GEOLOGY OF A LARGE INTACT EXTENSIONAL OCEANIC ARC CRUSTAL SECTION WITH SUPERIOR EXPOSURES: CRETACEOUS ALISITOS ARC, BAJA CALIFORNIA (MEXICO)

The Rosario segment of the Cretaceous Alisitos arc (Baja California, Mexico) is arguably the best-exposed structurally intact and unmetamorphosed upper to mid-crustal section of an oceanic arc on earth. The gently tilted, 50 km-long section exposes the transition from upper crustal volcanic rocks to mid-crustal plutonic rocks, formed in an extensional environment. It is the subject of a full GSA Special Paper¹ that includes a poster-sized geologic map insert, based on an exhaustive data set that includes geochemistry, geochronology, annotated outcrop photos and photomicrographs. In this talk, we summarize key topics of interest addressed in this study.

Subsegments within the Rosario segment include a central subaerial edifice, bounded on one side by a volcano-bounded basin and on the other by a fault-bounded basin, each underpinned by separate plutons. It thus preserves a diversity of depositional environments, providing exquisite examples of subaerial- to submarine-emplaced volcanic rocks derived from subaerial and submarine eruptions. The entire data set is integrated across these three subsegments, in a time slice reconstruction of arc evolution and the relationships between plutonism and volcanism. The arc crustal section was assembled in less than about 1.7 Ma. Silicic magmatism played a major role in construction of the extensional oceanic arc crust, similar to modern extensional oceanic arcs, although this switched to mafic magmatism as arc rifting proceeded.

The data set provides constraints on: (1) the evolution of large (15 km wide) silicic calderas in oceanic arcs, and extensional tectonic triggers for silicic caldera collapse; (2) caldera resurgence, in one case by emplacement of a sill complex and in another by incremental growth of a pluton; (3) construction of the arc section from multiple magmatic intrusions, which cooled at a rate that preserved this heterogeneity. All three plutons show that mafic and felsic magmas were coeval. Furthermore, incremental assembly of one of the plutons, which fed a silicic caldera-forming eruption, resulted in resurgence that tilted the overlying volcanic section and swept part of the caldera fill upward; (4) direct comparison of the Alisitos arc crustal section with velocity profiles in modern oceanic arcs.

¹2023, https://doi.org/10.1130/2023.2560(01)