THE TONALA SHEAR ZONE AND CAPTURE OF THE CHORTIS BLOCK BY THE CARIBBEAN PLATE SYSTEM

The Tonalá (TSZ) is a fossil plate boundary that is positioned between the Maya Block of Southern Mexico and the now active hanging wall block of the Middle America Trench. During the Miocene, a series of calc-alkaline plutons were emplaced along most of the exposed length of this 120 km long shear zone (U-Pb SHRIMP dates ca 10 Ma). Rocks immediately east of the shear zone consist of either undeformed Miocene plutonic rocks, or rocks associated with the Permian Massif, which is a complex suite of Permian-age granodioritic intrusions of arc affinity, metamorphic host rocks, and cross-cutting, Jurassic age, mafic to intermediate dikes. Exposures along the trace of the TSZ consist of carbonate mylonite, mylonite derived from granodorite, tectonite hosted in Miocene plutonic rocks, cataclasites, faults, and psuedotachylites. U-Pb Shrimp dates from a carbonate mylonite are similar to those of the Miocene plutons, although it is thought that these rocks were initially formed during the late Mesozoic when carbonte depositional environments were common throughout Mexico. All of the outcrop deformation fabrics of the TSZ demonstrate sinistral offset and are associated with a strikingly uniform, poorly lineated, foliation fabric that is parallel to the TSZ. Textural analysis of samples from deformed Miocene intrusive rocks shows that these rocks accumulated sinistral ductile strain at temperatures <400°C. All brittle structures found along the trend of the TSZ including tension gashes, pseudotachylytes, cataclasites, and fault planes also exhibit left-lateral offset. The sum of these observations is consistent with evidence of Cenozoic sinistral translation along a series of margin parallel regional shear zones such as Tierra Colorada (Solari et al., 2007, Rilller et al., 1992) and Chacalapa fault (Tolson, 2005; 2007). These faults are are found north of the TSZ and are associated with similar calc-alkaline intrusions that become progressively older to the north along the Mexican coast. In this context there is a clear pattern of sinistral shear along the coast margin of Mexico from 21°N to the Guatemalan border, which we interpret to indicate transfer of the Chortis block from the North American margin to its current position as part of the Caribbean plate system.