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What happens to Earth’s crust as it is stretched and thinned?

figure
Panorama of the central Reserve graben, with rift sediments exposed in the foreground and middle ground. Some of these sediments are gently folded, showing faults were still active after they were deposited. The cliffs in the background are made of pre-rifting andesite lava flows several hundred meters thick.
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Sam Martin
figure
An eroded-out normal fault in the Reserve graben, with very coarse rift sediments in the hanging wall (left side) and pre-rifting volcanic rocks in the footwall (right side). Faults are rarely this well-exposed in the field!
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Sam Martin
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The flat-topped mesas in the background are capped by a basalt flow dated at about 1.9 million years old. The older volcanic rocks and rift sediments beneath the lava flow are tilted down to the west (left), which makes the base of the lava flow an “angular unconformity.” The older rocks had to have been deposited, tilted, and then partially eroded off before the basalt flowed over them, so the surface between these rocks and the lava flow represents missing time in the rock record.
(click for a larger version)
Sam Martin

Reserve Graben, Catron County, NM
— April 14, 2021

New Mexico is a great place to study extensional tectonics, or what happens to Earth’s crust as it is stretched and thinned. The Reserve graben, a small rift basin in western Catron County, is part of a longer fault system, including the Plains of San Agustin, at the southeastern margin of the Colorado Plateau. It also sits within the massive Mogollon-Datil volcanic field, where some of the largest explosive eruptions in North America’s history have occurred. The San Francisco River and its tributaries have cut down through the sedimentary rock that filled in the basin, exposing many of the basin’s internal faults. We can measure the orientation of features such as striations on fault surfaces to tell which direction the faults slipped while they were active. Basalt flows are also interlayered with sedimentary basin fill in many places throughout the graben; through radiometric dating on these basalts, we can determine the timing of rifting and sedimentation. New 40Ar/39Ar dating at the New Mexico Bureau of Geology’s geochronology lab shows that the Reserve graben began to form around 16.4 million years ago, and that rifting had ended here by about 1.9 million years ago. We are working to compare the timing and direction of movement on the Reserve graben’s faults to the broader Rio Grande Rift and Basin and Range provinces for a more complete picture of the region’s recent tectonic history.

—Sam Martin, Geology M.Sc. student, NMT E&ES department