ISOLATION AND BREAK UP OF A MICROCONTINENT IN THE GULF OF CALIFORNIA: FAULT STRUCTURE AND VOLCANIC GEOCHEMISTRY

Isla Angel de la Guarda (IAG) is a young, subaerial microcontinent isolated by oceanic crust and transform faults within the Pacific-North American plate boundary zone. IAG and Baja California were part of the North American plate and were transferred as a block onto the Pacific plate in early rifting of the Gulf of California (GOC) during the late Miocene. Within the GOC, when the boundary between the North American and Pacific plates jumped from the east side of IAG to the west side during the Pliocene, breaking IAG off the Pacific plate and transferring it back to the North American plate. The southernmost part of the island, between the active Ballenas Transform Fault zone and the formerly active Tiburon Fracture Zone, hosts a ~5 km-wide zone of N-NE striking faults, with three recently active majorvolcanic centers, oriented N along strike from the North Salsipuedes Basin. During two field seasons, we have mapped faults and collected volcanic samples to better evaluate the relationship of these features to past, present, and future stages of rifting, the isolation of IAG within the rift of the GOC, and whether there are crustal weaknesses suggesting that IAG will continue to break up further in the future.

Satellite imagery, drone data, and field data are utilized to map faults at progressively finer levels of detail. Based on satellite data, we choose drone survey and field study locations. Drone photos are processed with structure from motion to see topography and identify features warranting further field investigation. Active depressions hosting sag ponds and accumulations of fine sediment indicate recent fault activity. Faulted terraces, dated to be as young as ~29 ka, suggest that break up is in its initial stages, and that faults may be linked to the North Salsipuedes Basin.

The three main volcanic centers are generally distinguishable by color in satellite imagery. Samples collected of lavas and inclusions have been analyzed both petrographically and with X-ray fluorescence (XRF). Petrographically, lavas all have plagioclase as the most common phenocryst, followed by varying amounts of pyroxene, amphibole, olivine, and possible alkali feldspar. XRF analyses of lavas and their inclusions will inform our understanding of the magma’s source, interactions with other bodies, and relation to rifting.