Active rift system in the basin and range province

Abstract The active rift system along the postulated landward extension of the East Pacific Rise into western North America (Heezen, 1960; Menard, 1960; Cook, 1961) consists of several active rift zones which follow belts of seismicity and probable high heat flow and include: (a)the belt of grabens along the eastern margin of the Sierra Nevada, the Walker Lane, and the Churchill Arc; (b)the system of great trenches extending through western Arizona, central Utah, southeastern Idaho, western Wyoming, western Montana, and British Columbia; and (c)the Rio Grande rift belt. The present paper reviews the following additional recent geophysical evidence that defines in more detail the active rift system in the eastern part of the Basin and Range province: 1. (1) In north central Utah, gravity surveys, together with refraction and/ or reflection seismic data along some profiles, indicate that the Wasatch Trench is a northward-trending belt of grabens about 160 miles long and up to 13 miles wide. 2. (2) In south central and southwestern Utah, gravity surveys indicate grabens adjacent to the northward-trending Sevier and Hurricane fault zones, which lie along the rift belt. 3. (3) In New Mexico, several gravity profiles across the Rio Grande rift belt indicate the width and depth of the grabens. 4. (4) Sparse data tentatively indicate high heat flow in the rift belts. The possible mechanism of the Late Cenozoic vulcanism and diastrophism in the Basin and Range province is explained by a postulated mantle convection current (associated with the formation of the Rise) that rose beneath the eastern part of the Basin and Range province (or perhaps the Colorado Plateau) and flowed westward (Cook, 1961). The active rift system is in the region of tension over the upwelling convection currents. To the west, in the region of translation and compression, it is postulated that the westward-moving mantle convection current dragged along the continental crust in the California region. The crustal drag caused a large continental-type block to move westward into the Pacific Ocean about 50–100 km in the California region between the landward projections of the Mendocino and Murray fracture zones, which were probably major lines of weakness existing before the formation of the East Pacific Rise. On the northern and southern margins of the block, respectively, the movement occurred along transcurrent and/or transform faults in: 1. (1) the Cape Mendocino area (Gorda escarpment). 2. (2) the Garlock fault, Big Pine fault, Channel Islands. Transverse Ranges, and Pinto Mountain fault areas. The westward movement of the block, which occurred along transcurrent and/or transform faults associated with these two fracture zones, probably caused: (a) the folding of the Coast Ranges in California along the frontal (i.e., west) segment of the block and (b) the bending of the San Andreas fault zone in two areas along the southern margin of the block. The right-lateral movement of the San Andreas fault zone is explained by a postulated second, northwestward-trending mantle convection current with a velocity gradient increasing southwest; this movement has apparently moved into the western Basin and Range province recently to cause transcurrent faulting there (Billings, 1960), and a change in the direction of the mantle convection currents there is postulated. The concept that a westward-moving continental-type crustal block has overridden the oceanic crust in the Pacific Ocean and that the driving force probably originated in the upper-mantle spreading beneath the Basin and Range province (or perhaps the Colorado Plateau) is fundamental to the interpretation of the Late Cenozoic tectonics in the southwestern United States. If the United States, along with the North American continent, has drifted westward as a consequence of the sea-floor spreading along the Mid-Atlantic Ridge and overriden the East Pacific Rise, as postulated by Vine (1966), the westward movement of the above-mentioned crustal block would constitute about 50–100 km of horizontal displacement in addition to the “normal” westward drift of the North American continent as a whole.