The fourth dimension
Boulder, Colo., USA – Remote sensing techniques facilitate observations and monitoring of ground displacements. In particular, space-borne Differential Synthetic Aperture Radar Interferometry (DInSAR) allows accurate measurements of ground deformation by properly analyzing multi-temporal satellite acquisitions over the region of interest. However, limitations of DInSAR may arise when large and/or rapid surface deformation occurs, including those caused by active rifting. Understanding the three-dimensional characteristics of the deformation field, as well as its temporal evolution, cannot be accomplished by DInSAR alone.
Accurate spatial and temporal dense information on the displacements is, however, crucial for the correct interpretation of complex geological phenomena. In this paper, Francesco Casu and Andrea Manconi propose an algorithm to retrieve the four-dimensional (i.e., along north, east, up, and time) surface deformation field over zones affected by active rifting.
In the Afar depression system, one of the locations worldwide where active rifting processes can be observed, Casu and Manconi retrieved information in areas where data was not previously recorded. Their method demonstrates its validity in complex situations such as rifting episodes, where the deformation associated to repeated intrusions, faulting, and lithospheric extension might overlap in space and time.
Four-dimensional surface evolution of active rifting from spaceborne SAR data
Francesco Casu, 1 IREA (Istituto per il Rilevamento Elettromagnetico dell'Ambiente), National Research Council, Via Diocleziano 328, 80124 Napoli, Italy; and Andrea Manconi and Dept. of Earth Sciences, Swiss Federal Institute of Technology, 8902 Zurich, Switzerland. Themed issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This article is OPEN ACCESS online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01225.1.abstract.
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Other recently published articles are highlighted below:
Implications for the structure of the Hat Creek fault and transfer of right-lateral shear from the Walker Lane north of Lassen Peak, northern California, from gravity and magnetic data
V.E. Langenheim et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane. This article is online at http://geosphere.gsapubs.org/content/early/2016/03/25/GES01253.1.abstract.
The region between two of California's major volcanoes, Mount Shasta and Lassen Peak, lies at the junction of several different geologic provinces. In this study, V.E. Langenheim and colleagues investigate the structure of the Hat Creek fault, the region's most prominent fault that is well expressed at Earth's surface. Yet how it projects into the subsurface is not well known. Their investigation shows that it dips steeply to depths where earthquakes nucleate. The spatial relationship of the fault at the surface with respect to micro-earthquakes suggests that deformation is stepping westward with time. The crust is not only being stretched in an east-west direction, but also sheared in a right-lateral sense, similar to deformation associated with the Walker Lane along the east side of the Sierra Nevada. Langenheim and colleagues write that their interpretations limit where Walker Lane right-lateral shear passes to the north of the Hat Creek Fault, indicating that the zone of right-lateral shear either ends abruptly near Mount Lassen, steps west south of the Klamath Mountains, or is geologically young, less than one million years old.
Paleogeographic implications of late Miocene lacustrine and nonmarine evaporite deposits in the Lake Mead region: Immediate precursors to the Colorado River
James E. Faulds et al., Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II. This article is online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01143.1.abstract.
This paper describes sedimentary deposits that accumulated in basins across the Lake Mead region in southern Nevada and northwestern Arizona immediately prior to development of the Colorado River. These deposits record the paleogeographic evolution of an important time period in the region from ca. 12 to 5 million years ago, including a sequence of events that ultimately led to development of the Colorado River. The deposits include thick sections (up to 2.5 km thick) of halite that formed in non-marine playas and limestone that accumulated in the lakes. The distribution and similar age of the limestone and evaporite deposits suggest a system of axial lakes near the future course of the Colorado River and extensive continental playas and salt pans in deeper satellite basins to both the north and south. The elevated terrain of the nearby Colorado Plateau was probably a major source of groundwater and possibly some surface water that fed the lakes and playas. Other basins farther south along the Colorado River may contain similar deposits. The Colorado River arrived in the Lake Mead region ca. 5.3-4.9 million years ago and may have initially emptied into the Las Vegas basin prior to spilling over into basins to the south along its present course.
Reevaluation of the Crooked Ridge River–Early Pleistocene (ca. 2 Ma) age and origin of the White Mesa alluvium, northeastern Arizona
Richard Hereford et al., U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II. This article is online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01124.1.abstract.
The essential features of the previously named and described Crooked Ridge River in northeastern Arizona (USA) are reexamined with new geologic and geochronologic data. Cenozoic alluvium at Crooked Ridge and White Mesa was proposed to be the product of a large, vigorous river draining the 35-23 million years old (Ma) San Juan Mountains volcanic field. The paleoriver was thought to be important in the early pre-Miocene evolution of the Grand Canyon. The results presented here by Richard Hereford and colleagues show that alluvial deposits of the ancient river (the White Mesa alluvium) accumulated in several low energy suspended sediment channel systems and that the drainage basin consisted mainly of Cretaceous bedrock. Dating detrital sanidine grains using the 40Ar/39Ar method demonstrates the alluvium is younger than about 2 Ma. Inset geomorphic relations show that the alluvium is older than the 1.2-0.8 Ma Bishop-Glass Mountain tuff. This young age of the alluvium is evidence of an unexpectedly young erosional history for northeast Arizona. The volume of eroded Cretaceous bedrock exceeds 3,000 cubic kilometers. Substantially more bedrock was removed to reach the present elevation of the Colorado and San Juan rivers.
Volcanic field elongation, vent distribution, and tectonic evolution of a continental rift: The Main Ethiopian Rift example
Francesco Mazzarini et al., Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, 56126 Pisa, Italy. Themed issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This article is online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01193.1.abstract.
Are volcanoes distributed randomly? Does volcanic vent pattern in volcanic fields reflect crustal and or lithospheric scale controls? To answer these questions, Francesco Mazzarini and colleagues studied the spatial distribution of volcanoes in a well-defined geotectonic setting: the Main Ethiopian Rift. Their new approach allows them to recognize different mechanisms acting at different spatial scales that rule the volcanoes distribution. The overall shape of volcanic fields reflects the deep control of magma storage by large lithospheric structures and geometries. On the other hand, inner structures of volcanic fields are controlled by crustal states of stress and strain. These methods can be successfully applied for studies in remote areas or planets where volcanism occurs.
Carboniferous basin in Holm Land records local exhumation of the North-East Greenland Caledonides: Implications for the detrital zircon signature of a collisional orogen
William C. McClelland et al., Dept. of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa 52242, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01284.1.abstract.
Carboniferous strata of the Wandel Sea basin, North-East Greenland, provide a record of unroofing of continental crust that was deeply subducted during collision of Eurasia and North America approximately 400 million years ago. The northernmost remnants of the metamorphic basement exposed in Holm Land give a U/Pb zircon age demonstrating that the crust formed as part of a 1.8 to 2.0 billion year old magmatic arc complex. The crust includes lenses of high-pressure metamorphic rock known as eclogite that formed at 423 plus or minus seven million years, thus documenting the northern extent of the North-East Greenland eclogite province during the Caledonian collision. U/Pb ages of detrital zircon from sandstones of the Sortebakker and Kap Jungersen Formations match the igneous and metamorphic ages observed in the depositionally underlying crystalline basement, reflecting a local provenance sourced in the North-East Greenland eclogite province. Other Devonian and Carboniferous basins within and peripheral to the Caledonides in Greenland and Scandinavia also show distinct signatures, demonstrating that there is not a simple, representative detrital zircon signature for the Caledonian orogen.
Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America
Gwen M. Linde et al., Dept. of Geological Sciences and Engineering, University of Nevada, Reno, Nevada 89557-0172, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/04/07/GES01252.1.abstract.
This study examines the sedimentary rocks of the Roberts Mountains allochthon (RMA) of north-central Nevada. The RMA is a package of rocks that geologists believe were emplaced onto the western margin of North America in Late Devonian-Early Mississippian time. The origin of these rocks has long been controversial; geologists have theorized that the rocks came from as far away as the ancient continents of Baltica or Gondwana. The RMA rocks are of great interest, as they are closely related to the rich gold-bearing strata of Nevada. The researchers analyzed the zircon grains in these rocks, and determined both the uranium-lead ages and the hafnium isotope ratios. Using these data, the researchers discovered the provenance of the sandstones. The RMA strata are from two distinct regions, neither of them exotic to North America. The Ordovician lower Vinini Formation originated in the central continent, while the remainder of the strata, from Ordovician to Devonian in age, originated near the Peace River Arch of western Canada. The researchers proposed a tectonic evolutionary history for the RMA rocks wherein the rocks were deposited from Ordovician through Devonian time and subsequently transported south along the western margin of North America by a transform fault system, and then emplaced onto the continent.
How diking affects the tectonomagmatic evolution of slow spreading plate boundaries: Overview and model
Valerio Acocella and Daniele Trippanera, Dipartimento Scienze, Università Roma Tre, L. S.L. Murialdo, 1, 00146 Roma, Italy. This article is online http://geosphere.gsapubs.org/content/early/2016/03/25/GES01271.1.abstract.
This article summarizes the state of our knowledge on how divergence may occur along oceanic and continental plate boundaries. Based on this, it proposes an original model stressing the importance of magmatic intrusions (dikes) in achieving such a divergence. This implies that non-magmatic processes may have a subordinate role in rifting plate boundaries.