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Department of Geology
Bryn Mawr College
101 North Merion Avenue
Bryn Mawr, PA 19010
Phone: (610) 526-7392
Fax: (610) 526-5086

Arlo B. Weil


Idaho-Wyoming Fold Thrust Belt

Most mountain belts display varying degrees of curvature over a range of scales, recording complex, three-dimensional deformation histories. However, despite their ubiquitous occurrence, few examples exist where deformation histories and processes responsible for orogenic curvature have been adequately quantified from geologic evidence. These histories are embedded in regional structural trends, paleomagnetic directions, mesoscopic structures, and grain-scale fabrics of deformed rocks. The Utah-Idaho-Wyoming salient of the Sevier orogenic belt is an ideal location to study processes that produce curvature over a range of scales. Regional trends of major thrust faults and folds curve through 90 degrees from the north to south end of the salient, with additional local curvature near cross-strike transfer zones. Rocks in the salient also carry multiple paleomagnetic components and display systematic suites of mesoscopic and microscopic structures (including cleavage, minor folds, minor fault networks, and deformed fossils), providing a record of rotations and internal strain within individual thrust sheets. Ongoing work is focused on combined paleomagnetic and structural studies of two stratigraphic levels (Jurassic Twin Creek Limestone and redbeds of the Triassic Ankareh Formation) that are well exposed, carry multiple magnetizations, and contain multiple strain markers. Analysis of slip lineation data from major thrust faults and construction of two-dimensional cross sections indicate that thrust transport directions change overall from ENE in the northern part to ESE in the southern part of the salient, and fault slip magnitude decreases toward the north and south ends of the salient. Three-dimensional cross sections, constrained by abundant seismic data, and incorporating new strain and rotation data from this study, are being constructed to develop a robust kinematic model of the salient.

Cantabria-Asturias Arc

The Cantabria-Asturias Arc (CAA) is the foreland fold-and-thrust belt of Iberia’s Variscan orogeny that makes up the core of the larger Ibero-Armorican Arc of Western Europe.  The arc is a unique structural feature with 180 degrees of arc curvature that is concave towards the foreland.  My investigation involved the disciplines of structural geology and paleomagnetism, with concentration on: 1) the large-scale geodynamic conditions responsible for the formation of the CAA, including investigation of a highly contentious dextral megashear, proposed to allow large-scale relative movements between Gondwana and Laurussia in the Permian; and 2) details of the timing of deformation and the kinematic history that created the CAA.  The results from these studies demonstrate that the CAA’s curvature is related to a change in regional stress field, from an initial east-west compressional phase (Late Carboniferous) to a later north-south compressional phase (Permian).  Geodynamically, this change in regional stress field was caused by the north-northeastward movement of Gondwana relative to Laurussia (a sinistral motion), and is in direct conflict with the dextral megashear proposed for the Early Permian.

Deformation related Carbonate Remagnetization

The amalgamation of Pangea during the Late Paleozoic is widely recognized as having caused remagnetizations in all of Pangea's major blocks.  The ubiquity of these remagnetizations has raised many questions concerning their relationship with orogeny, as well as their possible cause(s) and carrier(s).  To begin answering these questions, I have studied the remagnetized Devonian carbonates collected from the CAA.  These rocks experienced several sequential remagnetizations during the Variscan orogeny.  To characterize the distribution of crystal morphology and granulometry and to determine in which minerals the magnetic remanence resides, I determined rock magnetic properties and used scanning electron miscroscopy (SEM) on whole-rock samples and magnetic extract from the collection of Devonian carbonates.  Together, the results revealed that the late Paleozoic remagnetizations experienced by CAA carbonates are chemical remanent magnetizations facilitated by the presence of fluids that were activated during the Variscan orogeny.  Mega-scale tectonics likely increased fluid mobility as a product of tectonic thickening, tectonically induced permeability, and/or gravity driven flow, which ultimately facilitated the growth of new magnetic material within the carbonates.  Further work on remagnetizaed carbonates in presently underway.

Proterozoic Paleogeography

The existence of a long-lived Precambrian supercontinent, with Laurentia at its core surrounded by the Gondwana cratons, Baltica, and Siberia, has been the topic of considerable debate over the last decade.  Although still a hypothesis based on limited data, the existence of Rodinia has gained credibility within the paleomagnetic and geologic community.  In an attempt to compile all available paleomagnetic data for a coherent Rodinia reconstruction, I have synthesized apparent polar wander paths (APWP) for the major West Gondwana blocks, Laurentia, and Baltica.  The goal of this project is to create a viable geologic reconstruction for the Earth’s major tectonic blocks for the 1.10 to 0.80 Ga time interval.

Paleomagnetism of the Grand Canyon Supergroup

At the other end of the Rodinia debate are questions related to the timing and extent of breakup of the supercontinent and its subsequent influence on the Earth’s biosphere and hydrosphere.  This time interval (780 Ma - 560 Ma) encompasses a dramatic period of change in the history of our planet, and includes unparalleled carbon-isotope fluctuations, a hypothesized ice-covered “snowball” Earth, and the evolutionary radiation of eukaryotic organisms.  The influence of tectonism, in particular the breakup of a supercontinent, is essential to our understanding of this period of geological time.  As part of a large collaborative research effort with geologists from the University of New Mexico, Harvard, and MIT, I have worked on the paleomagnetism of the Proterozoic Grand Canyon Supergroup.  These new data fix Laurentia in a paleogeographic reference frame for the Late Neoproterozoic and fill missing parts of Laurentia’s APWP.  When these new data are compared with equivalent data from East Gondwana, it appears that rifting, and likely drifting, had initiated on the Cordillera margin of Laurentia by ~740 Ma.   This large-scale event could have provided adequate tectonic forcing for dramatic global climate and biological changes that characterize the late Neoproterozoic.

Paleomagnetism of the Uinta Mountain Supergroup

To further characterize the paleogeography of Laurentia in the Neoproterozoic a paleomagnetic project is underway with researchers from the University of New Mexico on the Uinta Mountain Supergroup, northern Utah.