Petrology and origin of basalts at DSDP Hole 66-487

The volcanism of Central America, according to current theory (Pichler and Weyl, 1973; Stoiber and Carr, 1974; Hey, 1977), is related to the subduction of the Cocos Plate under the North American lithospheric plate and the melting of ocean crust material in the subduction zone (Green and Ringwood, 1968; Dickinson, 1970, Fitton, 1971). Since Cocos Plate subduction occurs at the rate of more than 7 cm/y. (Hey et al., 1977), basalts underlying upper Miocene sediments of the Middle America Trench outer slope, penetrated in Hole 487 (Fig. 1) during Leg 66 (Moore et al., 1979), should have formed far from their present position if current theory is accurate. Present manifestations of basaltic magmatism in adjacent areas of the Pacific derive from the axial part of the East Pacific Rise, the Galapagos spreading center, and transform fracture zones. The question arises: Are there analogs of the Middle America Trench basalts among magmatic cock associated with these modern features, or do the trench basalts have some other origin?

Petrology, geochemistry, and geodynamics at DSDP Hole 66-493

At Site 493, DSDP Leg 66, dioritic basement was reached below lower Miocene (NN1 Zone, 22-24 Ma) terrigenous sediments. Petrographical, mineralogical (including microprobe analyses), and chemical features of the dioritic rocks reveal their magmatic affinity with the calc-alkaline series. Furthermore, their radiometric age (35.3 m.y.) links the basement to the Sierra Madre Occidental in Mexico and to mid-Tertiary volcanic arcs in Central America. The presence of Oligocene diorite 50 km from the trench axis confirms the truncation of the south Mexico margin, which we explain as the result of a 650 to 800 km left-lateral displacement of Central America relative to North America. Truncation must have occurred in the late Miocene, after the diorite intrusion and prior to the present subduction.

Grain size, mineral and chemical compositions of modern deposits fron the northern Caspian Sea and the Volga River delta

Comprehensive investigations revealed that modern deposits in the northern Caspian Sea involve terrigenous sands and aleurites with admixture of detritus and intact bivalve shells, including coquina. Generally, these deposits overlay dark grayish viscous clays. Similar geological situation occurs in the Volga River delta; however, local deposits are much poorer in biogenic constituents. Illite prevails among clay minerals. In coarse aleurite fraction (0.100-0.050 mm) heavy transparent minerals are represented mostly by epidotes, while light minerals - mostly by quartz and feldspars. Sedimentary material in the Volga River delta is far from completely differentiated into fractions due to abundant terrigenous inflows. Comparatively better grading of sediments from the northern Caspian Sea is due to additional factors such as bottom currents and storms. When passing from the Volga River delta to the northern Caspian Sea, sediments are enriched in rare earth elements (except Eu), Ca, Au, Ni, Se, Ag, As, and Sr, but depleted in Na, Rb, Cs, K, Ba, Fe, Cr, Co, Sc, Br, Zr, ??, U, and Th. Concentrations of Zn remain almost unchanged. Sedimentation rates and types of recent deposits in the northern Caspian Sea are governed mainly by abundant runoff of the Volga River.

Geochemistry and minerals of glass shards at DSDP Leg 66 Holes

Deep Sea Drilling Project Leg 66 drilled eight sites along a transect across the Middle America Trench off Mexico, including continental (Sites 493 and 489), oceanic (Site 487), and trench (Site 486) reference sites and four sites (490, 492, 491, 488) in the trench inner wall. Because of their location - close to volcanic sources and subject to prevailing winds and marine currents (N to S, NW to SE) - analysis of airborne ashes intercalated within the sediments at these sites provides a reliable record of explosive volcanism in the area. Intense onshore volcanic activity in Mexico during the Oligo-Miocene has been well documented by the andesites and ignimbrites of the Sierra Madre Occidental and Sierra Madre del Sur and in the Plio-Quaternary by the andesites and basalts from the Trans-Mexican Neovolcanic Belt and the eastern border of Baja California.