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Rates and directions of crustal extension in a continental rift vary in time and space as the rift evolves, and these geologic records are often preserved along fault planes. Some fault-kinematic studies have been undertaken in the central to northern segments of the Rio Grande rift, but similar studies from the southern part of the Rio Grande rift of western Texas, USA, and northern Mexico are fewer. We present new fault-kinematic data from six locations in the southern Rio Grande rift of Trans-Pecos Texas, combined with U-Pb dating of calcite slickenlines, to constrain the directions and time scales of extension. All locations preserve NE-SW−oriented extension, and locations within the Sunken Block graben preserve a more complex kinematic history of multiple extension directions. Four U-Pb ages range from 30.1 ± 3.1 Ma to 13.7 ± 0.9 Ma. Combined with fault-kinematic data and assuming a constant stress regime between 30 Ma and 14 Ma, these data support the interpretation that earliest extension in the southern rift was oriented NE-SW, and extension rotated clockwise to E-W and NW-SE after 13.7 ± 0.9 Ma. Based on available data, this rotation was broadly coincident with rotation in the extension direction in the southern Española basin and in the Basin and Range Province. These differences suggest that extension in the Rio Grande rift responded to the evolving western boundary of the North American plate but included initial underlying driving forces that were supplanted by lateral forces as the transform margin lengthened. Additionally, geochronologic and kinematic data across the Sunken Block graben of the southern Rio Grande rift indicate that the locus of rifting concentrated with time toward the center of this basin; such structural narrowing has previously been demonstrated in the northern segment of the rift. This study provides a much-needed comparison between the southern and northern segments of the rift but highlights the need for more collection of combined kinematic and geochronologic data.
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Paleomagnetic data bearing on vertical axis rotation of the Rio del Oso dike swarm, western Españ ola Basin, New Mexico
The Española Basin is one of a series of interconnected, asymmetrical basins in the Rio Grande rift that includes a number of north- and northeast-striking faults that accom- modated block tilting and basin subsidence. The western margin of the Española Basin, in particular, is characterized by a greater than 17-km wide zone of normal and oblique-slip faults. To clarify the involvement of block rota- tion in the tectonic evolution of the Española Basin, we carried out a paleomagnetic study of mafic intrusions (Rio del Oso dike swarm) that are genetically related to regionally exten- sive basalt flows of the mid-Miocene Lobato Formation. The primary hypothesis tested was that these intrusions experienced some degree of vertical axis rotation associated with mid-Miocene to recent continental rifting. In situ paleomagnetic results from forty-two sites yield a group mean declination (D) of 344.0°, an inclination (I) of 41.1°, α95 of 6.1°, and k of 14.1. The group mean result is discordant to the <10 Ma pole of D=356.0°, I=54.4°, α95 = 3.3° with a statistically significant inferred rotation (R) of -12.0°± 7.2° and flattening of +13.3° ± 5.5° relative to the <10 Ma pole field direction. These discordant results indicate that a modest degree of counter-clockwise vertical axis rotation occurred in this region, which is likely associated with Rio Grande rifting north of the Jemez Mountains. It is possible that oblique motion along the Santa Clara fault and/ or the Cañada del Almagre fault facilitated the vertical axis rotation. The results from this study imply that vertical axis rotation is common to extensional rift systems and should be considered when modeling continental extension.
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- Award ID(s):
- 1918357
- PAR ID:
- 10297106
- Date Published:
- Journal Name:
- New Mexico geology
- ISSN:
- 0196-948X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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