Paper No. 9
Presentation Time: 4:15 PM


IANNO, Adam J., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, ZHS 117, Los Angeles, CA 90089-0740 and PATERSON, Scott R., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Zumberge Hall of Science (ZHS), Los Angeles, CA 90089-0740,

We increasingly recognize the likelihood that large oceanic plateaus subducted beneath California during the Late Cretaceous. This caused the onset of flat-slab subduction similar to processes currently occurring underneath the central Andes. The oceanic plateaus are interpreted to represent conjugate structures to the modern-day Shatsky and Hess Rises found on the Pacific plate.

Our current understanding of Late Cretaceous tectonics does not adequately account for two widespread magmatic flare-ups that occur during periods of interpreted quiescence and form arc-perpendicular magmatic belts. The first flare-up occurred at 90-79 Ma across southern California. The second flare-up occurred at 79-67 Ma as a focused, roughly west to east belt of magmatism reaching from southern California to southwestern New Mexico. Both periods are represented by large volumes of metaluminous to peraluminous, gabbro to granite magmas created in part by pervasive partial melting of host gneisses at depth. Evidence of migmatite formation and partial melting can be observed in uplifted middle to lower crustal sections throughout the observed magmatic belt.

The currently accepted model does not provide a heat source sufficient to produce the widespread partial melting of host rocks that we observe. We propose that these two magmatic belts were created through two separate events of asthenospheric upwelling, with decompression melting producing basalt that infiltrated the crust, generating great volumes of hybridized magmas with varying percentages of crust and mantle components. Of particular note, we propose that the second event (79-67 Ma) resulted from upwelling following the tearing of the oceanic slab between the shallowly-dipping Shatsky conjugate plateau and the steeply-dipping Peninsular Ranges segment.

In addition to voluminous magmatism, the high heat flux from these events influenced coeval, regional crustal deformation across the southwestern United States and generated magmas and fluids that were the source of the economically valuable copper, gold, and other metal ore deposits (e.g. “porphyry deposits”) that parallel the tear zone across California and Arizona.