Abstract

Paleozoic sedimentary rocks in a wide area of the northern Sangre de Cristo Range show effects of heating during Tertiary time. Heating is tentatively interpreted as a response to burial during Laramide folding and thrusting and also to high heat flow during Rio Grande rifting.

The regional extent of heating is shown by the distribution of low-grade metamorphic minerals, altered conodonts, and reset fission-track ages throughout much of the study area. Alteration of conodonts to a conodont alteration index (CAI) of 4.0 suggests that temperatures reached ∼200 °C in the central part of the area. Temperatures may have reached 300 °C beneath Laramide thrusts on the west side of the range, where conodonts were altered to a CAI of 5.0, and where chloritoid and andalusite are found in sedimentary rocks of Pennsylvanian age. The lowest temperatures that were determined by conodont alteration (CAI = 1.0–2.0, <50–70 °C) are along the east side of the range, where rocks were evidently never buried deeply. Contact metamorphism was restricted to wall rocks of a few isolated stocks; near dikes and sills, it was not significant.

Fission-track ages of apatite across a section of the range show that rocks cooled abruptly below 120 °C, the blocking temperature for apatite, ∼19 Ma ago. Cooling was probably in response to rapid uplift and erosion of the northern Sangre de Cristo Range during early Rio Grande rifting.

First Page Preview

First page PDF preview
You do not currently have access to this article.