UPLIFT OF THE SOUTHERN ROCKY MOUNTAINS AND ALVARADO RIDGE

The Alvarado Ridge, so named because of its striking morphologic and dimensional similarity to ocean ridges, is the largest coherent epeirogenic feature in the conterminous U.S – a giant tractrix in profile -- that rose within the North American craton, embracing the Colorado Plateau, part of the southeastern Basin and Range province, the Wyoming Basin, the Southern Rocky Mountains, the Great Plains and the western part of the Central Lowland. The uplift history of all these provinces has been linked in a continuum since plate compression and probable crustal thickening first elevated the Laramide Southern Rocky Mountains along a N-S tract at the crest of what is now the Alvarado Ridge. Elements of the ridge crest were high much earlier, however, one as long ago as 400Ma, others appearing at circa 300 Ma and remaining high until 110 Ma, only to be renewed at 72 Ma and remaining high until today. Causes of rock uplift and elevation maintenance vary, but two of them here reflect the surprising rapidity with which a very profound change can take place, 3-7 m.y. A compilation of 1,389 radiometric dates of igneous rocks along the ridge indicates an abrupt increase in energy and mass transport in the shallow crust at 40 Ma, just 3 m.y.after a significant change in Pacific plate motion that reduced plate-plate compression. Extension at the summit appeared at least as early as 36 Ma. Magmatism culminated at 28-25 Ma, then rapidly declined to pre-40 Ma levels by 19 Ma, while extension continued. The relationship between elevation and crustal thickness on the ridge is remarkably poor, save locally. That between elevation and Bouguer gravity is better, but a coincident 17 m. anomaly of the geoid (NGS GEOID99) indicates an absence of complete isostatic compensation of the Alvarado Ridge. First and second order leveling along U.S. 50 indicates that the ridge is still rising epeirogenically.