STRUCTURAL KINEMATICS AND DEPOSITIONAL HISTORY OF A LARAMIDE UPLIFT-BASIN PAIR IN SOUTHERN NEW-MEXICO - IMPLICATIONS FOR DEVELOPMENT OF INTRAFORELAND BASINS

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 elastic rocks that accumulated in the complem entary Love Ranch basin. The synorogenic to postorogenic McRae and Lov e Ranch Formations are as much as 1460 m thick; they filled the Love R anch basin and onlapped the Rio Grande uplift. Present outcrops of the two formations cover an area of 100 km(2) and reveal the stratigraphi c architecture of the basin fill in three dimensions. Lithofacies dist ribution, 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 sho rtening began between Campanian and latest Maastrichtian time, and ini tially 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 a nd silicic composition, which were the primary source of volcanic detr itus in the latest Cretaceous-Paleocene(?) McRae Formation. At this st age of uplift, the northeastern flank dipped gently northeastward away from the crest and served largely as a sediment transport surface; Mc Rae strata accumulated only on distal parts of the surface in an embry onic Love Ranch basin to the northeast. Following an interruption in t ectonism 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, elastic Love Ranch strata of Paleoce ne-Eocene age accumulated in the basin. The Love Ranch lithofacies rec ord a gradual southwestward shift of alluvial-fan depocenters, which r esulted from growth of basin-margin structures and increasing basin as ymmetry. Syndepositional synclines and angular unconformities record t he growth of both intrabasinal folds and basin-margin thrusts. Clast c ompositions 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 Eoce ne time, Love Ranch piedmont-slope deposits onlapped the uplift, buryi ng all but the higher granite peaks. At this stage, the Love Ranch bas in broadened and was the site of broad alluvial plains crossed by brai ded rivers draining to saline lakes. Our analysis of syntectonic sedim entary rocks in the Love Ranch basin supports recent models of evoluti on of Laramide basement-involved block uplifts in which early stages p roduce approximately symmetrical structures, and sediment derived from the uplift is transported across most of the uplift flank to be depos ited in a distal setting. At this stage the fixture footwall of uplift -boundary faults dips basin-ward in a ramp-like fashion, providing a s ediment transport surface. As boundary thrust faults and fault-propaga tion folds evolve and grow, basin asymmetry rapidly develops, causing depocenters to shift toward footwall positions near the overthrust mar gins. This evolution from symmetrical to asymmetrical structures is re flected in an overall upward-fining sequence in the basin fill.