Tectonic regionalization of the Southern California crust from tomographic cluster analysis

William K. Eymold, & Thomas H. Jordan

Published December 1, 2019, SCEC Contribution #9008

We map crustal regions in Southern California that have similar depth variations in seismic velocities by applying cluster analysis to 1.5 million crustal velocity profiles from the three-dimensional tomographic model CVM-S4.26. We used a k-means algorithm to partition the velocity profiles into K sets that minimize the inter-cluster variance via a randomly seeded iterative process. The sequence of regionalizations with K up to 7 generates a coherent set of structural refinements; each increment of K introduces a new region typically by partitioning a larger region into two smaller regions or by occupying a transition zone between two regions without reorganizing the other preëxisting regions. The clustering is insensitive to initialization, and essentially identical results are obtained for K ≤ 7 if the compressional and shear velocity profiles are treated separately or jointly, indicating that the crisp clusters correspond to a global minimum in the Euclidian objective function. Application of a fuzzy c-means clustering algorithm to the same profiles validates this conclusion, and the results are used to measure cluster cohesiveness and identify clustering bias inherited from the tomographic starting model. The regions for K = 7 correspond to major physiographic provinces and geologic areas with known tectonic affinities. The region associated with the Salton Trough also includes the Los Angeles basins, whereas that associated with the Great Valley includes the Santa Maria and Santa Barbara basins. The regionalization splits the Sierra Nevada and Peninsular Ranges batholiths into western and eastern zones along the Sr-706 line. Three of the regions, taken together, form a continental domain that comprises almost all of the upper crust in the study region known to be composed of igneous and metamorphic rocks derived from continental lithosphere. Region-to-region variations in lower-crustal velocities and M-discontinuity depths within this continental domain can be explained by progressive crustal thinning accompanied by basaltic intrusion and densification of the lower crust. The regional boundaries from the whole-crust profile analysis often coincide with major faults, topographic discontinuities, or geochemical transitions mapped at or near the surface. This vertical correlation between surface features and deeper seismic properties indicates that regional boundaries mapped by crustal tomography are high-angle structures that separate regions with different tectonic histories. The regionalizations provide no direct evidence for the underplating of regions in the continental domain by Pelona-Orocopia-Rand schists, but such schist bodies may be locally present in the southern Sierra Nevada and western Mojave block, where a mid-crustal low-velocity zone at depths of 12-16 km is well expressed in the tomographic model.

Key Words
California, tomography, crustal structure, tectonic regionalization, cluster analysis.

Citation
Eymold, W. K., & Jordan, T. H. (2019). Tectonic regionalization of the Southern California crust from tomographic cluster analysis. Journal of Geophysical Research, 124.

Related Projects & Working Groups
Geological framework, SCEC Community Models (CXM)