Geology of the Agalteca quadrangle, Honduras, Central America



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The Agalteca quadrangle is located in the Sierras of Northern Central America and straddles the N60W-trending Montaña de Comayagua structural belt near the southeastern limit of its known 130 km extent. The structural belt may extend into unmapped areas to the northwest toward Guatemala and to the southeast toward Nicaragua. The structural belt has a width of approximately 30 km in the vicinity of the Agalteca quadrangle. Mapping of the Agalteca quadrangle has clarified that the Montaña de Comayagua structural belt consists of a series of left-stepping, en echelon strike-slip faults produced by probable dextral strike-slip displacement of unknown magnitude in the Late Cretaceous to early Tertiary. Associated with these strike-slip faults are syntectonic high-angle reverse faults, thrust faults, folds and antithetic shears. The assemblage is a "flower structure" in cross section, and is believed to be the product of transpression, or wrenching with a component of compression. The axis of the N60W-trending Laramide wrench zone in the Agalteca quadrangle is structurally high and exposes a conformable sequence of highly deformed Mesozoic sedimentary rocks. A Paleozoic (?) metamorphic basement, the Cacaguapa Schist, is known to unconformably underlie the Mesozoic sequence in central Honduras, but is not exposed in the Agalteca quadrangle. The Mesozoic sedimentary rocks include the Upper Jurassic (?) to Lower Cretaceous Todos Santos Formation conglomerate which is conformably overlain by the Valanginian (?) to Albian limestone of the Atima Formation. The Atima Formation is conformably overlain by the Albian to Late Cretaceous redbeds of the Valle de Angeles Group, which includes an intercalated limestone member, the Cenomanian Esquías Formation. The Mesozoic sedimentary rocks are intruded by mafic to felsic stocks and dikes of Late Cretaceous to Tertiary age, and are unconformably overlain by Tertiary volcanic rocks. The volcanic rocks include andesite of the early Tertiary Matagalpa Formation and the Oligocene-Miocene ignimbrites, basalt flows and volcanogenic sediments of the Padre Miguel Group. Terrace deposits of late Tertiary to Quaternary age unconformably overlie the older rocks in the Agalteca quadrangle. The Laramide structures of central Honduras are overprinted by north-trending grabens of the Honduras Depression which began to form in the mid-Miocene and are still active. In a regional context, the western tip of the Caribbean Plate (the Chortis Block) is undergoing east-west extension. North-trending grabens, including the Honduras Depression, cut the Chortis block from the Pacific Volcanic Chain to the Motagua transform boundary. The Honduras Depression is the most complex of these graben systems. The southern part of the Honduras Depression consists of grabens and half-grabens which trend north from the Gulf of Fonseca and are interrupted by the N60W-trending structures of the Montaña de Comayagua structural belt near Tegucigalpa. The northern part of the Honduras Depression trends north to the Caribbean coast from its termination against the Montaña de Comayagua structural belt near Lake Yojóa. The northern and southern segments of the Honduras Depression were born with an apparent left-lateral offset along the Montaña de Comayagua structural belt. The Honduras Depression is a developing rift. A pre-existing zone of weakness along the Montaña de Comayagua structural belt inhibited the formation of a through-going rift and rejuvenated the Laramide structural belt as a dextral transform fault zone. Late Cenozoic magmatic uplifts are present within the rejuvenated structural belt, and economically important base metal concentrations such as the El Mochito and El Rosario deposits are localized at the intersections of the northern and southern segments of the Honduras Depression with the Montaña de Comayagua structural belt. Quaternary alkalic basalt is associated with the Honduras Depression and is also restricted to the intersections of the depression with the structural belt in the vicinity of Tegucigalpa and Lake Yojóa.