Geological Survey Professional Paper Volume 438

Geological Survey Professional Paper Volume 438

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. Excerpt: ...Cretaceous(?) granitoid rocks crop out to the west. The gravity high over the metavolcanic rocks is caused in part by the density contrast between these rocks and the granitoid rocks farther west. The residual gravity relief caused by this density contrast (assumed to be about 0.2 g per cm3) is not more than 2 or 3 mgals, however, and this residual relief is not enough to have a serious effect on analysis of Cenozoic structures based on a single density contrast of--0.4 or--0.5 g per cm3. In the slopes of the Sierra Nevada southwest of Long Valley, alternating bands of metamorphic and granitoid rocks trend into the steep southern gradient of Long Valley. Although the gravity contours in this area reveal small maximums over the metamorphic rocks, the effects are small and do not influence the much larger minimum anomaly of the Long Valley block caused by the density contrast between the Cenozoic and pre-Tertiary rocks. The effect is merely to superimpose small anomalies of a few milligals on an anomaly of several tens of milligals. It is thus probably safe to ignore density changes within the pre-Tertiary rocks. H. W. Oliver (written commun., 1959) is making a detailed study of the significance of these changes. REGIONAL GRAVITY AND IS08TATIC COMPENSATION By subtracting 1,000 mgals from the contour values, the regional gravity over areas of pre-Tertiary rock outcrops with respect to the International Ellipsoid tends to be about--200 mgals. The regional gradient, decreasing to the west, is generally about 2 or 3 mgals per mile, although it is nearly zero in the Mono Basin area (Pakiser and others, 1960). The negative complete Bouguer gravity suggests that the crust is thicker than normal in this area and increases in thickness toward the west. Oliver...
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Product details

  • Paperback | 52 pages
  • 189 x 246 x 3mm | 109g
  • United States
  • English
  • black & white illustrations
  • 1236816498
  • 9781236816498