Synchronous opening of the Rio Grande rift ~20-10 Ma supported by apatite (U-Th)/He and fission-track thermochronology, and evaluation of possible driving mechanisms

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152 new apatite (U-Th)/He (AHe) dates are presented from 34 sample locations along the flanks of the Rio Grande rift in New Mexico and Colorado. These data are combined with apatite fission-track (AFT) analyses of the same rocks and modeled together to create well constrained cooling histories for Rio Grande rift flank uplifts. The data indicate rapid cooling from ~28 Ma to Recent in the Sawatch Range and the Sangre de Cristo Mountains, ~21 to 5 Ma in the Albuquerque basin, and ~17 to 8 Ma in the southern Rio Grande rift in southern New Mexico. Rapid cooling of rift flanks followed the Oligocene ignimbrite flare-up and the northern section of the Rio Grande rift in Colorado exhibits semi-continuous cooling since the Oligocene. Overall, however, rift flank cooling along the length of the rift was out of phase with high volume magmatism and hence is inferred to have been driven mainly by exhumation due to faulting. Although each location preserves a unique cooling history, when combined with existing AHe data from the Gore Range in northern Colorado and the Sandia Mountains in New Mexico together these data indicate ~ synchronous extension and rift flank uplift along >850 km of the length of the Rio Grande rift from ~20-10 Ma. These time-space constraints provide an important new dataset to develop geodynamic models for initiation and evolution of continental rifting. Models involving northward unzipping and Colorado Plateau rotation are not favored as primary mechanisms driving extension. Instead, a geodynamic model is proposed that involves upper mantle dynamics during multi-stage foundering and rollback of a segment of the Farallon plate near the Laramide hinge region that extended between the Wyoming and SE New Mexico high velocity mantle domains. First stage delamination accompanied and followed ~40-20 Ma volcanism in the San Juan and Mogollon-Datil ignimbrite centers. A second stage involved a ~30-20 Ma detachment of the remaining part of the Farallon slab. This produced renewed uplift of the Alvarado Ridge topographic high, enhanced surface uplift of rift flanks, developed a central graben with increased fault- related high strain rates, and resulted in maximum sediment accumulation in the Rio Grande rift. Our geodynamic model thus involves Oligocene removal of parts of the Farallon slab beneath the ignimbrite centers followed by a major Oligocene-Miocene slab break that instigated the discrete N-S Rio Grande rift, continuing upper mantle convection, and differential uplift of the southern Rocky Mountain - Rio Grande rift region.

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