GEOLOGICAL MAPPING OF ISLA MONSERRATE, BAJA CALIFORNIA SUR: HISTORY OF VOLCANISM, SUBSIDENCE, AND UPLIFT

Isla Monserrate, Gulf of California, records a history spanning the transition from convergent tectonics and volcanism to oblique rifting and marine deposition. This study involved geological and structural mapping, augmented by drone-yielded high-resolution orthomosaic and Digital Elevation Model (DEM). The geologic history of Isla Monserrate can be loosely categorized into three geological phases: pre-rift, syn-rift, and post-rift.

Five geological units were mapped on Isla Monserrate: (1) pre-rift Comondú Group volcanic and volcaniclastic rocks, (2) syn-rift pyroclastics, (3) syn-rift Pliocene marine deposits, (4) post-rift littoral and shallow-marine sediments covering coastal terraces (5) recent alluvium and coastal sediments. The Comondú Group consists of fluvial sandstones, debris flow deposits, and andesitic to mafic lavas. Pyroclastic units are composed of silicic tuff and welded ignimbrites. Pliocene sedimentary deposits record transgressive sequences into the region, including basal conglomerates, and fossiliferous marlstones.

Isla Monserrate has a sequence of coastal terraces and elevated platforms, documenting Quaternary uplift of the island. The lowest terrace level ranges ~15 m above sea level, while the highest terrace platforms in the island’s interior are above 200 m in elevation. The lowest terrace levels are covered in fossiliferous and coral-rich sand and gravel. Higher terrace platforms are capped by sheets of secondary carbonates, derived from either Pliocene or Quaternary carbonate-rich sediments.

The structural geology of Isla Monserrate documents progressive eastward tilting above a west-directed detachment system. At least three west-dipping, low-angle detachment faults have been found. The syn-extensional basins systematically increase in size to the west, which likely reflects a westward migration of deformation through the stacked system of detachment faults. High-angle faults with significantly smaller offset crosscut Pliocene rocks. The overall Monserrate geology shows eastward fanning-dip sequence: pre-rift Comondú strata dip ~50-70° to the east; syn-rift silicic tuff strata dip ~30˚; younger Pliocene deposits dip ~10-25°. Kinematic analysis of fault striae indicates that the principal axis of extension is oriented E-W to ESE-WNW, consistent with a transtensional shearing of the oblique rift.