Jean Aubouin

Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala

Drill cores from a transect of the Mid-America Trench off Guatemala were obtained at three sites on the oceanic Cocos plate, and at four sites on the continental Caribbean plate. An ocean sub-bottom seismometer was successfully emplaced in the deepest hole in the trench landward slope where it was left to record data after departure of the drill ship. Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation even at holes drilled against the trench9s landward slope. In contrast, the section cored on the trench9s landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence, broken by hiatuses but in a normal stratigraphic succession that is capped by Pliocene to Quaternary hemipelagic slope deposits. Seismic records show that the section overlies probable igneous oceanic crust from which it is separated by a few hundred metres. That thickness of undrilled section is insufficient to accommodate the potential offscraped volume of oceanic sediment carried into the trench during Neogene plate convergence. At the estimated 10 cm/yr rate of convergence, much of the oceanic sediment must have been subducted rather than tectonically accreted to the Guatemalan margin. Current models for convergent margin tectonics do not satisfactorily explain the surprising occurrence of Cretaceous to Miocene mudstone at the base of this trench slope. The recovery of gas hydrates prevented drilling to some landward-dipping reflections presumed to be imbricate thrust slices at two sites near the middle of the trench landward slope.

The geologic history of the Caribbean-Cocos plate boundary with special reference to the Nicoya ophiolite complex (Costa Rica) and D.S.D.P. results (Legs 67 and 84 off Guatemala): A synthesis

Abstract The Pre-Upper Senonian basement of Costa Rica crops out in the Santa Elena and Nicoya peninsulas. From south to north and from base to top the basement includes: the Esperanza, Matapalo and Santa Elena units. The Esperanza unit is Albian-Santonian in age and consists mainly of pillow basalt and massive basalt flows. The Matapalo unit includes Callovian to Cenomanian radiolarite and includes massive basalt flows, basalt, and dolerite basement. The Santa Elena unit contains ultramafic and mafic rocks in which harzburgite is the major component. The most important tectonic features of the Nicoya Complex are the large Santa Elena and Matapalo nappes. Nappe emplacement was from north to south during upper Santonian time. The sedimentary cover of the Nicoya Complex comprises: 1. (1) the Campanian El Viejo Formation that consists of shallow-water sediments in the north (Santa Elena Peninsula) and the Campanian-Maastrichtian Sabana Grande Formation of deep-water origin in the South (Nicoya Peninsula); 2. (2) Paleocene strata indicating deposition in a deep-water environment comprises the Rivas, Las Palmas and Samara Formations; 3. (3) a post-upper Eocene (?) sequence that consists of the shallow-water Barra Honda and Montezuma Formations. Two unconformities are significant geological features of the upper-Senonian to Tertiary history of Costa Rica. The lower one is at the base of the Sabana Grande Formation and marks a major change in the geologic conditions (basalt is scarce in the Campanian-Tertiary series); the upper unconformity at the base of the Barra Honda and Montezuma Formations is not as major as the lower one. During post-Campanian time, normal faulting occurred in two stages separated by a strong erosional phase. The geology of the landward slope of the adjacent Middle America Trench is outlined by interpreting multifold seismic reflection records off the west coast of Costa Rica and the DSDP Legs 67 and 84 transects off Guatemala. The western Caribbean plate boundary may have been under extensional stress for the last 75 m.y. The strong landward-dipping reflectors of the Middle America Trench landward slope off Guatemala could be equivalent to the on-land pre-Campanian overthrusts of Costa Rica. The available data are consistent with the Convergent Extensional margin concept.

The middle america trench as an example of a subduction zone

Abstract Short Sea-Beam survey cruises were conducted by the R.V. “Jean Charcot” in march 1980 along the Middle America Trench between Panama and Acapulco, with particular emphasis on the IPOD Legs 66 and 67 areas. These detailed mappings have revealed that the Cocos horst- and graben pattern, related to the bending of the oceanic plate, strikes generally 20°–30° oblique to the trench direction. This en-echelon pattern, closely parallel to the magnetic anomalies, is here proposed as being due to the reactivation of inherited fractures generated at the East Pacific Rise. Sea-Beam data, together with UTMSI multichannel seismic reflection profiles and IPOD drilling results, clearly show that at least two distinct geodynamic processes are presently working along the same trench: accretion and no accretion. They also show a relative structural homogeneity of the oceanic side of the trench in contrast to the dual aspect of the continental side.

The west Pacific geodynamic model

Abstract Between the Asian and Australian continents on the western side and the general subduction of the Central Pacific on the eastern side, the west Pacific presents two types of subduction: towards the continents and towards the ocean. As a consequence, at the present day there are two main types of marginal seas—the Japan Sea and the South China Sea—and, in the geological past, two types of orogenesis-the Japanese and the Taiwanese types. These are the models of the two main orogenic belts, respectively the Circum-Pacific and Tethyan. For a long period of the past, the Alps and the Mediterranean Sea were the tectonic model of reference. However with the increase in geological studies, oceanographic research, geophysical surveying, the west Pacific gradually became an alternative model which grew more useful with each passing day. The present-day trench-island arc-marginal sea system appears to be a good model for the first stages of orogenic belts, whereas the mountains of the east Eurasian margins provide an example of the completion of formation of such belts. The coexistence of different stages of mountain building is the main reason for the increasing interest shown by earth scientists in the west Pacific.

Some aspects of the tectonics of subduction zones

Abstract The main structures of a subduction zone are as follows. 1. (1) On the outer wall: faults, formed either by reactivation of the structural grain of the oceanic plate, when the latter is slightly oblique to the trench, or by a new fault network parallel to the trench, or both. The width of the faulted zone is about 50 miles. 2. (2) On the inner wall: either an accretionary prism or an extensional fault network, or both; collapsed structures and slumps are often associated, sometimes creating confusion with the accretionary structures. 3. (3) The overall structure of the trench itself is determined by the shape of the edge of the continental crust or of the island arc. Its detailed structure, however, is related to the oceanic plate, namely when the structural grain of the latter is slightly oblique to the trench, which then takes an “en echelon” form. Collapsed units can fill up the trench which is, in that case, restricted to an irregular narrow depression; the tectonic framework of the trench can be buried under a sedimentary blanket when the sedimentation rate is high and the trench bottom is a large, flat area. Two extreme types of active margins can be distinguished: convergent compressive margins, when the accretionary mechanism is strongly active; and convergent extensional margins where the accretionary mechanism is absent or only weakly active. The status of a given margin between these two extreme types is related to the convergence rate of the plates, the dip of the subduction zone, the sedimentation activity and the presence of a continental obstacle, because oceanic seamounts and aseismic ridges are easily subducted. Examples are taken from the Barbados, Middle America, Peru, Kuril, Japan, Nankai, Marianna, Manila, New Hebredes and Tonga trenches.

Subduction of the Cocos plate in the Mid America Trench

The Mid America Trench, off the Pacific coast of southern Mexico and central America, is the site of northeastward subduction of the Cocos oceanic plate under the North America and Caribbean plates1 (Fig. 1). In Guatemala, the North America and Caribbean plates are separated by the Polochic–Motagua left-lateral strike-slip faults (Fig. 1). The trench itself appears to be divided into two distinct sections at its intersection with the Tehuantepec Ridge. To the south the margin of the trench is wide and consists of a well developed shelf basin2 bounded on the east by a huge volcanic cordillera parallel with the trench axis at a distance of ∼200 km. In contrast, to the north, the margin is narrow and devoid of shelf basin; landwards, the trans-Mexican volcanic belt trends oblique to the trench. This volcanic chain would be anomalous if it were related to the subduction of the Cocos plate off southern Mexico. In 1979, as part of the International Phase of Ocean Drilling (IPOD) of the Deep Sea Drilling Project, Legs 66 and 67 respectively explored the northern (off Acapulco, Mexico) and southern (off San Jose, Guatemala) section of the Mid America Trench3–6 (Fig. 1). Since then short topographical surveys of the trench have been carried out by RV Jean Charcot8 using the sea-beam technique7. We report here that both sets of data, together with UTMSI multichannel seismic profiles10,11 can be used to reconstruct the processes of subduction along the Mid America Trench.

Preliminary Results, DSDP Leg 84, Middle America Trench Off Guatemala: ABSTRACT

The Middle America Trench off Guatemala, which was drilled on DSDP Legs 67 and 84, is part of a nonaccreting convergent margin. The igneous continental framework of Central America extends to the base of the landward trench slope. Ophiolitic rocks, which correspond to the acoustic basement seen in seismic records beneath a cover of Neogene slope deposits, were recovered at five sites (seven holes). This basement represents an ocean crust first tectonized in the pre-Campanian and emplaced in pre-early Eocene time; the basement is not a tectonic product of the present convergent margin. Subduction of sediment on the Cocos plate may result in the development of overpressures which was observed directly at four sites. On Leg 84, gas hydrate was recovered at three sites and detected at five of the six sites drilled. It occurs dispersed in muddy sediment or in porous lithologies and in fractures. A distinctive gas composition and low salinity pore water were found associated with the gas hydrate. The source of the gas was mainly biogenic, but thermogenic gas apparently was present in fractures of the basement rock. End_of_Article - Last_Page 1702------------

Sur la geologie de la zone littorale du Montenegro; les bouches de Kotor (Yougoslavie); comparaison avec les series helleniques correspondantes

The autochthonous zone of the Montenegro coast (Yugoslavia) is correlative with the Gavrovo zone of Greece, and the overthrust Cukali zone of Yugoslavia with the Pindus zone of Greece. The paleogeographic and structural consequences of this correlation are examined.