9. Geometry of the Nazca Plate and Its Geodynamic Implications

  1. George H. Sutton,
  2. Murli H. Manghnani,
  3. Ralph Moberly and
  4. Ethel U. Mcafee
  1. E. Rene Rodriguez,
  2. S. J. Ramon Cabre and
  3. Augusto Mercado

Published Online: 17 MAR 2013

DOI: 10.1029/GM019p0087

The Geophysics of the Pacific Ocean Basin and Its Margin

The Geophysics of the Pacific Ocean Basin and Its Margin

How to Cite

Rene Rodriguez, E., Ramon Cabre, S. J. and Mercado, A. (1976) Geometry of the Nazca Plate and Its Geodynamic Implications, in The Geophysics of the Pacific Ocean Basin and Its Margin (eds G. H. Sutton, M. H. Manghnani, R. Moberly and E. U. Mcafee), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM019p0087

Author Information

  1. Observatorio San Calixto, La Paz, Bolivia

Publication History

  1. Published Online: 17 MAR 2013
  2. Published Print: 1 JAN 1976

ISBN Information

Print ISBN: 9780875900193

Online ISBN: 9781118663592

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Keywords:

  • Geophysics—Pacific area—Congresses;
  • Woollard, George Prior, 1908

Summary

The geometry of the Nazca plate is established on the basis of two-dimensional seismicity sections using NOAA hypocenters from 1963 to 1973. Seismicity as mapped in the region is not a true indicator of the subducted slab due to systematic errors in hypocenter location because there are no seismic stations in the Pacific Ocean. Very narrow seismicity sections, however, are shown to be effective in describing the slab's geometry. Despite the uncertainties, it is clear that the Nazca plate underthrusts the South American continent in individual, discontinuous tongue-like pieces at different dip angles. It is especially true that between latitudes 19° and 21° S, a significantly large change occurs in the direction of underthrusting, from ENE to almost E-W. Accordingly, we should expect to find a transition region somewhere between latitudes 19° and 20° S. Such a region is not detectable from the seismicity alone, however. A simple theoretical model of the collision between the continental and oceanic plates shows two systems of principal stresses: one parallel to the coastline and the other perpendicular to it. The latter system shows a conical concentration of stress lines opening toward the continent, clearly defining a change in underthrusting direction, a conical transition zone near Arica, and the small tongue-like components of the plate coincident with the seismicity data. Thus the slab seems to act as stress guide down to a depth of 250 km. East-west gravity profiles show a minimum in the Bouguer anomaly curve of the order of 300 mgal and agree very closely with the surface topography and the inflexion region of the downgoing slab, but it seems likely that the plate effect is too small to account for the observed Bouguer minimum. The tectonic pattern of Bolivia, studied by means of ERTS satellite images and correlated with the theoretical distribution of stresses, shows that faults, lineations, and other geomorphological features agree well with the two systems of lines of stress derived from the collision of the oceanic plate against the continental lithosphere. Seismicity, gravity, and tectonic data combined with the spatial distribution of mines in Bolivia show very intriguing relationships between the geometry of the slab, gravity minimum, tectonism, and ore deposits. Many questions, however, need to be answered before definite conclusions can be drawn. The ages of most host rocks seem to be too old to have any direct relationship with the present geophysical patterns, but the age of ore mineralization itself is not known. The age of the ore is the critical parameter to be established to see whether or not the mineral deposits are young enough to be attributed to the geometry of the downgoing slab.