A variety of fresh to highly altered igneous rocks and sediments with minor to abundant volcanic components was cored at seven sites during Leg 17. Diagenetic alteration of the igneous rocks is ubiquitous and is particularly intense at Sites 167, 170, and 171. Zeolite facies metamorphism affected the lowest rocks at Hole 165A. Diagenesis is slow in the absence of high CO2 activity. However, in a Cθ2-rich environment it leads to rapid replacement of olivine, glass, and mesostasis, and slow replacement of pyroxene by montmorillonite. Plagioclase is only slightly to moderately attacked in the CO2-ri.cn environment and calcium is lost from pyroxene. Sodium, potassium, and water are gained in the montmorillonite, and iron is oxidized to the extent that available oxygen permits.The best methods of deriving original rock and magma types of altered specimens are through "refractory" (i.e., alterationresistant) trace elements, but even these appear to be lost when alteration is so intense that pyroxene is destroyed, such as at Sites 167, 170, and 171. Taking all available data into account, the following conclusions are drawn.True basement (ocean ridge basalts of Layer 2 of the oceanic crust, accreted at a spreading center) was cored at Sites 164 and 166 and possibly at the bottom of the hole at Site 169. At all other sites and higher in the hole at Site 169, igneous rocks were recovered that are more alkaline than ocean ridge basalts. They probably were emplaced at a distance from a spreading center above a section of older sediments and do not represent true basement. The evidence for this is conclusive at Site 169, where a sill of uncertain petrologic affinities was penetrated, and is very strong at Sites 167 and 171, which are located on large seamounts (Magellan Rise and Horizon Guyot, respectively). The evidence is weaker, but persuasive, at Site 165A and Site 170. Sills were definitely (Site 169) or probably (Sites 167 and 170) cored at three sites, and seem to be a common mode of emplacement when the true basement is covered by unconsolidated sediments.In addition to flows and sills, volcanic material appears in the sedimentary section as turbidites, hyaloclastites, pyroclastites, and dispersed detritus in pelagic sediments. Like the relatively alkaline rocks discussed above, the volcanic components in the sediments record activity that postdates the time of accretion of the underlying crustal plate at a spreading center. In general, the volcanic sediments were derived from nearby seamounts and ridges which yielded rubble and turbidites not only by slumping of unstable slopes but also by explosive activity as peaks approached sea level. Where the peaks emerged as islands, as at Site 171, subaerially eroded detritus and perhaps forereef calcarenites and calcirudites also were supplied.

SUMMARY Electron microprobe analyses of skeletal carbonate structures in coralline algae and pelecypods show that the magnesian content of the calcite depends on skeletal growth rates. Growth rate is a function of certain variables, including water temperature, light, and physiological cycles. The Mg‐content of calcareous sediment from specific organisms is not, therefore, mainly dependent on water temperature, as is generally believed, but also on the other factors affecting growth.

No abstract

The continental margin of southern Peru and northern Chile was surveyed during the 1973 and 1974 expeditions of the research vessel Kana Keoki. Seismic reflection profiles reveal three large basins at about 1000 m depth between Mollendo, Peru (17°00′ S) and Iquique, Chile (20°00′ S). Only small basins and discontinuous terraces are seen on profiles crossing the Iquique-to-Antofagasta, Chile (23°30′ S) segment of the continental margin.The structural cross-sections of the basins resemble those of arc-trench gaps. The undeformed uppermost reflectors probably represent turbidites, as evidenced by displaced shallow-water benthic foraminifera and coarse sands in cores. Deeper reflectors are generally inclined land-ward, with dips and deformation increasing in the lower reflectors down to about 1.5 seconds penetration. Seaward convergence of these reflectors indicates a progressive shoreward migration of the axis of maximum sedimentation. If the deeper beds are also turbidites, this axis marks the axis of the sediment trap on the continental slope.The structure is consistent with subduction of an oceanic plate and obduction of a portion of its sediment cover. The imbricate stacking of obducted material is lifting an anticlinal ridge visible in most traverses across the trench side of the basin. The growing ridge is deforming the older sediment trapped in the upperslope basin and shifting the locus of deposition shoreward. The irregular distribution of the basins apparently is a product of culminations and depressions of the surfaces of imbricate fault planes. These undulations may result from the transference of the irregular structure of the oceanic plate to the face of the continental block.

With the exception of the lower continental slope, or inner wall of the Peru Trench, normal faulting characteristic of extensional tectonics predominates in the shallow structure from the outer rise on the Nazca plate, across the trench, the contenental margin, the Coastal province, and into the High Cordillera province of the Andes of northern Peru. Marine seismic reflection and gravity traverses are used to trace the major basins, horsts, and large faults seaward from where they have been mapped on land and in the subsurface. Although these large-scale structures can be traced geophysically, neither single-channel nor multichannel seismic records reveal the wealth of detail of block faults and olistostrome-like low-angle slides known from subsurface studies of the onshore and offshore oil fields in the fore-arc basin along the coast at Talara, Peru.The subduction process has been efficient during the Cenozoic at the Peruvian margin. The accretionary wedge under the inner wall of the trench is small compared to the volume of sediment that entered the trench. Moreover, plutons near the coast and the trends that strike seaward in the structures of the igneous and metamorphic basement there indicate that a substantial width of the lower outer edge of the continental crust has been removed. The evidence strongly suggests that the crust has been stoped away by subduction.Whereas coastal Peru is underlain by a metamorphic, plutonic, and volcanic basement that is termed continental, coastal Ecuador has a belt of ophiolitic rocks regarded as former oceanic crust. The Dolores-Guayaquil Megashear separates the two terranes. It is probably a transform-fault boundary setting aside the ophiolites of coastal Ecuador and Colombia as a mini-plate. The Progreso Basin lies beneath the Gulf of Guayaquil and contains hydrocarbons. It was formed as a pull-apart basin by dextral movements along the Dolores-Guayaquil Megashear where that fault strikes westward into the trench. Banco Peru may be a horst of mafic or ultramafic basement left as a slice when the Progreso Basin was formed.