Time of sinistral slip along the polochic fault of Guatemala

Abstract

The Polochic fault of Guatemala and Chiapas, Mexico was a Neogene transform segment of the North American — Caribbean plate boundary, with a total sinistral slip of 130 km. Principal activity was within the interval from about 10.3 to 6.6 m.y. B.P. or less, a time bracket based upon K-Ar ages from volcanic rock clasts in the Colotenango beds of northwest Guatemala. Earliest possible movement was established from ages of volcanic rock constituents of the older Colotenango conglomerates, deposited by streams that flowed north from a terrane now in Central Guatemala and onto terrane that was subsequently offset along the Polochic fault. It is believed that the Selegua River was connected to the Chixoy River of Central Guatemala. This river system deposited the older part of the Colotenango conglomerate now in western Guatemala about 130 km west of its source area. Two volcanic clasts from the conglomerate yield ages of 12.3 ± 1.0 and 10.3 ±1.0 m.y. The younger age is believed to set the maximum age of sedimentation of this older part of the conglomerate and is probably close in time to initiation of fault activity. Distinctive brown serpentinite clasts in the conglomerate have been shown to be petrographically, mineralogically, and geochemically similar to serpentinite from the presumed source area north of Salamá, Guatemala. A youngest age of major offset is established by the younger part of the Colotenango beds, a volcaniclastic deposit which fills much of the ancient Polochic fault-controlled Cuilco River Valley. Our studies of volcanic clasts of the younger Colotenango conglomerate reveal pristine glass, attesting to its relatively young age. K-Ar determination establishes an age of 6.6 ± 0.3 m.y. B.P. for these clasts. The youngest age of movement has not been established with certainty. It lies between the 6.6 m.y. age and the age of the uppermost volcaniclastic valley fill. The synchroneity of spreading in the Cayman Trough (Macdonald and Holcombe, 1978) with transform activity along the Polochic suggests strongly that the two were genetically related. The earliest data in the bracketed time interval correlates with the age of ongoing spreading for the Mid-Cayman Rise. The Mid-Miocene to Pliocene volcanic arc (9–19 m.y. B.P.) of Mexico and its probable continuation in Guatemala (8–19 m.y. B.P.), sinistrally offset across the Polochic, fits well with the chronology discussed above for the Polochic.