TECTONIC HISTORY OF TWO DISPARATE HIGH-PRESSURE, LOW-TEMPERATURE METAMORPHIC BELTS ALONG THE MOTAGUA FAULT ZONE, GUATEMALA

The boundary between the North American and Caribbean plates in Guatemala is defined by the Motagua fault zone, one of three left-lateral, subparallel strike-slip fault systems.  Tectonic slices of serpentinite and high-pressure, low-temperature (HP-LT) rocks occur both north (Maya block) and south (Chortís block) of the Motagua fault. Until recently the HP-LT belts have been accredited to one event.  North of the Motagua fault, serpentinite mélange hosts omphacite metabasite, albitite, jadeitite, and garnet amphibolite.  Assemblages south of the fault include eclogite, blueschist, and jadeitite in serpentinite matrix.  Recent ⁴⁰Ar/³⁹Ar geochronology on phengitic mica and amphibole yields two distinct ages for northern and southern belts: 65-77 Ma and 116-125 Ma respectively.  Varying ages, in addition to dissimilar HP-LT assemblages, indicate that the two exhumed belts are due to two separate tectonic events.

We present new structural data, both ductile and brittle, that indicate a complex tectonic history for the Maya and Chortís blocks.  Five ductile phases (D₁-D₅) are present both north and south, most of which indicate north-south contraction.  D₂, both in the north and south, is characterized by a strong foliation with stretching lineations formed by left-lateral shearing due to displacement partitioning.  The northern D₄ structures may have formed as a result of a restraining bend.  Four phases of brittle faulting resulted from progressively increasing counterclockwise rotation both with relative age and with proximity to the Motagua fault.  Additional ⁴⁰Ar/³⁹Ar and apatite and zircon fission-track geochronology provide insight to the absolute timing of ductile and brittle events.  These data support a tectonic model in which the Chortís block is located alongside southwestern Mexico approximately 120 Ma.