AN INTEGRATION SCHEME FOR GEOPHYSICAL STUDIES OF THE CONTINENTAL LITHOSPHERE: AN UPDATE

Modern studies of the processes that create and deform the continental lithosphere have both fundamental scientific and societal implications. With the development of ambitious projects such as EarthScope, the data emerging make it possible, in fact essential, to build 3-D models that have the highest spatial resolution possible, are tied to geologic constraints, and provide information on the composition and physical sate of the materials that constitute the lithosphere. This ambitious can obviously be best accomplished by measuring a broad range of measurements of Vp, Vs, density, magnetic properties, electrical properties, thermal properties, seismic anisotropy, attenuation (Q), temperature, etc. for volume elements. In addition, interfaces that represent features such as stratigraphic boundaries, the Moho, faults, and boundaries of magmatic bodies and other discrete masses must also be mapped in order to properly characterize a region of the lithosphere. This goal can only be achieved through a highly integrated approach that takes advantage of all of the geological and geophysical constraints available. In most cases, controlled source and natural source seismology have the potential to provide the greatest spatial resolution of discontinuities and regions with characteristic seismic velocity, anisotropy, or Q. However, each of these types of seismic data are measured and analyzed by a variety of techniques. Thus, developing an integration scheme for seismic results is an important first step in building integrated models of lithospheric structure.

The diverse types of seismic data and analysis techniques each have their own sensitivities and spatial resolution, and when used alone, can constrain some aspects of the lithospheric structure. However when used together with other types of geophysical and geological data, the combined approach has the potential to produce a better constrained model that also reflects multiple physical parameters. For example, controlled source experiments yield the Vp structure, and sometimes Vs structure, of the crust and uppermost mantle with the analysis of refraction/wide-angle reflection data. In particular, analysis of the PmP phase (Moho reflection) yields a good estimate of the average Vp of the crust (Vpave) for the crust. In addition to providing an independent measure of crustal thickness that complements the wide-angle data, receiver function analysis can constrain the Vp/Vs ratio utilizing full-crustal reverberation(s) from teleseismic earthquakes. Thus, a simple form of integration involves using the Vp/Vs ratio from receiver functions and Vpave from refraction measurements, to solve for the Vsave of the crust. When refraction/ wide-angle reflection data and several receiver functions nearby are available, we have devised schemes where by 3-D voxel-based models and 2-D models with interfaces can be derived using tomographic inversion in the first case and ray-based techniques in the second case. In either case, gravity, magnetic, and electromagnetic data can easily add extra constraints. The ultimate goal is to add geologic and geodynamic results to make the result 4-D in nature.