Seismic refraction, reflection, and gravity data obtained across the Peru continental margin and Nazca plate at lat 9°S permit a detailed determination of crustal structure. The western portion of the continental shelf basement consists of a faulted outer continental shelf high of Paleozoic or older rocks. It is divided into a deeper western section of velocity 5.0 km/ sec. The combined structure forms a basin of depth 2.5 to 3.0 km which contains Tertiary sediments of velocity 1.6 to 3.0 km/ sec. The 3 km thick, 4.55 to 5.15 km/ sec basement of the eastern shelf shoals shoreward. Together, this basement and the eastern section of the outer continental shelf high form a synclinal basin overlain by Tertiary sediment which have a maximum thickness of 1.8 km and a velocity range of 1.7 to 2.55 km/ sec. The gravity model shows a large block of 3.0 g/cm 3 lower crustal material emplaced within the upper crustal region beneath the eastern portion of the continental shelf.Refraction data indicate a continental slope basement of velocity 5.0 km/ sec overlying a slope core material with an interface velocity of 5.6 km/sec. The sedimentary layers of the slope consist of an uppermost layer of slumped sediment with an assumed velocity of 1.7 to 2 km/ sec that overlie an acoustic basement of 2.25 to 3.6 km/sec. The high velocities (and densities) of the slope basement suggest the presence of oceanic crustal material overlain by indurated oceanic and continental sediments. This slope melange may have formed during the initiation of subduction from imbricate thrusting of upper layers of oceanic crust.A ridgelike structure within the trench advances the seismic arrival times of deeper refractions and supports the suggestion that it is trust-faulted oceanic crust which has been uplifted relative to the trench floor. The model of the descending Nazca plate consists of a 4 km thick upper layer of velocity 5.55 km/sec and a thinner (2.5 km) but faster (7.5 km/sec) lower layer that overlies a Moho of velocity 8.2 km/ sec. The gravity model indicates that the plate has a dip of 5° beneath the continental slope and shelf. West of the trench, the lower crustal layers rise upward, which may represent upward flexure of the oceanic plate due to compressive forces resulting from the subduction process.The upper crustal layers of the 120 km long oceanic plate portion consist of a thin, 1.7 km/sec •Present Address: Downloaded from 424 JONES, P. R" III sedimentary layer overlying a 5.0 to 5.2 km/sec upper layer. Immediately beneath these layers, a 5.6 to 5.7 km/sec lower layer becomes more shallow to the east within 60 km of the trench, and a deeper 6.0 to 6.3 km/sec layer thickens to the east. The lower crustal model consists of a 7.4 to 7.5 km/sec high velocity layer that varies in thickness from 2.5 km to 4.0 km. The 8.2 km/ sec Moho interface varies not more than ±0.5 km from a modeled depth of 10.5 km.
Tripartite arrays of sonobuoys were used to investigate microseismicity at two locations on the central portion of the northwest trending Blanco fracture zone 450 km west of Oregon. The two arrays operated for 14 and 12 hours at locations separated by 50 km. Individual sonobuoys were tracked by radar as they drifted across the fracture zone. Epicenters determined for 32 of the 38 events detected show close association with topographic features of the fracture zone, in contrast to teleseismically determined epicenters, which are offset approximately 30 km from the fracture zone. The events at array 3 do not lie in the central trough but along the southern flank of a linear ridge that is parallel to the trend of the fracture zone. The events at array 4 lie within a topographic depression that offsets the center of the fracture zone and are probably related to steep scarps observed on the bathymetry. Magnitudes assigned to 23 events at array 4 range from −0.5 to +0.5.
The observed mismatch between the topography and the epicenter pattern associated with the Gorda Ridge spreading center was studied with two small arrays of four sonobuoys each deployed over the rift valley at 42ø37'N and 43øN. Observed microearthquake activity originates from the crestal region and supports earlier suggestions of systematic mislocation of teleseismica!!y determined epicenters on the Gorda Ridge. The observed seismic activity, which includes swarm events, averaged 3.5 events per hour over a total array recording time of 19.3 hours. Located microearthquakes originated from the median valley floor, valley walls, and crestal mountains. Other events, whose location could not be computed, appeared to originate from the surrounding crestal mountains with a predominance of events from west of the intersection of the Gorda Ridge and the Blanco fracture zone. Of 69 events detected, 5 were suitable for the calculation of focal depth. Focal depths at the intersection of the Gorda Ridge and Blanco fracture zone are 6.5-10 km below a 3.5-kin datum, while those farther to the south at a linear portion of the ridge range in depth from 2.5 to 6.5 km below datum. This may imply more rapid cooling near the fracture zone. A composite fault plane solution for three events on the eastern valley wall indicates movement on a high angle, with the inner wall moving upward with respect to the crestal mountains. This is the first direct evidence for uplift of median valley walls, a process which must occur if median valleys are steady state features of slowly spreading ridges.
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