Recognizing Fossil versus Remobilized Crustal Boundaries in Alaska through Earthscope TA Combined with Geologic History
Abstract
The assembly of Alaska's crust is often referred to as occurring by a series of accretionary orogens, and much of northern and southern Alaska preserves and/or is being modified by these processes. However these subduction and collisional assemblages have been significantly modified by local crustal thinning and numerous strike-slip faults. We compare the geologic history of some of these faults and the rocks they juxtapose with a new interpretation of the Moho of all Alaska to address these questions: to what extent is the Moho surface a relic of pre-Cenozoic tectonic processes vs. a surface reflecting late Cenozoic - modern processes?
The Earthscope TA full deployment in AK produced an unprecedented view of the crustal thickness of the whole region, as determined in this presentation by P-wave receiver functions. The map view of hand-picked individual stations' depth to Moho reveals an irregular Moho surface, both the previously known N-S variations but also significant changes in depth from E to W. Some of the abrupt changes in Moho depth are across known strike-slip faults that have not been active in 10s of millions of years, suggesting that "relict" or inherited crustal features can remain well-preserved, despite evidence of active tectonics across most of the region. The lack of a smooth Moho in any part of the state and the coincidence of steps in the Moho across active and defunct strike-slip faults does not support a décollement or slip transfer surface within any crustal layer as a way to transfer strain far inboard from the Pacific margin. One of the notable Moho features is a significant region of thick (> 40 km) crust in SW Alaska, in the back-arc of the modern Alaska - Aleutian arc. This region includes Paleoproterozoic basement (the Farewell terrane) juxtaposed with Mesozoic arcs along poorly exposed boundaries. This thicker crust area coincides with a region of very low seismicity and no evidence of internal deformation, and we interpret that the crust and possibly lithosphere may not have undergone significant modification since Paleozoic time. Its unique properties and geologic history relative to terranes around it indicate that it has been translated many 100s of kms away from a continental margin in the late Mesozoic, and abrupt changes in Moho thickness coincide with parts of this boundary.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT050...12R
- Keywords:
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- 1209 Tectonic deformation;
- GEODESY AND GRAVITY;
- 8102 Continental contractional orogenic belts and inversion tectonics;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS