volcanoes and, therefore, the types of effects they might have from a risk assessment perspective.
- Eruptive history and geochronology of Mount Mazama and the Crater Lake region, Oregon
Charles R. Bacon and Marvin A. Lanphere, U.S. Geological Survey, Volcano Hazards Team, Menlo Park, CA 94025-3561, USA. Pages 1331-1359.
- Keywords: geochronology, volcanology, argon, calderas, arc volcanism, Crater Lake, Mount Mazama.
- The volcano known as Mount Mazama collapsed to form Crater Lake caldera during a violent explosive eruption approximately 7700 years ago. Geologic mapping of Mount Mazama and potassium-argon dating of its lava flows are used to reconstruct the eruptive history of this Cascade volcano, and of smaller volcanoes nearby, during the past 400,000 years. Physical characteristics of the lavas indicate extensive ice cover during independently established glacial intervals. Changes in erupted magma volume and composition through time suggest that Mazama grew in pulses, and that the silica-rich magma of the climactic eruption accumulated at a few kilometers depth between about 30 and 8 thousand years ago during a period of especially intense regional volcanism.
- Emplacement of the Kodiak batholith and slab-window migration
David W. Farris, University of Southern California, Earth Sciences, Los Angeles, CA 90089, USA; et al. Pages 1360-1376.
- Keywords: slab window, pluton displacement, Alaska, Kodiak, tectonics, ridge subduction.
- This paper describes the physical evolution of the Kodiak batholith located on Kodiak Island, Alaska, and how its magma interacted with and penetrated through the surrounding accretionary prism sedimentary rocks. The Kodiak batholith is one of the largest near-trench intrusions in the world, and it has been previously theorized to have formed at the intersection of three tectonic plates migrated eastward along the Alaskan margin. New geochronologic data yielded ages ranging from 59.2 to 58.4 ± 0.2 million years old, which supports the above interpretation. These data suggest that the location of the triple junction is marked by Kodiak batholith plutons on a time and length scale of tens of kilometers and hundreds of thousands of years, respectively.
- Stratigraphic record of Pleistocene faulting and basin evolution in the Borrego Badlands, San Jacinto fault zone, Southern California
Andrew T. Lutz, PO Box 185, Eugene, OR 97440, USA, and Rebecca J. Dorsey (corresponding author), University of Oregon, Geological Sciences, Department of Geological Sciences, Eugene, OR 97403-1272, USA; et al. Pages 1377-1397.
- Keywords: stratigraphy, San Jacinto fault, Pleistocene, California, basin analysis.
- The San Jacinto fault is a young, seismically active strand of the southern San Andreas fault system, yet until recently its age of initiation in the western Salton Trough was poorly known. This paper presents a detailed study of the Pleistocene Ocotillo Formation and Fonts Point Sandstone in the Borrego Badlands, Southern California, where well exposed sedimentary rocks are cut by the Coyote Creek and Clark faults. Lutz et al. documented variations in grain size, clast composition, and paleocurrents in the Ocotillo Formation, used paleomagnetism to determine the age and accumulation rates of the deposits, and used the data to extract a record of sedimentation and basin evolution as controlled by crustal deformation in the San Jacinto fault zone. Results of this and two companion studies show that Ocotillo gravel and sand prograded rapidly across a large area of the western Salton Trough at about 1.1 million years ago in response to initiation of the San Jacinto and other strike-slip faults. Local subsidence and sedimentation in the Borrego badlands was controlled by slip on several strands of the fault zone, and by northward tilting on the north limb of a large regional anticline. A second fault reorganization at about 0.6 million years ago resulted in the end of basin subsidence, deposition of a thin sheet-like alluvial deposit (Fonts Point Sandstone), and onset of modern uplift and erosion that has produced the Fonts Point escarpment.
- Middle-Late Permian mass extinction on land
Gregory J. Retallack, University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, USA; et al. Pages 1398-1411.
- Keywords: Permian, Triassic, extinction, paleosol, palynology, vertebrates.
- Mass extinction at the end of the Permian was the greatest catastrophe in the history of life in the sea, but another mass extinction 9 million years earlier, at the boundary between middle and Late Permian, was comparable in magnitude to the end-Cretaceous mass extinction in the sea. New studies by an international team of scientists working in Antarctica and South Africa now indicate that both Permian crises were severe on land as well. Unlike the Cretaceous disaster, which destroyed dinosaurs and flowering plants, the Permian crises wiped out archaic kinds of reptiles and plants unfamiliar to us today. All three disasters were accompanied by a variety of global environmental changes, including warmer and wetter climate at high latitudes and soaring levels of atmospheric carbon dioxide. Furthermore, all three catastrophes were geologically abrupt and short-lived, not the culmination of long-term environmental deterioration. Asteroid impact is suspected for the end-Cretaceous crisis, but the evidence for impact remains elusive for both Permian catastrophes. Unusual carbon chemical composition at both Permian boundaries indicate a carbon cycle crisis involving methane pollution of the atmosphere. Outbursts of methane from marine clathrates are unlikely to have created such large perturbations of the carbon cycle, but thermal alteration of coals intruded by feeder dikes to flood basalts could have been a significant planetary hazard. Life of the Permian, like the life of a soldier, may have endured long periods of boredom punctuated by short periods of terror.
- Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washington
Jennifer E.P. Matzel, Berkeley Geochronology Center, Department of Earth, Atmospheric, and Planetary Sciences, Berkeley, CA 94709, USA; et al. Pages 1412-1430.
- Keywords: geochronology, magma reservoir, plutons, time scales, crustal origin.
- This paper presents a detailed history of the growth of two magmatic intrusions in the North Cascades of Washington State. The Mount Stuart batholith was constructed over an approximately 5.5-million-year time interval that was punctuated by four intervals of high magma flux. The durations of the high-flux periods are relatively short (a few hundred thousands of years) relative to the duration of the entire batholith. In contrast, the Tenpeak intrusion was constructed over a shorter, approximately 2.6-million-year time period with magma influx distributed throughout its history. The punctuated history of magmatism in the Mount Stuart batholith means that at least four distinct magma reservoirs formed, but at any given time during its history, the volume of low-crystallinity magma was modest in comparison to the size of the entire batholith. The more distributed magma influx into the Tenpeak intrusion suggests that the melt-rich portions of the magma reservoir were most likely smaller and more ephemeral in nature than the Mount Stuart batholith.
- Rates of active faulting during late Quaternary fluvial terrace formation at Saxton River, Awatere fault, New Zealand
Dougal P.M. Mason, Opus International Consultants, Geotechnical Engineering, Wellington 6001, New Zealand; et al. Pages 1431-1446.
- Keywords: coseismic processes, neotectonics, paleoseismicity, river terraces, slip rates, strike-slip faults.
- This paper focuses on a flight of river terraces in northeast South Island, New Zealand. These terraces have been displaced by the Molesworth section of the Awatere Fault, a major active strike-slip fault in the Pacific-Australia plate boundary zone in this part of New Zealand. The slip rate and earthquake history are reevaluated for this site based on a comprehensive resurvey of the terrace displacements, new dating of the terrace surfaces, and excavation of a paleoseismic trench. These new data suggest the frequency and magnitude of earthquakes on this fault have been variable over the past 15,000 years, but that the slip rate has remained approximately constant.
- Geomorphic analysis of the Central Range fault, the second major active structure of the Longitudinal Valley suture, eastern Taiwan
J. Bruce H. Shyu, California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA; et al. Pages 1447-1462.
- Keywords: Taiwan, tectonic geomorphology, Central Range fault, Longitudinal Valley, sutures, exhumation.
- The mountainous island of Taiwan is being created by the ongoing "collision" between two geologic plates. The boundary between these two plates, the Longitudinal Valley in eastern Taiwan, is one of the most seismically active areas of the world. Whereas a major fault along the valley's eastern side is well studied, little is known about the valley's western side. In this paper, Shyu et al. described geomorphic evidence for another major fault along the western side of the valley. Such evidence includes deformed landforms and young river deposits. This other major fault, named as the Central Range fault, not only is important in the assessment of seismic hazard in eastern Taiwan, but may also play important roles in the mountain building processes of the island.
- Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S)
Daniel Melnick, GeoForschungsZentrum Potsdam, PB 3.1 Dynamics of the lithosphere, Telegrafenberg, Potsdam, Brandenburg D-14473, Germany; et al. Pages 1463-1480.
- Keywords: Chile margin, forearc deformation, subduction earthquakes, tectonic inversion, reverse faults, crustal seismicity.
- The Chile convergent margin, where the Nazca oceanic plate is subducted under the South American continent, has been affected by some of the largest earthquakes on Earth. Melnick et al.'s study describes the geology and geomorphology of Isla Santa MarÝa, a small island where Charles Darwin measured 3 m of uplift caused by a great earthquake in 1835. Geophysical data sets reveal that active faults cut the entire continental plate in this region and extend from the island to the city of Concepción. Melnick et al. integrate the surface and subsurface observations to propose that these faults have been responsible for uplifting and titling the island over the past 50,000 yr and of its emergence above sea level about 30,000 yr ago. Furthermore, they suggest that the large earthquakes, which nucleate in the interface between the subducted oceanic plate and the overriding continent, can trigger movement of the faults imaged below the island. This would cause localization of surface deformation along these faults as well as of strong ground motion. Thus, identification and mapping of these structures will help to mitigate seismic hazards in this populated coastal region.
- Cenozoic exhumation of the northern Sierra Nevada, California, from (U-Th)/He thermochronology
M. Robinson Cecil, University of Arizona, Geosciences, Tucson, AZ 85721, USA; et al. Pages 1481-1488.
- Keywords: Sierra Nevada, tectonics, exhumation, paleotopography, thermochronology.
- The Cenozoic exhumation of the northern Sierra Nevada mountain range is constrained by dating the cooling of granitoid rocks through shallow crustal levels using (U-Th)/He thermochronology. Estimated ages from this study range from approximately 100 to 45 million years ago and suggest a two stage history of Sierra Nevadan exhumation. The first stage is characterized by rapid cooling and denudation following arc formation and emplacement of the Sierran batholith. Rapid exhumation during this time (approximately 90-60 million years ago) agrees with high rates of sedimentation in the Great Valley. The second stage is characterized by slow exhumation rates and suggests tectonic quiescence of the range since about 60 million years ago. These low rates, while they agree with shorter-term erosion rates, are puzzling given the high average elevation and topographic relief of the Sierra Nevada, which are often attributed to recent tectonic uplift of the range.
- 40Ar/39Ar thermochronology constraints on the timing of Proterozoic basement exhumation and fault ancestry, southern Sangre de Cristo Range, New Mexico
Robert E. Sanders, New Mexico Tech, Earth and Environmental Science, Socorro, NM 87801, USA; et al. Pages 1489-1506.
- Keywords: 40Ar/39Ar, fault reactivation, K-feldspar thermochronology, Precambrian, New Mexico.
- Proterozoic basement exhumation history and fault ancestry and reactivation in north-central New Mexico are investigated using 40Ar/39Ar geochronology and K-feldspar thermochronology. The southern Sangre de Cristo Range basement experienced 5 to 7 km of denudation between 1000 and 800 million years ago, interpreted as the result of intracratonic tectonism related to the Grenville orogeny. In contrast, basement rocks to the east of the range front in the Las Vegas basin were not exhumed until later, between 750 and 600 million years ago, in response to the break up of the Rodinian supercontinent. The Montezuma fault that separates these two structural blocks therefore facilitated differential displacement twice during the Neoproterozoic and was subsequently reactivated during Ancestral Rocky Mountain and Laramide tectonism.
- Thermal histories from the central Alborz Mountains, northern Iran: Implications for the spatial and temporal distribution of deformation in northern Iran
Bernard Guest, University of Hannover, Institute for Geology, Hannover, Lower Saxony 30167, Germany; et al. Pages 1507-1521.
- Keywords: Exhumation, (U-Th)-He, thermochronology, collision, Alborz Mountains, Iran.
- Guest et al. have presented radiometric cooling-age data that, by their interpretation, show when rocks from the Alborz Mountains in northern Iran cooled due to mountain-building and exhumation from depths greater than ~2 km. They observe three cooling episodes that are consistent with the patterns of deformation preserved within the rocks of the Alborz. The earliest occurred between 60 and 80 million years ago and probably records uplift of portions of the proto-Alborz in response to a major contractional deformational event that occurred along the then-Andean type margin of Iran during this time interval. An intermediate cooling event that occurred between 40 and 50 million years ago correlates with a time of crustal extension in central Iran and the Alborz region. The final cooling stage began about 12 million years ago and most likely records the development of the present Alborz Mountains in response to the collision of Arabia and Eurasia. These results help us to understand the geologic and tectonic evolution of Iran (a poorly understood region), the timing of the Arabia-Eurasia collision, and the behavior of Earth's crust in collisional settings.
- 40Ar/39Ar ages of detrital white mica constrain the Cenozoic development of the intracontinental Qaidam Basin, China
Andrea B. Rieser, GeoForschungsZentrum Potsdam, Section 3.3, Telegrafenberg, Potsdam, Brandenburg 14473, Germany; et al. Pages 1522-1534.
- Keywords: Tibetan Plateau, geochronology, intramontane basin, provenance, denudation.
- The Qaidam Basin in western China has been completely surrounded by mountain ranges for about 20 million years and has accumulated a large amount of sediments. Large strike-slip faults run parallel across the Altyn and the Kunlun mountains bordering the basin in the north and the south, respectively. This paper shows the four 40Ar/39Ar age groups represented by detrital white mica from the western Qaidam Basin. Compared to basement age data from the surrounding mountains, potential source areas for the four age groups can be distinguished indicating the importance of tectonics and strike-slip faulting during the infilling process. Theoretical calculations regarding infilling and erosion volumes further support the findings from the age distribution.
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