Boulder, Colo., USA - New Geosphere research posted online ahead of print interprets the Eocene-Early Miocene paleotopography of Nevada, examines the origin of the Colorado Mineral Belt, compares mountain building processes in Alaska, uncovers more about the dynamic Antarctic ice from the AND-1B borehole, and more.
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Eocene-Early Miocene paleotopography of the Sierra Nevada-Great Basin-Nevadaplano based on widespread ash-flow tuffs and paleovalleys
Christopher D. Henry et al., University of Nevada, Reno. Posted online 23 Jan. 2012; doi: 10.1130/GES00727.1
In as study by Henry et al., as much as 3000 km3 of the tuff of Campbell Creek erupted 28.9 million years ago from a supervolcano about 35 km in diameter in central Nevada. The tuff flowed down paleovalleys as much as 280 km to the west, into the western foothills of the Sierra Nevada, and 300 km to the northeast, to what is now the Ruby Mountains. The distribution of the tuff of Campbell Creek and other widespread ash-flow tuffs demonstrate that the Sierra Nevada was a lower, western ramp to a higher plateau in what is now central Nevada, that a north-south "paleo-continental divide" existed through central-eastern Nevada, and that the characteristic basin-and-range topography of Nevada did not exist until after 29 million years ago. The plateau and paleodivide connected northward at least into Idaho and southward into northern Sonora, Mexico.
Origin of the Colorado Mineral Belt
Charles E. Chapin, New Mexico Bureau of Geology and Mineral Resources (retired). Posted online 12 Jan. 2012; doi: 10.1130/GES00694.1
Ogden Tweto stated the puzzle of the Colorado mineral belt succinctly in 1975: "The problem ... is not so much why or how magmas were generated, but why magmatic activity took the pattern it did--that is, of a rather sharply defined belt diagonal to all major tectonic elements in an extensive region that elsewhere is nearly devoid of contemporaneous igneous rocks." The Colorado mineral belt is a 500-km-long, 25-km-wide belt of igneous intrusions and mining districts that trends northeastward from the Four Corners area on the Colorado Plateau to the eastern edge of the Rocky Mountains near Boulder, Colorado. In plate tectonics terms, the mineral belt is located within a 1200-km-wide gap in the volcanic chain that marked the western edge of North America during the Laramide orogeny (approximately 80 to 40 million years ago). As the North American plate moved southwestward, the Farallon oceanic plate was subducted beneath it on a northeastward trajectory. Observations that major differences in volcanism, sedimentation, and trends of mountain ranges occur on opposite sides of the Colorado mineral belt led Chapin to suspect that the mineral belt was located above a segment boundary in the subhorizontally subducted Farallon plate. Coincidence in timing of the beginning of magmatism along the mineral belt (75 million years ago) with accelerated Farallon-North American convergence and major tectonic deformation of the Rocky Mountain area added credence to the plate tectonic interpretation. As the thicker lithosphere of the North American plate rapidly overrode the underlying Farallon plate, tensional stresses dilated the segment boundary, allowing fluids and magmas to rise into the crust of the Rocky Mountain region, thus forming the Colorado mineral belt.
Miocene magmatism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc
David A. John et al., USGS. Posted online 23 Jan. 2012; doi: 10.1130/GES00674.1
The Miocene Bodie Hills volcanic field is a >700-square-kilometer eruptive center of subduction-related magmatism in the southern segment of the ancestral Cascades arc north of Mono Lake. It consists of about 20 major eruptive units, including four relatively mafic composition (basaltic andesite-andesite) stratovolcanoes emplaced along the margins of the field, and numerous, more centrally located intermediate to silicic composition (dacite-rhyolite) flow dome complexes. Volcanism was episodic with two peak periods of eruptive activity: an early period at about 14.7 to 12.9 million years ago that mostly formed stratovolcanoes, and a later period between about 9.2 to 8.0 million years ago dominated by large dome fields. Following an approximately 2-million-year hiatus in magmatic activity, post-subduction volcanic rocks of the Pliocene-Pleistocene (about 3.6 to 0.1 million years ago) Aurora volcanic field were deposited on the east side the Bodie Hills volcanic field. Geophysical data from John et al. suggest that many of the Miocene volcanoes have shallow plutonic roots that extend to depths ≥2 km below the surface, and much of the Bodie Hills may be underlain by low-density plutons likely related to Miocene volcanism. Numerous hydrothermal systems were operative in the Bodie Hills during the Miocene volcanism, including systems that formed large gold-silver vein deposits in the Bodie and Aurora mining districts. Economically important mineral deposits in the Bodie Hills are temporally related to dome complexes. Rock compositions and volcanic center landforms in the Bodie Hills are broadly similar to these features in other parts of the southern part of the ancestral arc south of Lake Tahoe (approximately latitude 39° to 39.5°N); dome fields among less abundant stratovolcanoes are common, intermediate compositions (andesite-dacite) are abundant, and mafic compositions (basalts) are scarce. The scarcity of mafic volcanic rocks is likely a consequence of thick crust that prevented ascent of mantle-derived basalt magmas. Farther north along the Miocene arc between Lake Tahoe and southern Oregon, the crust was thinner, basalt melts rose directly to the surface, and ancestral arc eruptions formed mafic shield volcanoes.
Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska
James B. Chapman et al., SandRidge Energy. Posted online 23 Jan. 2012; doi: 10.1130/GES00677.1
Not all mountains are created in the same way. Chapman et al. examine an intersection between two mountain chains in southern Alaska. One set of mountains formed as a result of strike-slip faulting, and the other formed from contractional faults in a head-on collision of tectonic plates. By examining the fault surfaces involved within this intersection and studying the rocks that were folded and offset by these faults, we can unravel the history of these mountain chains and understand how they formed. In this instance, the mountains that formed by strike-slip faults were built first. These mountains formed several million years ago as a small tectonic plate was sliding northward along the west coast of North America. As this mountain chain pushed further north toward Alaska, the geometry of the plate boundary changed and the small tectonic plate could no longer easily slide northward. As a result, a new mountain chain began to form that involved different types and orientations of faults. This new mountain chain is the second type of mountains that formed from contraction. Chapman et al. can demonstrate how the new mountain system overprinted the older one and make an estimate of how long ago this process started to occur. Combining these age estimates with additional studies of the degree of faulting in the mountain chains provides important rates on faulting.
Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica
Trevor Williams et al., Lamont-Doherty Earth Observatory. Posted online 23 Jan. 2012; doi: 10.1130/GES00655.1
The ANDRILL McMurdo Ice Shelf (MIS) project drilled a 1285-meter-deep borehole containing a remarkable record of Antarctic glacial history. It shows that the West Antarctic ice sheet retreated and advanced about 38 times over the last 6 million years, with consequent sea level changes modeled to reach 3 to 7 m higher than today. This history of dynamic ice comes from the alternating sedimentary rock types: diatomite, formed from open-water plankton, and diamict, made of clays, sands, and gravel from under past ice sheets. There are other rock types, like mudstones and sandstones, and gradations between them all. Williams et al. describe how physical properties help to distinguish them--properties such as magnetic susceptibility, natural gamma radiation, potassium content, and electrical resistivity all have characteristic signatures in the different rock types. They can be used to describe features such as the increased clay content at the top of the diamicts and the base of the diatomites, and thus describe the transition from ice-covered to ice-retreated conditions. The properties measured in place by downhole logging tools represent the only information for the few intervals unrecovered by coring, and we interpret lithology for those intervals. In this way, we help to tell the story of dynamic Antarctic ice from the AND-1B borehole.
Crustal structure and signatures of recent tectonism as influenced by ancient terranes in the western United States
Hersh Gilbert, Purdue University. Posted online 23 Jan. 2012; doi: 10.1130/GES00720.1
The western portion of North America has experienced a long history of deformation and is now characterized by diverse topography. Observations from data recently collected by the EarthScope USArray provide the first uniform seismic data set designed to investigate this region. Results presented here by Gilbert indicate that the crust varies in thickness across the western United States and exhibits distinct structures within the Basin and Range, Snake River Plain, the Sierra Nevada, and the active Cascade volcanic arc. These distinct features illustrate that recent tectonic processes have affected the structure of the crust. Additional trends in crustal characteristics align with the boundaries between the terranes that accreted together to form North America. The preservation of these ancient features suggests that they influenced subsequent deformation.
LaDiCaoz and LiDARimager--MATLAB GUIs for LiDAR data handling and lateral displacement measurement
Olaf Zielke and J Ramon Arrowsmith, Arizona State University. Posted online 23 Jan. 2012; doi: 10.1130/GES00686.1
This software contribution publication provides Matlab GUIs (graphical user interfaces) for LiDAR (light detection and ranging) data handling and lateral displacement measurement. In recent years, digital elevation models (DEMs) generated from high-resolution LiDAR data have become a powerful tool for many scientific disciplines that investigate earth surface related processes such as tectonic geomorphology, hydrology, vegetation dynamics, or civil engineering. For example, they can be used in tectono-geomorphic studies to identify and measure geomorphic features such as fluvial channels that are laterally displaced as they cross an active fault zone. Such offset data may be used to reconstruct the earthquake history along a given fault (e.g., the San Andreas Fault, California). However, the high resolution and thus large volume of LiDAR data makes their analysis somewhat cumbersome. Here, Zielke and Arrowsmith provide Matlab GUIs that enable fast and uncomplicated LiDAR data visualization and processing as well as offset measurements. Key features of the provided GUIs include (A) analysis of large (>108 data points) DEMs on standard desktop PCs, (B) automated generation of *.kmz files from LiDAR-derived DEMs for import into GoogleEarth, and (C) slicing and lateral back-slipping of the DEM to assess offset measurement reliability.