THE RIO GRANDE RIFT IN NORTHERN COLORADO

In his paper that first extensively described the Rio Grande rift in Colorado, Tweeto (1979) applied the term "Rio Grande rift proper" to the 'more or less continuous graben of Neogene age that terminates northward at the head of the upper Arkansas Valley near Leadville.' He applied the term "Rio Grande rift system" to 'the much more extensive belt of Neogene block faulting, including not only the rift proper but neighboring and northward-continuing faults that were active concurrently with the rift proper.' Decker et al. (1988) published heat-flow data demonstrating that the heat-flow anomaly characteristic of the Rio Grande rift proper is also associated with the Rio Grande rift system, and extends north to the Wyoming state line. New data have been collected, including geothermal gradients estimated from deep, corrected bottom-hole-temperatures and radiogenic heat production measurements from silicic plutons and basement rocks. These data confirm that a major thermal anomaly in northern Colorado is associated with the Rio Grande rift system. This anomaly predicts super-solidus temperatures in the lower crust if steady-state conditions are assumed, as originally demonstrated by Decker et al. (1988). The upper crustal structural elements and the thermal anomaly associated with the Rio Grande rift system truncate close to the Colorado-Wyoming state line. Minor volcanic rocks associated with the Rio Grande rift system in northern Colorado are predominantly late Oligocene/early Miocene in age, although the youngest volcanism in Colorado (4,000 a, at Dotsero) is also in northern Colorado. There is no volcanism to correlate with very young crustal intrusions that could be the source of the contemporary thermal anomaly, however. Topography along the Rio Grande rift system in Colorado is consistent with isostatic support provided by thin lithosphere as predicted by the thermal anomaly.