Paleomagnetic and Structural Investigation of the Wyoming Salient: Insights into the Curvature of Mountain Belts

Posted May 28th, 2010 at 10:24 am.

Melissa Lindholm

Curved mountain belts are common throughout the world, but the controlling factors and mechanisms in their formation are not well understood and likely vary from location to location. The Wyoming Salient (term for curved orogen that is convex towards the mountain belts interior) in the Sevier Mountains of the Western United States is an example of a classic curved mountain belt on which much research has been done, making it an ideal location to explore questions about the kinematics and mechanics of curved orogens. The first order question for any curved mountain belt is whether curvature was attained at the beginning of mountain belt formation (primary arc), formed after mountain-building was complete (orocline), or curved progressively as the belt developed (progressive orocline). If the rocks making up the mountain belt rotated about a vertical axis (suggesting curvature occurred after or during initial formation of the orogen), this will be revealed in the orientations of magnetic minerals in rocks within the arc. Paleomagnetism, the study of the Earth’s ancient magnetic field as it is recorded in the rock record, is an ideal tool to quantify these rotations.

Paleomagnetic samples will be collected throughout the arc to determine the amount of vertical-axis rotation recorded in the Wyoming salient. These samples will be collected mainly from two formations: from limestone members of the Jurassic Twin Creek formation and redbeds of the Triassic Ankareh formation. Rocks of the Wyoming salient display multiple magetizations, including primary and secondary components that reveal different stages in the growth and development of the salient. Synorogenic Cretaceous strata will also be sampled to constrain the timing of rotation. The results of these paleomagnetic studies will be integrated with strain analysis and three-dimensional restorations to create a kinematic model for the development of the salient. This model can then be used to identify mechanisms and controlling factors in the development of the salient, which may include variations in layer thickness, foreland buttressing, detachment strength, and the shape of indenting thrust sheets in the hinterland. The development of this model will be the end product of a long-term collaborative project Dr. Arlo Weil is involved in to determine the three-dimensional kinematic history of the Wyoming Salient. The project will give insight into the processes involved in the evolution of curved orogens.

This summer, I will be collecting field observations and structural measurements from the salient and collecting paleomagnetic samples from throughout the arc. I will then bring these samples back to Bryn Mawr, where I will help prepare them for analysis and eventually analyze them in the new paleomagnetic lab, using a spinner-type magnetometer.

Filed under: 2004,Lindholm, Melissa,Weil, Dr. Arlo by Ann Dixon

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