Seismic refraction and reflection measurements in the Middle America Trench offshore Guatemala

Eleven serpentine samples from DSDP Leg 84 and four serpentinized ultramafic samples from Costa Rica and Guatemala were described and their relict mineral compositions measured by electron microprobe to try to determine the origin of the Leg 84 serpentinites and their relationship to the ultramafic rocks of the onshore ophiolites. The Leg 84 samples comprise more than 90% secondary minerals, principally serpentine, with hematitic and opaque oxides, and minor talc and smectites. Four distinct textural types can be identified according to the distribution of opaque phases and smectite. Remnants of spinel, olivine, orthopyroxene, and clinopyroxene occur variously in the samples; spinal occurs in all the samples. Textural evidence suggests that the serpentinites were originally clinopyroxene-bearing harzburgites. Relict mineral compositions are refractory and relatively uniform: olivine, Fo^ ^^ 9 ; orthopyroxene, En^^; clinopyroxene, Wo 47 En 50 Fs 3 ; spinels, Cr/Cr + Al = 0.4-0.6. 567A-29-2, 30-35 cm has slightly more magnesian olivines (Fo 92 ) and orthopyroxene, and more aluminous spinels (Cr/Cr + Al = 0.3). These compositions are similar to those inferred for refractory upper-mantle materials and also fall within the range of compositions for relict minerals in abyssal peridotites. They could be of oceanic origin. The onshore samples include serpentinites, a clinopyroxene-bearing harzburgite, and a clinopyroxenite. They too have magnesium-rich silicate assemblages, but relative to the drilled samples have more iron-rich olivines (Fogo) and more aluminous and sodic pyroxenes; spinels which are clearly relicts are very aluminum-rich (Cr/Cr + Al = 0.1-0.25). These samples are most likely mantle materials, but significantly less depleted. Their relationship to the drilled samples is unclear. Serpentinites were the most common basement materials recovered during Leg 84, and there appears to be a bimodal assemblage (basalt/diabase and serpentine) of igneous rocks sampled from the trench slope. Diapirism of serpentine throughout the trench slope and forearc is suggested as an explanation for this distribution of samples.

Section: Introductionsupporting

confidence: 61%

Section: Discussionmentioning

confidence: 99%

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

Bloomer¹

1985

“…Ladd et al (1978) and Ibrahim et al (1979) showed that seismic refraction and reflection interval velocities within the Figure 5. The imbricate underthrust accretionary model of Seely et al (1974).…”

Section: I-i-i-ihr23rmentioning

confidence: 99%

Tectonic Synthesis, Leg 66: Transect and Vicinity

Watkins¹,

McMillen²,

Bachman³

et al. 1982

Cores recovered from eight sites drilled along a transect across the Middle America Trench off southwestern Mexico show that accretion and offscraping of trench turbidites and the pelagic-hemipelagic blanket covering the ocean crust has been active for approximately 10 m.y. Active offscraping and imbricate underthrusting have been limited to the lower slope, whereas middle and upper slopes show uplift due to underplating by subducted sediments and younging of ocean crust. Offscraped trench sands of Pleistocene to Miocene age have been uplifted as much as 2 to 3 km above the present-day trench floor by combined accretion and underplating.Sediment input into the subduction zone is partitioned, with roughly equal amounts being offscraped, underplated, and subducted. The offscraped component appears to consist primarily of trench turbidites, whereas the subducted component is thought to consist primarily of pelagic-hemipelagic sediments. The trench turbidite component probably dominates the underplated component.Partitioning values based on mass balance budget and on theoretical lithospheric bending were comparable. This comparability lends confidence to our theory of flux partitioning.Although we find no direct evidence of tectonic erosion, there is indirect evidence to support the feasibility of the mechanism, and although there is no present forearc basin, differential uplift rates suggest incipient development.Thus we infer the following history of the region: 23-20 Ma-reorganization of plate motion: relative movement of Pacific and North American plate changes from oblique normal (subduction) to parallel (sinistral strike-slip) with margin subsidence and marine transgression.19-17 Ma-margin sinks rapidly to, or just below, CCD, then begins slow rise. Sinistral strike-slip motion continues.10 m.y.-plate motion changes from strike-slip to normal-oblique; accretion begins and continues to present.

“…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). Seismic refraction studies off Guatemala indicate two landward-dipping units with velocities and thicknesses appropriate for oceanic crust (Ibrahim et al, 1979). Sediments drilled on the shelf edge by Exxon almost immediately above acoustic basement were deposited at abyssal depths, indicating that thin, probably oceanic crust forms basement (Seely, 1979).…”

Section: Introductionmentioning

confidence: 99%

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

Ladd¹,

Schröder²

1985

Seismic reflection data on the shelf offshore Guatemala reveal the stratigraphy of the forearc basin that has developed as the result of Tertiary subduction on the site of a Cretaceous passive margin. Abyssal Cretaceous and Paleocene sequences that presently dip landward to the northeast probably originally downlapped across a seaward-dipping continental rise surface. The termination of the thick Paleocene sequence at the shelf edge is probably a postdepositional tectonic truncation. A bathyal Eocene-Oligocene sequence deposited in the forearc subsequent to the initiation of subduction is truncated by a sharp angular unconformity, which indicates severe tilting and erosion of the forearc basin prior to subsidence and the deposition of the seaward-thickening early Miocene sequence. A late Miocene and Pliocene unit thins seaward, indicating renewed uplift of the shelf edge. A Quaternary sequence progrades across the gently tilted upper Miocene-Pliocene sequence toward San José Canyon.Major unconformities of the forearc basin were coeval with major plate reorganizations. A major reorganization of the Pacific-Farallon spreading center, which occurred in the late Paleocene, may have coincided with or preceded the uplift of the Guatemala shelf edge and the inception of subduction in this region. A late Oligocene disruption of the Farallon Plate forming the Nazca and Cocos plates coincided with renewed subsidence of the shelf edge associated with the seaward-thickening early Miocene sequence. In the middle Miocene another plate reorganization, which led to a much shortened Pacific-Cocos plate boundary, may have coincided with renewed uplift of the shelf edge. The regional Pliocene-Pleistocene unconformity may be associated with an eastward jump in the Pacific-Cocos spreading center from the Mathematician spreading center to the present East Pacific Rise.The causal mechanism relating plate reorganizations to ocean margin crustal dynamics may involve the changing pattern of forces driving an oceanic plate as the size and shape of the plate change.