Interpretation of Seismic-Reflection Data of the Middle America Trench offshore Guatemala

Abstract

A geophysical and geological survey conducted over the landward slope of the Middle America Trench offshore Guatemala, together with published well information from the outer shelf and Leg 67 drilling results from the toe of the slope indicate that imbricate slices of oceanic crust were emplaced in the landward slope offshore Guatemala in the Paleocene or early Eocene. Since that time, sediment apparently has accumulated on the landward slope primarily as a sediment apron blanketing an older, tectonically deformed prism of sediments and crustal slices. There is little or no evidence for continued tectonic accretion seaward of the volcanic arc during the late Tertiary.Seismic reflection and refraction surveys have revealed landward-dipping reflections that are associated with high compressional wave velocities, large magnetic anomalies, and basic-ultrabasic rock. Multifold seismic reflection data reveal that the edge of the continental shelf is a structural high of Cretaceous and Paleocene rock against which Eocene and younger sediments of the shelf basin onlap and pinch out. The upper part of the continental slope is covered in most places by a 0.5-to 1.0-km-thick sediment apron with seismic velocities of 1.8 to 2.6 km/s. The base of the sediment apron commonly coincides with the base of a gas hydrate zone where water is 1500 to 2300 meters deep. Immediately beneath the sediment apron an irregular surface is the top of an interval with velocities greater than 4 km/s. Within this interval, landward-dipping reflections are traced to about 6 km below sea level. These reflections coincide with the top of seismic units having oceanic crust velocities and thicknesses.The sediment apron pinches out on the lower continental slope where refraction results indicate only a few hundred meters of 2.5-km/s material lying over about a kilometer of 3.0-km/s sediment. Between the 3.0-km/s sediment and a landward continuation of ocean crust, an interval of 4.1-to 4.7-km/s material occurs that thins seaward. Near the interface between the 4 +-km/s material and oceanic crust with velocities of 6.5 to 6.8 km/s, reflection records indicate a landward-dipping horizon that can be followed about 30 km landward from the Trench axis.Coring on the continental slope returned gravels of unweathered metamorphosed basalt, serpentine, and chert, unlike rock generally found onshore in Guatemalan drainage basins feeding the Pacific coast. These gravels, which were probably derived from local subsea outcrops, are similar to lithologies found on the Nicoya Peninsula farther south.A canyon cut in the outer continental shelf and upper continental slope may be associated with faulting, as indicated by an offset of linear magnetic anomalies at the shelf edge.In a general way our observations are consistent with previous suggestions that slices of rock, some of which may have oceanic crustal lithologies, are imbedded in the upper slope. However, the reflection data collected for the Deep Sea Drilling Project site survey do not show the many conc...

Petrology and Relict Mineralogy of Serpentinites from Deep Sea Drilling Project Leg 84

“…There are, however, some aspects of the drilled assemblages which contradict a model based only on imbricate stacking. As pictured by Ladd et al (1978) or Azema and Tournon (1982, p. 740), the most common materials sampled ought to be basalts, diabases, and gabbros, with lesser amounts of peridotitic rocks. In contrast, coring recovered serpentinite as the sole basement rock at Holes 566, 566A, 566C, and 570; serpentinite in blocks and between units of basalt and gabbro in Hole 567A; and gabbro and basalt alone in Hole 569A (site chapters, this volume).…”

“…Seismic profiles of the forearc have been interpreted as showing two or more large imbricate slices of ocean crust in the forearc (Ladd et al, 1978). Such imbricate stacking might well expose some upper mantle materials at shallow depths.…”

“…It had been suggested, on the basis of geophysical data, that this slope was composed of imbricated, landward-dipping slices of ocean crust (Seely et al, 1974;Ladd et al, 1978). Gravity and magnetic data, particularly a magnetic high parallel to the trench-slope break, are consistent with the presence of high-density crustal rocks at shallow depths, and indicate that these rocks may be an extension of mid-Tithonian-upper Santonian ophiolitic rocks exposed in coastal Costa Rica (Woodcock, 1975;Couch 1976;Couch and Woodcock, 1981).…”

Seismic Stratigraphy of the Continental Shelf offshore Guatemala: Implications for Vertical Tectonics Related to Subduction

“…The nature of the shelf differs dramatically along strike: off Mexico the shelf is narrow and apparently underlain by a granitic extension of continental rocks (Shipley et al, 1982), whereas farther south off Guatemala the shelf is underlain by a thick sediment forearc basin of Cretaceous and Tertiary sedimentary rocks probably lying on a piece of oceanic crust (Seely et al, 1974;Seely, 1979;Ladd et al, 1982). A structural high onlapped by Tertiary sediments is generally found at the seaward edge of the forearc basin offshore Guatemala.…”

“…Subsequent to this rupture the structural high at the shelf edge developed as a result of progressive underthrusting of sediment and lithosphere beneath the isolated slice of lithosphere, which led to progressive uplift and tilting of this residual forearc basin (Seely, 1979;Ladd et al, 1978Ladd et al, , 1982Couch and Woodcock, 1981). Magnetic and gravity data indicate that a basement high follows discontinuously the continental shelf edge from the Gulf of Tehuantepec off Mexico southward to the Nicoya Peninsula off Costa Rica where an Early Cretaceous ophiolite complex crops out (Dengo, 1962;Couch and Woodcock, 1981;Lund-berg, 1982).…”