Boulder, Colo. -- The February issue of GEOSPHERE, published in electronic format only by the Geological Society of America, is now available online. Geology topics of interest include: first documentation of the eruptive history of the USA's southern Rocky Mountains, which includes some of the largest subvolcanic magma chambers known on Earth; new data illustrating tectonic evolution of the seismically active Gargano Promontory in central Italy; and formation of Canada's Monashee Mountains in southern British Columbia. Articles are open access and may be viewed or downloaded at http://www.gsajournals.org/perlserv/?request=get-toc&issn=1553-040X&volume=3&issue=1.
Neogene-Quaternary intraforeland transpression along a Mesozoic platform-basin margin: The Gargano fault system, Adria, Italy
Andrea Billi, Università Roma Tre, Rome, 00146, Italy; et al. Pages 1-15.
This article deals with the tectonic evolution of the Gargano Promontory in central Italy. This is a seismic area that has recorded several destructive earthquakes in historical times. The last important seismic event in 2002 caused the death of 27 children and one of their teachers in the collapse of the San Giuliano School. In this paper, on the basis of field geological data and offshore geophysical evidences, the Miocene-Recent history of the seismic Gargano fault system is reconstructed. The tectonic activity of this fault is connected with the complex plate tectonics of the central Mediterranean area that is part of the broader Africa-Eurasia convergence zone.
Paleomagnetic dating of fault slip in the southern Rocky Mountains, USA, and its importance to an integrated Laramide foreland strain field
T.F. Wawrzyniec, University of New Mexico, Albuquerque, NM 87107, USA; et al. Pages 16-25.
Using the record of Earth’s magnetic field found in fault zone materials of the Picuris-Pecos fault system, which is the largest strike-slip fault found in the eastern flank of the western U.S. Cordillera, Wawrzyniec et al. have demonstrated that no significant modification of this record has taken place since the ancestral Rocky Mountain orogen (about 300 million years ago). Additional field based observations further indicate that the largest phase of deformation took place during the Precambrian eon (over a billion years ago). These findings have important implications for tectonic models that describe the formation of Cordilleran foreland uplifts that lie east and north of the Colorado plateau, which is a relatively coherent fragment of crust that makes up a large part of the southwest United States. Researchers know very little about the chronology of large slip events and therefore there is very little agreement over the relative magnitude of major deformation events that have affected this part of the western Cordillera, particularly over the past 300 million years. Specifically, the plateau during this time frame has not been strongly translated or rotated with respect to the North American craton, and there may be no direct link between the Cenozoic foreland uplifts and the forces that produced these features. The latter has important implications for our understanding of how foreland deformation may be coupled to plate boundary conditions over 1000 km inboard of the plate margin.
The Monashee reflection: Re-examination of a Lithoprobe crustal-scale seismic reflection in the southern Canadian Cordillera
Stefan. Kruse and Paul F. Williams (corresponding author), University of New Brunswick, Geology, Fredericton, NB E6K 2L2, Canada. Pages 26-41.
This research is aimed at understanding how the Southern Canadian Rocky Mountains were formed and has implications for mountain building processes in general. The area of interest lies in the Monashee Mountains of southern British Columbia. Geological mapping, observations made on Earth’s surface, and subsurface evidence from Lithoprobe (a Canadian National research project) seismic sections is combined in an attempt to interpret the structure of Earth’s crust and the history of development of the structure. A major crustal-scale structure is observed in the seismic sections, below the Monashee Mountains. This structure extends to a depth of ~30 km and is interpreted as a product of horizontal stretching of the crust. The stretching is a manifestation of the collapse of an earlier mountain chain. [This article includes three animations.]
Incremental assembly and prolonged consolidation of Cordilleran magma chambers: Evidence from the Southern Rocky Mountain volcanic field
Peter W. Lipman, U.S. Geological Survey, Menlo Park, CA 94025, USA. Pages 42-70
The overall eruptive history of the enormous composite volcanic field that blanketed the Southern Rocky Mountains in the western USA between 37 and 26 million years ago, including at least 30 large ignimbrite sheets (individually with volumes of 100–5000 km3) and associated caldera collapses with dimensions as much as 75 km across, is documented in this paper for the first time, based on recent isotopic age determinations and new volume estimates. These results provide a basis to infer processes involved in the incremental growth and prolonged consolidation of some of the largest subvolcanic magma chambers known anywhere on Earth, processes typical of Cordilleran magmatism elsewhere such as in the Sierra Madre of northern Mexico and the central Andes of South America. Granitic plutons such as those in the Sierra Nevada batholith of California are interpreted as the solidified remnants of similar subvolcanic magmatic activity; the plutons providing an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages.
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