NSF Award
2220233
The formation of mountain belts is one of the most important processes that takes place on Earth. Mountain belts affect the distribution of key Earth resources, including economically important minerals, petroleum, and water. They form as a result of plate tectonics, along boundaries between converging plates. When mountain ranges become too high to be stable, the mountain belt may expand laterally, or material may ‘escape’ from the belt, due to the force of gravity. This escape may occur along faults that are exposed at the surface, where material is pushed away sideways from the mountain belt, or simply by collapse of material from higher to lower elevation. At mid-to deep-crustal levels (more than ten kilometers below the Earth’s surface) rocks flow in a fluid-like manner. In present-day mountain belts such as the Himalaya, these fluid-like rocks can be squirted away from the mountain belt, either toward the surface or entirely below the surface along tabular channels. These are important processes that contribute to the formation and modification of mountain belts, but they remain imperfectly understood. Specifically, because these rock flow zones are often below the surface, they are difficult to investigate in present-day mountain belt systems. The purpose of this project is to investigate an ancient flow zone in eastern Massachusetts that formed about 420 to 360 million years ago and today is partially exposed at the surface. Geological techniques will be used to investigate the zone at the surface and geophysical imaging techniques to elucidate the subsurface geometry. A better understanding of the formation of these flow zones will help us understand both ancient and modern mountain building processes in more detail, with important implications for our understanding of how Earth resources are distributed. This project will involve multi-disciplinary research that brings together geologists and geophysicists working in the Appalachians, and is synergistic with ongoing national and international collaborations. This project will contribute to the training of undergraduate and graduate students, with a focus on training students from historically untapped groups through various programs at the Colorado School of Mines and Yale University.<br/><br/>An integrated geophysical and geological approach will be used to test a model of channel flow and ductile extrusion for one of the Appalachian terranes, the Nashoba terrane, in SE New England. Channel flow is flow of a weak, partially molten mid- to lower crustal layer between more competent overlying and underlying crust as a result of crustal thickening and pressure gradients. Localized denudation at the surface may cause ductile extrusion towards the surface. The purpose of this project is not only to further test a hypothesis for the evolution of the Nashoba terrane based on field and geochronology data, but also to visualize ductile flow of rocks during the geologic past below the surface using geophysical data. To do this, a tightly spaced array of six broadband seismic stations will be deployed across the Nashoba terrane in eastern Massachusetts, complementing currently available data in the area. Additionally, existing data from the Putnam terrane in eastern Connecticut, as sampled by the SEISConn array, will be used. While the top of the interpreted ductile extrusion zone is well constrained along the NW boundary of the Nashoba terrane, the SE part of the zone may have incorporated part of the Avalon terrane SE of the Nashoba terrane. New structural mapping and geochronology will be carried out in this part of the Avalon terrane to constrain the boundary of the ductile extrusion zone better. Combined new and existing structural, geochronological, and seismic imaging constraints will be used to test the channel flow hypothesis against alternative hypotheses, including thrust and normal faults, a positive flower structure, or a metamorphic core complex. The evolution of the Nashoba and Putnam terranes will then be placed in the overall context of the tectonic history of the SE New England Appalachians. Methods used and potential outcomes may provide new evidence for fundamental processes behind evolution of orogenic systems, enabling comparisons with modern systems (the Himalayas) as well as other ancient orogens (e.g., Canadian Cordillera) where channel flow and ductile extrusion have been proposed.<br/>Funding for this project is provided by NSF EAR Tectonics and Geophysics Programs.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
