Crustal Low‐Velocity Zones Under the Peru‐Bolivia Altiplano

Abstract

In 1968, the Carnegie Institution of Washington together with North and South American collaborators carried out a reconnaissance explosion seismic experiment to investigate the apparently highly anomalous crustal structure under the Peru–Bolivia altiplano. The data of this experiment have been reinterpreted by ray‐tracing in a spherical Earth so as to fit as closely as possible arrival times, relative amplitudes, cusps, etc., of seismograms displayed in record section. The resultant model confirms the previous average model consisting of three major refractors: the sedimentary‐metamorphic layer 4–9 km thick and 4.5–4.9 km s‐1 velocity; the ‘granitic’ layer with 6.0–6.1 km s‐1 velocity down to 26–30 km depth; and the ‘gabbroic’ layer reaching depths of 68–70 km below sea level with 6.8–6.9 km s‐1 velocity. However, in order to account for relatively large amplitudes in the secondary arrivals with apparent velocities close to the first arrivals, two low‐velocity zones are postulated within the crust under the Peru–Bolivia altiplano. In Peru, the shallow and thinner low‐velocity zone with boundaries at 9 km and 12 km depth is between materials of 6.0 km s‐1 and 6.1 km s‐1. The deeper and thicker low‐velocity zone with upper and lower bounds at about 30 km and 40 km under Bolivia, and more approximately at 36 km and 46 km under Peru, is embedded in 6.8 and 6.9 km s‐1 materials. The shallower low‐velocity zone is conceivably related to the parent magma of volcanic and intrusive acidic rocks with the deeper low‐velocity zone related to the volcanic and intrusive basic rocks in accord with petrological and geochemical findings of Pichler and Zeil in the Andes of northern Chile. The presence of velocity inversions above the 50 km depth is also in harmony with the postulated existence of a high electrical conductivity zone shallower than 50 km depth under the Andes as postulated recently by Schmucker to explain magnetic ‘day‐time fluctuation’ anomalies. Copyright © 1972, Wiley Blackwell. All rights reserved