Five lithologic units, ranging in age from Middle Miocene to Recent,
are defined on the basis of continuous seismic reflection profile records.
Two of the units are Miocene sedimentary and volcanic rocks that have been
truncated to form a major unconformity (post-orogenic surface) related to
the most recent major tectonism of the region. The remaining units are
post-orogenic unconsolidated sediments.
The fault pattern offshore is generally related to that exhibited on
the island. The pattern conforms to a wrench-fault system hypothesized
by Moody and Hill (1956) modified by a general north-south tensional fracturing.
The San Clemente Fault is assumed to be the primary wrench fault
of the system. Anomalies in the thicknesses and the structure of the
unconsolidated sediment and rock units tend to confirm the structural model.
A canyon (Eel Ridge Canyon) off the west side of San Clemente Island
appears to have been caused by pivotal faulting and erosion, and represents
a boundary between different structural trends north and south.
A prominent terrace around the island is postulated to have been
wave-cut during and since the Late Pleistocene. The island has been
tilted slightly to the west by Recent tectonism.
A steep magnetic gradient off the east side of the island is considered
the consequence of faulted volcanic flows comprising the island
itself and a deep basic rock mass responsible for a large positive magnetic
anomaly off the northwest side. Other magnetic anomalies reflect
major structural trends.
Earthquake epicenter data suggest a recent and possibly cyclical
occurrence of fault activity in the northern Continental Borderland
region and the study area. Fault offsets at the sea floor and earthquake
epicenters along the San Clemente fault zone imply recent adjustments
along the fault.
Wrench-fault movement resulting from a simple shear or shear couple
is considered to have caused the zone of brecciation along the San
Clemente Fault and produced the fault-trace curvature so evident in a
series of en echelon, northwest-striking major faults of the Borderland.
Tensile release during periods of wrench-fault development has been a
fundamental factor in the structural development of the Borderland
basins. The entire structure of the Continental Borderland is believed
to have developed by right-lateral movement along the series of wrench
faults. These faults are believed to have resulted from a translation
of force by sea-floor spreading originating on the East Pacific Rise in
the Gulf of California region. This force is considered to have moved
a northern Continental Borderland crustal plate westward by east to west
release along major wrench faults bordering the north and south ends of
this plate.
Sediments, transported along channels developed along faults in the
island block, were deposited in basins developed by faulting and folding
of the pre-orogenic rocks. Transportation appears to have been by means
of turbidity-current flows, sand flows, and slides. A maximum average
depositional rate of 35 to 47 centimeters per 1,000 years is estimated
for post-orogenic sediments.
The following findings are suggested for inclusion in the Neogene
history of the island: (1) the top 365 meters of Miocene andesitic lavas
were deposited above sea level and tend to become slightly more basic in
composition with depth; (2) subsidence of the island region and temporary
sea-level stand(s) occurred after the deposition of the volcanic rocks,
with possible periods of foundering to about the Late Pliocene; (3)
emergence, lengthy subaerial exposure, and a period of partial submergence
took place from about Late Pliocene through Early Pleistocene;
(4) a north-south compressive force developed or recurred across the
Borderland during Late Pliocene, developing the present northwest-southeast
and east-west wrench-fault systems that have been intermittently
active to the present time; (5) much of the present Borderland topography
formed during the Pleistocene to Recent. / Graduation date: 1969
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29378 |
| Date | 24 November 1968 |
| Creators | Ridlon, James Barr |
| Contributors | Byrne, John V. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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