Late‐Cretaceous allochthons and post‐Cretaceous strike‐slip displacement along the Cuilco‐Chixoy‐Polochic Fault, Guatemala

Abstract

The disposition of allochthonous masses, emplaced in late Cretaceous to Early Tertiary time, along the Cuilco‐Chixoy‐Polochic strike‐slip fault zone in western Guatemala, constrains the amount of Tertiary displacement. The absence of major disruption of the allochthons argues against lateral displacements of hundreds of kilometers during the Tertiary. In westernmost Guatemala, along the flank of the Cuilco‐Chixoy‐Polochic fault, granite, volcaniclastic and volcanic rocks, minor carbonate and phyllite comprise a sequence of south‐dipping allochthons. These tilted slabs lie against almost horizontal Jurassic and Cretaceous beds to the north. Field relations indicate that the slides were emplaced across the Cuilco‐Chixoy‐Polochic fault from south to north probably during late Cretaceous or early Tertiary time. Other fault‐bounded masses, which crop out tens of kilometers eastward, near San Sebastián Huehuetenango, probably were emplaced contemporaneously with those to the west. Locally, these allochthons lie astride the active trace of the Cuilco‐Chixoy‐Polochic fault along which offset streams record about 1 km of left‐lateral motion. Total displacement recorded by the apparent offset of allochthonous sheets is no more than several kilometers. Farther east, along much of the Cuilco‐Chixoy‐Polochic fault zone, a throughgoing strike‐slip trace has not been identified. We postulate that the pre‐Tertiary tectonic record is obscured by an extensive series of allochthons composed of regional stratigraphic suites emplaced during late Cretaceous time. In western Guatemala stratigraphic relationships between serpentinized peridotite and carbonate beds of late Cretaceous age indicate Turonian as a maximum age for initial emplacement of allochthonous material. Regional relationships indicate that deformation culminated during latest Cretaceous ‐ earliest Tertiary time. The emplacement of thrust sheets along the Cuilco‐Chixoy‐Polochic fault is a likely product of contemporaneous Cretaceous convergence along the Motagua zone to the south. Collision of the Chortis terrane against southern Yucatan resulted in crustal shortening, uplift, and consequent northward‐directed emplacement of slides. Allochthons, composed of ultramafic rocks, sedimentary cover and, less commonly, granitic rocks, were probably derived from the suture zone and the uplifted region between the Cuilco‐Chixoy‐Polochic and Motagua faults.