The crustal structure and tectonic framework of the central Peru Margin, between lat 7° and 10°S were interpreted using mainly a 102 km long, multichannel seismic section and existing geologic and geophysical data in the region. Thrust faulting occurs in upper layer 2 basalts as the Nazca plate descends beneath the continental slope. This produces basaltic ridges (slabs) within the trench axis and at least 26 km landward beneath the overriding continental plate. Broken low-frequency reflectors within this diffracting subduction complex suggest that ophiolitic slivers of basalt are being incorporated into portions of it, forming a sediment-basalt melange. Three prominent forearc basins, Salaverry, Trujillo, and Yaquina Basins, occupy the central margin from east to west, respectively. Drill holes penetrated Tertiary sediments on the outer shelf and the nearby eastern flank of the Trujillo Basin and bottomed in a metamorphic arc massif. The massif is correlated with seismic refraction velocities greater than 5.7 km/sec and a density of 2.72 to 2.80 g/cm 3 which underlie the continental shelf. Our interpretation of the seaward limit of the massif is uncertain and depends upon the geophysical cirteria used. Each of three forearc models position the arc massif at about 26, 61, or 115 km landward of the trench, with the subduction complex occupying the region between the trench and the massif. The massif-subduction complex interface should be located through drilling to test the proposed models. The intramassif basins, Salaverry and Trujillo, have subsided during Tertiary time to allow the accumulation of 2 to 4 km of marine sediment. The Trujullo Basin apparently has not experienced much vertical movement since the late Miocene, based upon microfossil paleodepth indicators found in dolomicrites and glauconitic micrites dredged from the basin. However, abundant brecciated dolomicrites in the same dredges and disturbed strata in reflection records from both the Yaquina and Trujillo Basins suggest deformation of the basins during the Pliocene-Pleistocene by a compressional regime.

The morphology and shallow structure of the Peru continental margin has been mapped using bathymetric and seismic reflection profiles from lat 6°S to 16°S. Other geophysical and geologic data are used to constrain interpretations of the margin’s deeper structure and to relate the offshore to the onshore Andean geology. Two prominent structural ridges, subparallel to onshore Andean trends, control the distribution of the offshore Cenozoic sedimentary basins. The Coastal Cordillera, which surfaces north of lat 6°S and south of lat 14°S, can be traced onto the offshore as an Outer Shelf High (OSH); it is evidently cored with Precambrian and Paleozoic metasediments and crystalline rocks. A series of shelf basins is situated between the Coast Range/OSH and the Andean Cordillera: from north to south, these are the Sechura, Salaverry, and East Pisco Basins. A second set of upper-slope basins flanks the Coast Range/OSH to the southwest, limited seaward by an Upper-Slope Ridge (USR) of deformed sediment: from north to south, these are the Trujillo, Lima, and West Pisco Basins. The Yaquina Basin lies within divergent arms of the USR. The shelf and upper-slope basins are set on continental massif. An anastomosing network of elongate ridges and ponded sediments is the surficial expression of the subduction complex, which apparently begins just seaward of the USR. The effect of the late Paleocene/Eocene Andean orogeny has been extrapolated offshore as a distinct interface of seismic velocity in the Salaverry Basin. Though Cenozoic marine sedimentation in the shelf basins did not begin until after this event, sedimentation in the upper-slope Trujillo Basin may have been more continuous through the early Tertiary. In the Trujillo Basin, the bulk of the nearly 4 km thick sedimentary section is of Paleogene age, while in the adjoining upper-slope Lima Basin to the southeast, the bulk of the nearly 2 km thick sedimentary section is of late Miocene or younger age. Apparently, post-Oligocene tectonism caused uplift, deformation, and a gross reduction of sedimentation in the Trujillo Basin; this event is evidenced by boundaries of differential structural deformation in seismic reflection profiles. In middle to late Miocene time, while orogenic activity affected the inland Andean Cordillera, the upper-slope Lima Basin subsided and began its depositional record. Unconformites in shelf basins apparently reflect the inland tectonism at this time. The boundary between the Lima and Trujillo Basins, and between the contrasting styles of upperslope tectonic movement, is near lat 9.5°S, coincident with the present day intersection of the Mendaña Fracture Zone with the continental margin. A final phase of upper-slope deformation closed the Pliocene. Like earlier tectonic activity, the major break in structural style of this epoch occurs near lat 9.5° S: compressional faulting and folding characterize the younger sediments of the Trujillo Basin, while the Lima Basin appears as a broad, open syncline, distrubed only in its southernmost occurrence.

ABSTRACT Investigation and design-build construction of the Highway 20 realignment through the Oregon Coast Range provides new insight into paleo-landslides of the Tyee Formation and their slope stability. They are widespread, often extending outside of current drainage basins, and much of their morphology has been almost completely hidden by surficial processes. Radiocarbon tests indicate that some of the slide features are older than the testing limits, while other results range from approximately 18,000 to 40,000 yr B.P. The depth of erosion suggests that the paleo-slides may be as old as Pliocene. Geotechnical models of the paleo-slides, needed to analyze potential construction impacts, are developed from subsurface explorations, construction outcrops, radiocarbon testing, monitoring of geotechnical instruments, and geomorphology revealed by light detection and ranging (LIDAR). The process of predicting landslide boundaries (head scarps, toes, lateral and basal shear zones, etc.) for stability analysis of specific landslides has revealed details of their geologic evolution. This field trip provides background on (a) the investigations that have exposed numerous giant paleo-landslides, (b) findings and interpretations of the age of the landslides and (c) methods that are being employed to mitigate landslide reactivation.

ABSTRACT Regional-scale processes of tectonism, late Quaternary marine transgression, and patterns of aeolian deposition and erosion largely control the geoarchaeological character of the Oregon coast. Dramatic changes to the landscape of the Oregon coast since the Last Glacial Maximum drove the evolution of terrestrial and marine environmental processes which in turn conditioned the location and nature of prehistoric human activities. Due to the geologic complexities of Oregon's coast, archaeological investigations must address a broad range of geological factors that worked to greatly modify the ancient coastal landscape. In many ways, the modern Oregon coastline bears little resemblance to that associated with prehistoric coastal peoples prior to 3000 years ago, requiring geoscientific perspectives to reconstruct the late Quaternary environmental context. Through the integration of geologic concepts and information, geoarchaeology offers an effective means of finding early sites in the modern coastal landscape and in the now-submerged paleocoastal landscape.