A geophysical investigation of the Peru-Chile Trench

Abstract

Twenty-six profiles of topography, free-air gravity anomaly, and total intensity magnetic anomaly across the Peru-Chile Trench are presented and discussed. The trench is obliterated as a topographic feature south of about 40°S, but a well-defined negative gravity belt and seismic reflection profiles indicate that the trench continues as a structural feature at least into the Drake Passage. The trench can be broadly divided into two main provinces, the main sediment-free province which extends from 8°S to 32°S and the sedimentary province which extends from 33°S to 57°S. Three other minor provinces are also defined. A free-air gravity anomaly map of the offshore area of the entire west coast of South America is presented. A secondary negative gravity belt is found that roughly parallels the prominent negative gravity belt associated with the trench. This secondary belt is thought to be a reflection of the Bolivar Geosyncline which lies very near the present coast of Colombia, Ecuador, and Peru. Studies incorporating seismic refraction and reflection measurements and gravity measurements indicate a pronounced crustal thinning beneath the offshore flank of the trench. Crustal thicknesses beneath the axis of the trench appear to be normal for the transition region of an ocean-continent margin. Seismic refraction measurements and continuous gravity observations are in close mutual agreement. It is suggested that the trench may originate by high angle normal faulting near the base of the continental slope accompanied by a downward flexure of the crust further offshore. The gravity data indicate that there are no radical variations in the crustal structure along the entire length of the western continental margin of South America. The magnetic anomalies cannot be correlated over large distances. The magnetic signature is considerably smoother over the shoreward flank of the trench than over the offshore flank and the trench axis roughly defines the boundary. This points to a sharp contrast in the magnetic state of the material underlying the two flanks of the trench. The relatively large amplitudes of some observed magnetic anomalies strongly suggest that they are not produced primarily by induced magnetization of bodies in the crust. In certain areas the observed anomalies correlate well with the basement topography. There is good evidence that some of these anomalies are caused by remanent magnetization of magnitude many times larger than the predicted induced magnetization and in a direction opposite to the earth's present field.