Abstract

The kinematic and erosional history of the Rio Grande uplift, a large northwest-trending, basement-involved, thrust-bounded, block uplift of Laramide age (latest Cretaceous–early Tertiary) in south-central New Mexico, is documented by clastic rocks that accumulated in the complementary Love Ranch basin. The synorogenic to postorogenic McRae and Love Ranch Formations are as much as 1460 m thick; they filled the Love Ranch basin and onlapped the Rio Grande uplift. Present outcrops of the two formations cover an area of 100 km2 and reveal the stratigraphic architecture of the basin fill in three dimensions. Lithofacies distribution, clast size and composition, paleoflow data, syndepositional structures, nature of the basal unconformities, and ages of basin fill provide essential data for constraining uplift history.

Laramide shortening began between Campanian and latest Maastrichtian time, and initially created open, symmetrical, northwest-trending folds, as well as a broad, approximately symmetrical uplift. This incipient Rio Grande uplift was capped by Upper Cretaceous volcanic rocks of intermediate and silicic composition, which were the primary source of volcanic detritus in the latest Cretaceous–Paleocene(?) McRae Formation. At this stage of uplift, the northeastern flank dipped gently northeastward away from the crest and served largely as a sediment transport surface; McRae strata accumulated only on distal parts of the surface in an embryonic Love Ranch basin to the northeast. Following an interruption in tectonism lasting as much as 10 m.y., renewed shortening in Paleocene(?) time elevated the Rio Grande uplift and formed the Love Ranch basin. At least 900 m of upward-fining, clastic Love Ranch strata of Paleocene-Eocene age accumulated in the basin. The Love Ranch lithofacies record a gradual southwestward shift of alluvial-fan depocenters, which resulted from growth of basin-margin structures and increasing basin asymmetry. Syndepositional synclines and angular unconformities record the growth of both intrabasinal folds and basin-margin thrusts. Clast compositions document progressive erosional unroofing of the Rio Grande uplift from Upper Cretaceous volcanic rocks into Precambrian granite and metamorphic rocks. Canyons 0.4 km deep locally drained the uplift, and maximum topographic relief may have approached 2 km. By late Eocene time, Love Ranch piedmont-slope deposits onlapped the uplift, burying all but the higher granite peaks. At this stage, the Love Ranch basin broadened and was the site of broad alluvial plains crossed by braided rivers draining to saline lakes.

Our analysis of syntectonic sedimentary rocks in the Love Ranch basin supports recent models of evolution of Laramide basement-involved block uplifts in which early stages produce approximately symmetrical structures, and sediment derived from the uplift is transported across most of the uplift flank to be deposited in a distal setting. At this stage the future footwall of uplift-boundary faults dips basinward in a ramp-like fashion, providing a sediment transport surface. As boundary thrust faults and fault-propagation folds evolve and grow, basin asymmetry rapidly develops, causing depocenters to shift toward footwall positions near the overthrust margins. This evolution from symmetrical to asymmetrical structures is reflected in an overall upward-fining sequence in the basin fill.

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