1. Using Stochastic Point Pattern Analysis to Track Regional Orientations of Magmatism During the Transition to Cenozoic Extension and Rio Grande Rifting, Southern Rocky Mountains.
- Author
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Rosera, J. M., Gaynor, S. P., Ulianov, A., and Schaltegger, U.
- Abstract
The southern Rocky Mountains in Colorado and northern New Mexico hosted intracontinental magmatism that developed during a tectonic transition from shortening (Laramide orogeny, ca. 75 to 40 Ma) through extension and rifting. We present a novel approach that uses stochastic weighted bootstrap simulations of a large set of new and historical geochronology data to better understand how regional anisotropies responsible for focusing magma emplacement evolved through time. This technique can detect subtle trends in directional distributions, including multi‐modal orientations, and can be filtered from regional to local scales. Our results indicate that magmatism followed first the northeast trend of the Colorado mineral belt between 75 and 40 Ma and deviated afterward. These deviations vary depending on the scale of the analysis. At the smallest scale we evaluated (<75 km), the orientation of magmatism from 45 to 30 Ma rotated counter‐clockwise before aligning with the north‐south trend of the modern Rio Grande rift. Larger, regional‐scale analyses indicate magma centers between 40 to 35 Ma and 25 to 20 Ma were dominantly oriented southwest‐northeast, whereas magmatism between 35 and 25 Ma had north‐south orientation. The large areal footprint of magmatism and shifting regional patterns suggest that ancient zones of weakness in the North American lithosphere accommodated magma flow at different moments in time, rather than controlled by a retreating interface of the Farallon and North American plates. Plain Language Summary: Continents can be deformed by compression or extension, and this deformation is commonly accompanied by magma ascension and crystallization in the upper crust. In many cases, magmas generated during a given time period are emplaced and crystallize aligned to their dominant stress fields, such as compression or extension. It is also possible for magmas to solidify aligned along ancient faults (and associated weaknesses), and therefore their spatial orientations may reflect tectonic events much older than magma crystallization. However, these "magma alignments" are not always obvious in maps, because there are many factors that go into dating magma systems, and magma flow is not always limited to one or a few obvious faults or related features. This study introduces a technique that treats sample locations for dated rocks as a point pattern and applies previously developed point pattern analyses to detect subtle alignments in magmatism. We apply this tool to a well‐known example in the western United States, where magmatism was continuous from tectonic compression through extension, to evaluate how magma alignments shifted through time. Key Points: Fry analyses reveal shifting regional orientations in Eocene‐Oligocene magmatism in the southern Rocky MountainsRegional‐scale magmatism followed ancient continental structures at various time periodsSmaller‐scale analyses indicate magma orientations rotated counter‐clockwise between 45 and 30 Ma [ABSTRACT FROM AUTHOR]
- Published
- 2024
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