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ABSTRACT

Welcome to our three-day field trip aimed at exploring Mesozoic magmatism and tectonism in southern California. The broader southern California area is a great place to study Cordilleran-style continental margin arcs built across diverse basements ranging from thinner oceanic to thicker continental crust. We will weave together several themes during this trip: (1) spatial variations, including in the vertical dimension, of arc magmatic and tectonic systems; (2) volcanic-plutonic links; (3) pluton sizes and shapes; (4) emplacement and tectonic/plutonic mass balance issues; and (5) arc magmatic and tectonic tempos. While doing so we plan to discuss/ examine the main components of southern California Mesozoic magmatic systems while discussing temporally and spatially variable (1) melt sources, (2) processes in crustal columns, and (3) tectonic activity, all of which lead to the final preserved arcs.

Bedrock and detrital zircon age patterns in a broad corridor across southern California define magmatic flare-up maxima in the Mojave area at ca. 250 Ma in a SE migrating arc, at ca. 170 Ma in a westward migrating arc, at ca. 100 Ma in the eastward migrating Peninsular Ranges Batholith (PRB), extending from just south of the Transverse Ranges and into Baja California, and additional smaller maxima at 81 and 73 Ma in the Transverse Ranges and Joshua Tree regions that may reflect the western edge of a broad region of melting and magmatism throughout the Basin and Range area.

On this trip we will integrate studies from different areas of these Mesozoic arcs including Baja California (Paterson), the northern PRB (Clausen), the San Gabriel Mountains (Schwartz), and the Joshua Tree National Park area (Memeti and Paterson) with a wealth of new data being published by other groups. Day 1 will examine aspects of the Cretaceous PRB in the Perris block where the edge of the western belt and transition zone is exposed. On Days 2 and 3 we will examine parts of two tilted crustal sections, one in the eastern San Gabriel Mountains and the other in the Little San Bernardino Mountains and Joshua Tree National Park. While visiting these crustal sections we will discuss Late Cretaceous to Early Tertiary events to explore why crustal melting and magma emplacement occurred both inboard and post the main magmatic flare-ups in regions that usually are thought to have transitioned into cold fore arc.

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