Seismic refraction, reflection and gravity data obtained
across the Peru continental margin and Nazca Plate
at 9° S. permit a detailed determination of crustal structure.
Complex structures normal to the profile require the
development of a ray trace technique to analyze first and
later arrivals for eleven overlapping refraction lines.
Other data integrated into the seismic model include velocities and depths from well data, near surf ac sediment structures
from reflection profiles and velocities obtained from
nearby common depth point reflection lines. Crustal and
subcrustal densities and structures were further constrained
by gravity modeling to produce a detailed physical model of
a convergent margin.
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 and a shallower,
denser eastern section of velocity 5.65 to 5.9 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. In this area, near-surface sedimentary structure
suggests truncated sinusoidal features caused by exposure
to onshore-offshore bottom currents.
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 sediments 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/cm³ lower crustal material emplaced within the
upper crustal region beneath the eastern portion of the continental
shelf.
Refraction data indicates a continental slope basement
of velocity 5.0 km/sec overlying a slope core material with
n 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 which overlies 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 over
lain 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. Once created, the melange forms a trap and
forces the subduction of most of the sediments that enter
the trench.
A ridge-like structure within the trench advances
the seismic arrival times of deeper refractions and supports
the suggestion that it is thrust-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 which 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 shallow, 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 sedimentary layer
overlying a 5.0 to 5.2 km/sec upper layer. An underlying
5.6 to 5.7 km/sec lower layer becomes more shallow to the
east within 60 km of the trench while 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 which
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. / Graduation date: 1979 / Best scan available for figures.
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29371 |
| Date | 09 August 1978 |
| Creators | Jones, Paul Roy III |
| Contributors | Johnson, Stephen H. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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