||24 February 2006
GSA Release No. 06-05
March / April Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. — The March-April issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include: new insights into controversies regarding the origins of fjords and whether they were filled by ice sheets during the last Ice Age; and new evidence from the Basin and Range Province regarding the relationship between tectonic processes and long-term climate evolution.
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- The role of material anisotropy in the neotectonic extension of the western Idaho shear zone, McCall, Idaho
Scott Giorgis, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53703, USA, et al. Pages 259-273.
- Keywords: reactivation, western Idaho shear zone, anisotropy, paleomagnetism, Neotectonics, transtension.
- This paper addresses why and how continents fall apart. The eastern edge of Basin and Range extension in Eastern Oregon and western Idaho occurs at the edge of the Idaho batholith. The western edge of the Idaho bathlith contains a plate-scale shear zone, known as the western Idaho shear zone. The present-day extensional deformation in the Northwest has exploited the crustal flaw created by this shear zone, utilizing the well-developed rock fabric to develop faults. The strong control on the orientation of faults, results in an expected set of E-W-trending normal faults, which are not observed in non-reactivated regions. This research suggests that the eastern edge of the zone extensional break-up of North America is controlled by the western Idaho shear zone.
- Temporal and spatial records of active arc-continent collision in Taiwan: A synthesis
Chi-Yue Huang, Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, et al. Pages 274-288.
- Keywords: Taiwan, arc-continent collision, oblique collision, stratigraphic records, collision suture.
- This article describes time and space records of an active mountain building history in the Western Pacific, where a volcanic arc chain has been colliding against the Eurasian Continent since 6 Ma. The history of this active mountain-building can be recognized from ocean to land with a same tectonic event repeatedly occurring in different time from north to south. The active Taiwan arc-continent collision can play as a standard example for the ancient mountain ranges that also experienced oblique arc-continent collision worldwide.
- Near-tip stress rotation and the development of deformation band stepover geometries in mode II
Chris. H. Okubo and Richard A. Schultz, Geomechanics-Rock Fracture Group, Department of Geological Sciences and Engineering/172, University of Nevada, Reno, Nevada, 89557-0138, USA. Pages 343-348.
- Keywords: deformation band, damage zone, plastic deformation, faulting, sandstone, stress rotation.
- The propagation of deformation bands into compressive and extensional stepover geometries is investigated by integrating field observations with numerical model simulations of the effective stress state due to shear along the bands. Deformation bands are tabular discontinuities, with mm- to cm-scale thicknesses, of localized volumetric strain and shear. Deformation bands are precursors to faulting in porous granular rocks and soils. Systematic rotations in near-tip principal stress orientation due to shear along the overlapping deformation bands are shown to predict band propagation paths that are consistent with characteristic stepover geometries, as viewed in the mode II direction, the orientation in which the observation plane is parallel with the displacement direction. Therefore, propagation paths for deformation bands can be predicted from knowledge of the effective near-tip stress state. These results establish a mechanics-based framework for investigations of fault growth and fault-controlled fluid flow in porous granular rocks and soils.
- Eastward migration of the Qaidam basin and its implications for Cenozoic evolution of the Altyn Tagh fault and associated river systems
Erchie Wang, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China and Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, People's Republic of China, et al. Pages 349-365.
- Keywords: Tibetan plateau, Qaidam basin, depocenter eastward migration, longitudinal river.
- The Qaidam basin is the largest Cenozoic basin within the Tibetan plateau, known for containing abundant oil and natural gas resources. The deformational and sedimentary evidences indicate that the depocenters of the basin have migrated southeastward for 400 km from Oligocene to Quaternary. We infer that the sediments within the basin were not shed from the surrounding mountains but from the West Kunlun range, transported by a 2000-km-long longitudinal river along the Altyn Tagh strike-slip fault.
- Extensional shear zones, granitic melts, and linkage of overstepping normal faults bounding the Shuswap metamorphic core complex, British Columbia
Bradford J. Johnson, Goleta, California 93117, USA. Pages 366-382.
- Keywords: extension tectonics, normal faults, transfer zones, melts, metamorphic core complexes, Cordillera.
- This is an investigation of relationships between emplacement of granitic melts and the evolution of a large fault system during regional extension of the continental crust. The Shuswap metamorphic core complex is an extensive exposure of rocks in southeastern British Columbia that were formerly part of the middle crust, where they were hot enough to partially melt. The Shuswap complex was exhumed from the middle crust in early Tertiary time (approximately 56 to 42 m.y. ago), as the upper crust moved laterally and down along faults and associated zones of ductile deformation (shear zones). The western margin of the Shuswap complex is the west-dipping Okanagan Valley fault system, the focus of this investigation. More than 450 km long, it consists of two offset segments connected by a stepover zone called the Shuswap Lake transfer zone. Deformed rocks called mylonites define shear zones associated with the fault system. Most of the mylonite was derived from the deformation of metamorphosed sedimentary rocks containing small pods of partial melt. In the Shuswap Lake transfer zone, however, the mylonite was mostly derived from granitic intrusions that originated as large batches of crustal melt. An argument is made that physical properties of the granitic intrusions (e.g., low viscosity relative to the metamorphic host rocks) controlled the dynamics and, to some extent, the geometry of the transfer zone. The results of this study provide evidence that granitic melt plays an important role in the evolution of stepovers in large extensional fault systems.
- History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau
Margaret E. McMillan, Department of Earth Sciences, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, USA, et al. Pages 393-405.
- Keywords: Cenozoic, tectonic uplift, climate change, incision, Rocky Mountains, Colorado Plateau.
- The Western U.S. is full of dramatic evidence of erosion. Ranges in the Rocky Mountains are sliced by streams forming steep gorges like the Black Canyon of the Gunnison and the Royal Gorge. The Colorado Plateau is dissected by big rivers creating a labyrinth of canyons including the most well-known, the Grand Canyon. Even parts of the western Great Plains have been cleaned out by erosion, with only scattered mesas and buttes left to stand as reminders of a former landscape. We use these remnants of former topography in order to study when, where, and how much erosion has occurred. Why study what is not there anymore? The pattern and timing of erosion help us to unravel the upward-directed mountain building forces and the downward-directed mountain erasing forces. This information is key to our understanding of the elevation history of the region that now forms the backbone of our continent.
- Cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets
Jason P. Briner, Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80303, USA, et al. Pages 406-420.
- Keywords: fjord, cosmogenic radionuclide, Laurentide Ice Sheet, weathering zone, glacier erosion, basal thermal regime.
- Fjords are spectacular features that fringe many of the world's continents. This paper sheds light on a long-standing debate regarding the origin of fjords and their occupation by ice sheets during the last Ice Age. Contrasting views of the occupation of fjords by glaciers are that either (1) only the bottoms of fjords were filled by ice, or (2) that ice filled fjords up to and beyond their rims. Supporters of scenario one noted that upland terrain adjacent to fjord rims was highly weathered, and thus, not covered by ice recently. Alternatively, supporters of scenario two advocated that ice did cover the uplands, but that the ice was frozen to its bed (so-called "cold-based" ice), and simply preserved the highly weathered terrain. This is important because these two different scenarios have differing implications for fjord formation and evolution, ice sheet history and dynamics, and ultimately the global climate system. Standard methods used by glacial geomorphologists have failed to differentiate the two scenarios. By applying cosmogenic radionuclides to fjord landscapes in Arctic Canada, we were able to solve this debate by demonstrating that the weathered uplands near fjord rims were indeed covered by cold-based ice. The results of this paper will be of interest to those who study the formation of these ubiquitous landscapes, reconstruct the extent of Ice Age glacier, and seek to understand past and present ice sheet processes.
- Multiple constraints on the age of a Pleistocene lava dam across the Little Colorado River at Grand Falls, Arizona
Wendell Duffield, Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA, et al. Pages 421-429.
- Keywords: Grand Falls lava dam, Quaternary, 40Ar/39Ar, 3He, magnetic secular variation, optical luminescence.
- During Pleistocene time, popularly known as the Ice Age, a lava flow spilled into and filled the canyon of the Little Colorado River, thus quickly creating a dam, about 50 kilometers (30 miles) east of the place that later became the town of Flagstaff, Arizona. Geologists have long wondered about the age of the dam, especially because the lava looks young enough to have happened "almost yesterday." The first attempt to calculate a numerical age, from the radioactive decay of potassium in the lava, was done in the 1970s and resulted in 150,000 years, older than the lava appears to the naked, yet calibrated geologic eye. Four subsequently developed techniques (40Ar/39Ar, 3He, luminescence, magnetic secular variation) to date young lava flows all converge on an age of about 20,000 years (reported in this paper), a result that is far more reasonable to those calibrated eyes. This well-established, new and much younger age is important to studies of landscape and river evolution for the surrounding region. The new age also adds to a growing level of confidence in the accuracy of results from each of the four techniques used. As a serendipitous bonus, it's now intriguing to wonder if humans watched the lava dam form, in light of emerging archaeological evidence that people may have first inhabited North America much earlier than the once widely accepted age of 12,000 years ago.
- Relay ramps in active normal fault zones: A clue to the identification of seismogenic sources (1688 Sannio earthquake, Italy)
D. Di Bucci, Dipartimento della Protezione Civile, Servizio Sismico Nazionale, Rome, Italy, et al. Pages 430-448.
- Keywords: transfer zone, fault interaction, seismogenic fault system, Benevento, southern Apennines.
- The paper considers for the first time the field evidence of an active relay ramp (area of complex deformation which accommodates the extension between normal faults) in a seismic area as a clue for the definition of a seismogenic normal fault. The study area is the Calore River valley, Italy, which was struck by a destructive earthquake in 1688. The area is located at the core of the Apennine chain, which is characterized by active extension and associated severe seismicity. The Italian record of strong earthquakes, which covers about two thousand years, describes many destructive events on still unknown seismogenic fault systems. This is because seismogenic faults usually have no clear field evidence in the Southern Apennines. The paper outlines that in this case, precious information can be obtained from the analysis of the surficial deformation that accompanies the seismogenic structures, and also shows that the detailed analysis of active transfer zones is a useful tool for the definition of the seismogenic potential of an area.
- Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand
Rupert Sutherland, Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand, et al. Pages 464-474.
- Keywords: Alpine fault, tectonics, earthquakes, glacial deposits, plate collision, strike-slip faults.
- The Alpine fault is the most active geological structure in New Zealand and bears striking similarity to the San Andreas fault in California. This paper describes how landforms that were created during ice-ages have been offset by the Alpine fault, and uses these observations to precisely determine the long-term slip rate on the fault to be 23 ± 2 mm/yr. The improved definition of slip rate allows better estimates of how often earthquakes rupture the Alpine fault, and means that geologists can now understand with much greater certainty how nearby faults must be moving to accommodate the remainder of the plate boundary motion through New Zealand.
- Stable isotopic evidence for Neogene surface downdrop in the central Basin and Range Province
Travis W. Horton, Geology Department, University of Puget Sound, Tacoma, Washington 98416-1048, USA and C. Page Chamberlain, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA. Pages 475-490.
- Keywords: isotope ratios, lacustrine sediments, Basin and Range Province, paleotopography, Neogene.
- The chemical composition of minerals preserved in ancient rocks suggests that the central Basin and Range Province has experienced a regional decrease in elevation since approximately 20 million years ago. This finding helps clarify the links between tectonic processes and the long-term climate evolution of the region. Specifically, extension of Earth's crust in response to tectonic and/or gravitational forces caused a net elevation decrease throughout the region, possibly allowing for intensified atmospheric flow from the south.
- Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India
P.M. Myrow, Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA, et al. Pages 491-510.
- Keywords: Cambrian, Parahio Formation, India, Tethyan Himalaya, stratigraphy.
- The Himalaya formed during Cenozoic collision of India and Asia, but an earlier enigmatic Cambrian-Ordovician (ca. 490 Ma) event may have strongly influenced its regional geology. Analysis of well-preserved Cambrian deposits is critical for understanding the nature of this early tectonic event and its influence on the later tectonic evolution of the Himalaya. We studied the Parahio Formation of the Parahio Valley, Spiti region of India, which is the best and most fossiliferous Cambrian section of strata in the entire Himalaya. This formation consists of over 1350 m of dominantly sandstone, siltstone, and shale that was deposited in a storm-influenced ancient deltaic environment, and recorded the switching of river channels and related sea level changes. This interpretation is at considerable odds with previous interpretations. The top of the section shows evidence for a collisional event that caused uplift and erosion of some of the Cambrian rock. This event affected much of the Himalaya and the nature of this event has been widely debated. We provide evidence to suggest that previous models indicating uplift to the north of this part of India are unlikely, and that uplift may have instead taken place to the south in a different tectonic setting.
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