|7 July 2011
GSA Release No. 11-44
Director - GSA Communications & Marketing
Lithosphere Highlights: New research posted 7 July
Boulder, CO, USA – Highlights for articles published online 7 July 2011 are provided below. Keywords include: Coast Mountains batholith, Anderson Reservoir, Canada, Alaska, Chugach terrane, Valdez Group, Basin and Range province, and Sierra Nevada.
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U-Pb-Hf characterization of the central Coast Mountains batholith: Implications for petrogenesis and crustal architecture
M. Robinson Cecil et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA; doi:10.1130/L134.1.
The Coast Mountain batholith of coastal British Columbia and southeastern Alaska is a large belt of granitoid rocks (over 1500 x 100 km) that was intruded over the course of more than 100 million years. Formed as the result of long-lived subduction along the margin of western North America, it is an important recorder of the geologic processes responsible for the growth of a large continental magmatic arc. M. Robinson Cecil of the California Institute of Technology and colleagues widely sample granitoid intrusions making up the central portion of the batholith. Crystallization ages and hafnium isotopic compositions of the granitoids were then determined, in order to identify spatial and temporal trends in batholith petrogenesis. Hafnium isotopic signatures, which are used to infer the type and age of the source rocks from which the granitoid melts formed, were found to have a strong spatial dependence and to generally become more juvenile eastward. The distribution of hafnium isotope values was used to infer the country rock assemblages into which the granite plutons were intruded. Additionally, they were used to provide constraints on the nature of accreted crustal panels at depth and the structural boundaries that separate them.
No correlation between Anderson Reservoir stage level and underlying Calaveras fault seismicity despite calculated differential stress increases
Tom Parsons, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi:10.1130/L148.1.
Anderson Reservoir sits atop the San Francisco Bay region's most active fault. Seasonal rainfall changes the weight of the reservoir that presses down on the Calaveras fault; but does it affect the rate of earthquakes? Numerical models simulating seasonal water level increases calculated the added stresses, which are compared by Tom Parsons of the U.S. Geological Survey with observed earthquake rates. Parsons found that there is no apparent link between water level increases and earthquake rates beneath the reservoir.
Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska
Evan J. Kochelek et al., Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA; doi:10.1130/L131.1.
The Chugach terrane in southern Alaska is nearly all that remains of the paleo-Pacific seafloor that has been subducted since the Jurassic. Rocks of the Valdez Group represent the majority of the volume of the Chugach terrane but also represent the shortest time interval of all of the Chugach terrane. Using U-Pb laser ablation-multicollector-inductively coupled plasma-mass spectrometry analyses of detrital zircons collected from sandstones in the Valdez Group, research by Evan J. Kochelek of New Mexico State University and colleagues sheds light on the complex processes surrounding subduction-driven accretion of material onto the continental margin. They propose that continuous deposition and accretion of sediment occurred from approximately the Cenomanian age until at most the Maastrichtian age. Furthermore, provenance analysis of these detrital zircons indicates that sedimentary rocks of the Valdez Group were derived from the Coast Mountains batholith in western British Columbia.
Updated paleomagnetic pole from Cretaceous plutonic rocks of the Sierra Nevada, California: Tectonic displacement of the Sierra Nevada block
John W. Hillhouse and Sherman Grommé, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi:10.1130/L142.1.
Magnetite crystals in Sierra Nevada granites hold clues to the plate-tectonic deformation of California. Paleomagnetism preserved in magnetite provides a means for measuring vertical axis rotation by using the long-term dipole nature of Earth’s magnetic field as a frame of reference. Although the San Andreas fault is generally viewed as the major plate-tectonic boundary between the Pacific sea floor and North America, geologists have found evidence that areas east of the fault accommodate a portion of the relative plate motion. Multiple fault offsets in the Basin and Range province of California and Nevada indicate that substantial west-northwest displacement of the Sierra Nevada has occurred during the past 16 million years. The motion continues, as shown by global positioning satellite measurements. The mobile "Sierra Nevada block" is bounded on the west by the Great Valley of California and on the east by a complex of steep faults at the edge of the Great Basin. Whether the Sierra Nevada block has undergone vertical-axis rotation as the Basin and Range province formed is critical to estimates of extension of the deep continental crust and to regional tectonic modeling. The principal finding of this study by John W. Hillhouse of the U.S. Geological Survey and colleague Sherman Grommé is that vertical-axis rotation of the Sierra Nevada block since about 80 million years ago is probably less than six degrees, given confidence limits of the paleomagnetic data and uncertainty concerning the post-emplacement westward tilt of Sierra Nevada granite bodies. If tilt is near zero as some suggest, the magnetic data indicate no significant vertical-axis rotation of the block. Other evidence implies tilt of the block up to three degrees, which would favor a small rotation in the counterclockwise sense.