MAPPING CONTINENTAL LITHOSPHERE – 25 YEARS OF SEISMIC IMAGING IN THE ANDES

Mapping the complete seismic structure of the continental lithosphere has long been a major goal of Geoscientists. Surface mapping, drill hole data and seismic reflection data have aided in mapping the upper crust with high resolution, but mapping the mid and lower lithosphere has remained a challenge. In the last 25 years major advances in passive source seismology have revolutionized our ability to image the mid and lower lithosphere. This improvement has come from a wealth of new data as well as improved tomography and imaging techniques. One such example is ambient noise tomography, a technique that uses “noise” from atmospheric disturbances recorded as surface waves across a seismic network and can provide us with an estimate of the shear-wave velocity structure of the crust. In combination with earthquake generated surface waves, the entire lithospheric column can be imaged in 3D. Surface waves are most sensitive to smooth variation in velocity structure, so they must be complemented with another technique more sensitive to rapid change in velocity structure, such as converted phases. By utilizing P-S and S-P converted phases we can now map the Moho and lithospheric discontinuities. By combining the two sets of techniques, we now have the capability of imaging the complete lithosphere in 3D for large regions of the continents.

Many of the biggest advances have come from imaging lithosphere in tectonicly active regions including the North and South America Cordilleras. The high elevations of Andes have crustal thicknesses of ~70 km nearly double the average continental crust but with overall low crustal seismic velocities more indicative of upper crust than typical lower crust. This suggests that thick crust forms by shortening and thickening the upper and mid crust and removing the more mafic lower crust often observed beneath cratons. Much of the western U.S. had high elevations and thick crust prior to the extension that resulted in thin low velocity crust (~35 km). The Andean and western U.S. crust have evolved through time by processes including crustal thickening by tectonic shortening, lithospheric removal, fluids and hydration, and magmatism. Mapping the seismic velocities of continental lithosphere has improved our understanding about how continental crust is modified in the orogenic process.