CIDER 2011 Summer Program

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The CIDER 2011 Summer Program theme is the dynamics of mountain building. Mountain belts are one of the most obvious manifestations of the interior dynamics of our planet. The globe is characterized by 2 great mountain belts, the east-west trending Himalayan-Tethyan system running from Gibraltar to eastern Asia, and the north-south trending American Cordillera, extending from Tierra del Fuego to the Aleutian islands. The exploration of these mountain belts during the last two centuries, and modern mountain climbing, capture the public's imagination in the manner of astronomy and space exploration ¹.

Mountain belts and the dynamics of their formation are a fascinating subject in Earth science. Developing an understanding of the dynamics of orogenesis requires a broad multi-disciplinary approach; one that includes geodynamics, seismology, geodesy, metamorphic and igneous petrology and geochemistry, structural geology, sedimentology, tectonic geomorphology, and mineral physics. EarthScope, particularly the Plate Boundary Observatory and the recently completed USArray deployment across the western U.S. orogenic plateau, as well as recent large-scale geodetic and seismology experiments in the Andeas, Himalayas, and Tibetan Plateau, offer us an unprecedented opportunity to apply whole new datasets to understanding mountain forming processes.

Most mountain belts result from plate interactions across broad areas now termed diffuse plate boundaries, in which the assumption of rigid plates is relaxed or cast off entirely, however, no systematic theory exists to explain the behavior of these boundaries. The two great mountain belts owe their existence to long-lived subduction zones. Early in plate tectonic theory, the mountain ranges found at convergent boundaries were thought to form largely by compression between the plates with magmatic additions to the crust coming from slab melting in the downgoing plate. This view has been shown to be flawed in many respects: slabs do not melt, rather they dehydrate causing basaltic melting in the overlying mantle wedge, which migrates upward and subsequently melts crustal rocks; formation of large igneous bodies, e.g., the Sierra Nevada batholith, are the culmination of a multi-step process. Although the convergent structures in the accretionary wedge at the suduction zone result from compression between plates they are modulated by the addition or removal of subducted sediments from below. The larger structures found inboard of the active magmatic arcs can result from inter-plate compression, from collision with an atypical oceanic plate whose buoyancy resists subduction, for example a large igneous province, or sea-mount, which may lead to flat slab subduction, resulting in hydration and deformation in of the over-riding plate, as in the Laramide uplifts in the western U.S, and from internal collapse of large orogenic plateaus, e.g., the fold and thrust belts along the eastern Andean plateau. The circumstances under which an orogenic plateau forms are still poorly understood, particularly for ocean-continent convergence. Moreover a number of surprising results have emerged from research in the last two decades suggesting that orogenesis is a more complex dynamical system than had been previously thought:

   1) Erosion, and therefore climate, can greatly affect the form mountain belts assume and in turn rapidly ascending mountains greatly affect climate resulting in a positive feedback loop. Erosion not only controls the depths of rocks exhumed, as measured by metamorphic grade, but the prevailing climate systems can also control the overall vergence of uplift. As an example of the effects of climate on orogeny and orogeny on climate, the Andean plateau actually is a series of very deep, filled, internally drained sedimentary basins, the plateau is so high that it blocks most Pacific moisture from the reaching the interior of the plateau and the rest of South America, which in turn limits sedimentation off of the plateau.

   2) The strength of the megathrust zone in subduction zones is thought to relatively low, but its behavior can vary greatly, from being completely locked to completely slipping. Explanations for the degree of plate coupling include sediment or water lubrication, the thickness of the sediment load lying on the thrust zone, and the presence or absence of structural heterogeneities in the subducting plate.

   3) Almost all orogenic belts are capped by extensional, not compressional structures, resulting from collapse as gravitational energy gained by uplift overcomes rock strength.

   4) A process that is increasingly recognized as important in the growth of mountains is delamination of the base of the crust and lithosphere as orogenic plateaus grow and collapse due to their gravitational potential energy. This process can include "eclogitization" of mafic lower crust as mafic mineralogies are forced into a higher pressure field, metamorphose to garnet and other dense minerals, resulting in an unstable negative buoyancy in the middle of the lithosphere. When occurring locally, it can lead to downwelling of the lower lithosphere, while the overlying crust rapidly uplifts. In places is appears that this process can cascade, leading to wholesale loss of the lithosphere beneath a mountain belt, with attendant uplift and widespread volcanism from decompression melting as the asthenosphere replaces sinking lithosphere.

   5) Although long recognized beneath different mountain belts, intermediate depth seismicity (from ~100 km to ~400 km) has no satisfactory physical explanation. Either resulting from delaminating or subducting lithosphere it is difficult to develop a rheology that will permit brittle failure in a relatively hot mantle.

¹ One of the first widely known scientists in history, Alexander von Humboldt, owed his public fame largely to having climbed what was thought to be Earth's highest peak, Chimbarozo, in the Ecuadorian Andes. Chimbarozo is in fact the point on Earth farthest from Earth's center. Although failing to reach the summit, Humboldt reached an elevation of 5,878 meters, the highest ascent to that time. Following reports of his climb the European public followed Humboldt's South American expedition avidly. At his death Humboldt was considered to be the most widely known person on Earth.

For inquiries and practical questions, contact Micaelee Ellswythe (micaele AT berkeley DOT edu).

For questions on the scientific program, contact Alan Levander, Rice University (alan AT rice DOT edu).

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