2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM


AVE LALLEMANT, H.G.1, FRANCIS, A.H.1, SISSON, V.B.2, HEMMING, S.R.3, RODEN-TICE, M.4, BRUEKNER, H.K.3, HARLOW, G.E.5 and CHIQUIN, M.6, (1)Earth Science, Rice University, 6100 Main, Houston, TX 77005, (2)Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th St, New York, NY 10024, (3)Earth and Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia Univ, 61 Route 9W, Sparkill, NY 10976, (4)Center for Earth and Environmental Science, Plattsburgh State University of New York, 1001 Broad Street, Plattsburgh, NY 12901, (5)Dept. Earth & Planetary Sci, AMNH, New York, NY 10024-5192, (6)Centro Universitario del Norte, Finca Salamacha, Coban, 16002, Guatemala, ave@rice.edu

The Motagua Valley in Guatemala is underlain by a major fault zone that is the boundary between the Maya block of the North American plate and the Chortís block of the Caribbean plate. The geological evolution of the Maya block is quite different from that of the Chortís block. The Motagua fault zone is a serpentinite-matrix mélange containing tectonic inclusions of High Pressure / Low Temperature (HP/LT) metamorphic rocks that were thrust both northward onto the Maya block, and southward upon the Chortís block.In the north, the HP/LT inclusions are omphacite metabasite, jadeitite, garnet amphibolite, and albitite, whereas the HP/LT rocks in the south are lawsonite eclogite, blueschist, and jadeitite. The HP/LT rocks in the north are altered in contrast to those in the south.

Structural and geochronological data indicate that at least four tectonic events (T1 to T4) have occurred in the study area. The first event (T1) occurred between 135 and 113 Ma (Nd/Sm and 40Ar/39Ar); these ages have only been found in the Chortís block in the south. The second event occurred between 77 and 53 Ma (40Ar/39Ar); these ages were found only in the Maya block in the north. The third event (T3) took place in both blocks between 43 and 23 (?) Ma, as shown by 40Ar/39Ar and apatite fission-track analyses. The fourth event (T4) is related to recent deformation.

Mega- and mesoscopic deformation structures suggest that during T1 the SE moving Chortís block was subducted underneath and collided with western Mexico after which the HP/LT rocks were exhumed and emplaced. The Chortís block continued moving to the SE until a subduction reversal of the Central American arc occurred. Subsequently, the Chortís block as part of the Greater Antilles arc migrated to the NE. During T2 it collided and was thrust over the Maya block (southern Mexico). It appears that a second group of HP/LT rocks were exhumed and thrust to the north. An alternative model is that these rocks are the same rocks that formed during T1 but were strongly deformed, altered, and metasomatized. The third and fourth events (T3 and T4) are probably related to changes in plate motions along the Caribbean - North American plate boundary.