Cathy J. Busby
Santa Barbara, CA
Evolutionary Model For Convergent Margins Facing Large Ocean Basins: Mesozoic Baja California, Mexico
Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 m.y.), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted.
Phase 1 consists of intra-oceanic intra-arc to backarc basins that were isolated from continental sediment sources. Progradational backarc arc-apron deposits record growth of adjacent volcanoes toward and above sealevel, followed by influx of silicic pyroclastics due to arc rifitng; then the remnant backarc basin was blanketed by sands.
Phase 2 consists of fringing arc basins adjacent to a continent; these show at least one cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites.
Phase 3 consists of a high-standing, compressional continental arc, with abundant coarse sediment supplied to a forearc basin. Strongly-coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench-slope) basins atop the gravitationally collapsing accretionary wedge.
Strongly coupled subduction together with oblique convergence during phase 3 also resulted in development of forearc strike-slip basins upon arc basement. Phase 3 basins are predominantly deepwater basins with narrrow shelves. One of the phase 3 basins contains a coastal paleovalley that was catastrophically cut and filled by landsliding and resedimentation events triggered by seismicity during the K/T boundary bolide impact.
The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional, and so evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California.
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