Geology of the Burica Peninsula, Panama-Costa Rica : neotectonic implications for the southern Middle America convergent margin




Corrigan, Jeffrey Delon

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The Burica Peninsula, situated along the Pacific coast on the Panama-Costa Rica border, extends to within 15 km of the projected trend of the Middle America trench axis and provides the thickest exposure of Neogene, marine sedimentary rocks along the Middle America margin. A thick section (~3,000 m) of gently folded, and intensely faulted, Pliocene-Early Pleistocene marine sedimentary rocks unconformably overlie a discontinuous, Paleocene-Eocene limestone unit which, in turn, overlies a highly fractured, mafic igneous basement. The basement is similar to late Cretaceous-Eocene igneous exposures (the Nicoya Complex) on the Osa and Nicoya Peninsulas, 50 and 280 km, respectively, to the northwest. The Plio-Pleistocene marine sedimentary rocks consist predominately of laterally continuous, thinly bedded (2-7 cm thick), very fine-grained volcaniclastic sandstone, siltstone, and mudstone. Benthic foraminifera in this turbidite section indicate paleo-water depths of 2-3 km at the base of the section, and ≤1.5 km at the top. Lithofacies mapping and paleocurrent patterns indicate at least two major channe-fill complexes (≥300 m wide), as well a number of smaller (3-15 m wide) channel-fill deposits. Both the large and small channel-fill deposits contain clasts up to 15 cm long (a-axis) and exhibit predominately clast-supported textures. Measurements (231 total) of sole marks (flute and groove casts), imbricated clasts in conglomerates, and aligned wood fragments suggest that paleoflow (S-SSE) was at a high-angle to the margin. Debris-flow deposits and slump horizons also occur and account for approximately 3% of the section. Facies patterns, normal-to-margin paleoflow, and paleobathymetry suggest that the sediments were deposited on the landward trench-slope of the Middle America Trench. The well-bedded, Plio-Pleistocene turbidite sequence reveals a monoclinal structure that dips to the E-NE at 10°-40° away from a basement highland defined by poorly exposed high-angle reverse(?) faults. The Medial Fault Zone (MFZ), an inferred 2-3 km wide, north-trending, right-lateral strike-slip fault system, is approximately coincident with the Panama-Costa Rica border in the study area. The MFZ separates the well-bedded turbidite sequence to the east from the pre-Paleocene basement and overlying Pliocene massive, fossiliferous, shallow-water(?) muddy siltstones to the west. Numerous normal faults throughout the Neogene sedimentary cover east of the MFZ show two dominant trends, N45°E and N80°E, and appear to have moved contemporaneously. Displacements along individual faults averages 0.5-3.0 m. I interpret the observed structures to indicate localized NNE-directed subhorizontal shortening superimposed on regional NNW-SSE directed extension in direct response to uplift of the peninsula. Based on magnetic anomaly reconstructions, impingement of the aseismic Cocos Ridge on the Panama-Costa Rica volcanic arc is spatially and temporally correlative with the uplift of the Burica Peninsula. A 2 km paleodepth, with uplift initiating at the end of the Pliocene, suggests a long-term uplift rate of 1.25 mm/yr. I suggest that the magnitude of uplift, as well as the Neogene fault patterns, are best explained in terms of isostatic adjustments resulting from subduction of the Cocos Ridge beneath the Burica Peninsula.