TECTONIC EVOLUTION OF THE ANCESTRAL CASCADES ARC SENSU LATO: EOCENE TO PLIOCENE, US TO MEXICO

We present an overview of the tectonic setting for Cenozoic arc magmatism in the Cordilleran U.S. and Mexico. The pre-existing crustal architecture of this region was strongly influenced by ca. 75-50 Ma flat-slab subduction and crustal thickening. Then, from ca. 50- 25 Ma, the tectonic setting of arc magmatism differed along the margin in these ways: (1) In the northern Cordilleran U.S., the Siletzia oceanic plateau was accreted at 48 Ma, causing slab breakoff and rollback of the Farallon slab. This produced the Challis episode (ca. 50-39 Ma), a broad, inboard, SW-younging belt of syn-extensional magmatism with (inherited?) arc geochemical affinities. Meanwhile, arc magmatism began in the western Cascades of Washington by 46 Ma, precluding the “trench jump” model; instead subduction was initiated outboard of Siletzia by northward propagation of the Farallon trench. (2) In the southern Cordilleran U.S. and in Mexico, prolonged and widespread slab rollback resulted in broad westward/southwestward sweep of arc magmatism and extension. Giant continental calderas formed where the inherited crust was thickest (ca. 40-20 Ma), and this was accompanied by extension and dominantly juvenile magmatism in Mexico. This transitioned to dominantly andesite magmatism as the arc swept west onto thinner continental crust. A slab tear near the California-Oregon border accommodated these two contrasting subduction domains.

By ca. 25 Ma, one continuous andesite arc extended from the U.S. through Mexico above the subducting Farallon slab. When the Farallon-Pacific ridge was subducted at the latitude of Los Angeles at ca. 25 Ma. this arc became divided into two segments– the Comondú arc (Mexico) and the Ancestral Cascades arc sensu stricto (U.S.). Orthogonal extension continued in both arc segments until 12 Ma, when two factors became important: the Pacific - North American transform plate boundary lengthened, and the Pacific plate vector shifted to a more northerly direction. Thus, the Walker Lane-Gulf of California oblique rift was born within the thinned and thermally weakened lithosphere of the Comondú arc and the southern Ancestral Cascades arc. This presentation ends with a description of controls of Walker Lane dextral transtension on the magmatic evolution of Ancestral Cascades arc volcanic centers in California.