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The late Cenozoic geology of Cajon Pass : implications for tectonics and sedimentation along the San Andreas fault

Citation

Weldon, R. J. (1986) The late Cenozoic geology of Cajon Pass : implications for tectonics and sedimentation along the San Andreas fault. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-08302006-135307

Abstract

The geology in Cajon Pass, southern California, provides a detailed record of strike slip activity on the San Andreas fault, compressional deformation associated with the uplift of the central Transverse Ranges and an excellent Cenozoic record of syntectonic sedimentation. Age control was established in all of the sediments deposited since the late Early Miocene, using biostratigraphy, magnetostratigraphy, fission-track dating of volcanic ashes, radiocarbon dating, soil development, and the relative stratigraphic and geomorphic position of the units. Tectonic deformation and sedimentation styles varied through time, reflecting the evolution of the San Andreas fault zone within the Pacific - North American plate boundary. Particular attention was paid to determining rates of tectonic deformation and establishing the timing of changes in deformational and depositional styles in the area. Progressive offset of radiocarbon-dated alluvial and paludal sediments have been used to determine the Holocene slip rate on the San Andreas fault in Cajon Pass. Four independent measurements of the slip rate yield an average of 24.5 ± 3.5 mm/yr. The similarity of the four values, which span different intervals of time up to 14,400 years ago, suggest that the slip rate has been constant during this period. An excavation across the San Andreas fault provided some constraints on the timing of paleoearthquakes. Coupled with the historic record, this investigation indicates that the last earthquake associated with rupture on the fault in Cajon Pass occurred around 1700 AD. At least 2 earthquakes caused rupture on the San Andreas fault after 1290 AD and perhaps 6 earthquakes are recorded in the thousand year period before European settlement of southern California in the 1770s. Downcutting and erosion into the western San Bernardino Mountains, during the last 700,000 years, has created Cajon Pass as it exists today. The downcutting was punctuated by at least four pulses of channel aggradation that provide stratigraphic markers throughout the area. They are dated at 0.5 ± 0.1 million, 55,000 ± 10,000, 17,000 to 6,000, and 2000 to 300 years ago. These aggradational periods were caused by order of magnitude increases in sediment production associated with changes in the climate from relatively wet to dry conditions. The locus of the latest Pleistocene to early Holocene fill migrated upstream through time, with aggradation lasting only a few thousand years at any point in the drainage. Incision of the fill also migrated upstream, beginning long before the fill pulse reached the headwaters of the system. The fill terrace, or upper surface of the fill deposit, does not represent a time line or a surface down which water flowed everywhere at once. Thus, the use of a fill terrace as either a time or spacial reference line for tectonic studies, without accounting for the its transgressive character, can result in erroneous conclusions. During the early to middle Pleistocene, prior to the erosion of Cajon Pass, the southern part of the area was uplifted and coarse fan deposits were shed across the northern part of the area onto the Mojave Desert. Some of these sediments were derived from distinctive sources in the San Gabriel Mountains southwest of the San Andreas fault zone. Matching these distinctive facies in the deposits with their sources established offsets across the fault zone and made it possible to tie the uplift northeast of the fault to activity on the San Jacinto fault as it passed by across the San Andreas fault. The fan deposits are dated by a combination of bio-stratigraphy and magnetostratigraphy. The average slip rate across the combined San Andreas and San Jacinto faults is 37.5 ± 2 mm/yr during the Quaternary Period. The six determinations of the slip rate show no evidence for rate changes during the Quaternary Period. The slip rate on the San Andreas fault alone was determined by one offset of be 21 ± 7 mm/yr. The record of contemporaneous activity on the San Jacinto fault to the southeast requires that the San Andreas fault's rate be close to the upper limit of this range. Contemporaneous activity on the San Andreas and San Jacinto faults is uplifting the high, eastern San Gabriel Mountains and deforming the San Andreas fault plane. The geometry of this deformation is such that uplift of the country on the northeast side of the San Andreas fault occurs. This hypothesis is supported by the northwest migration of the uplift at the slip rate on the San Andreas fault, and the style of surface deformation that is characteristic of folding over a steeply dipping lateral ramp at depth. A kinematic model was constructed to determine the role of the San Andreas fault in the Pacific - North American plate boundary. The Quaternary slip rates determined for the San Andreas fault in Cajon Pass and the slip vectors associated with the geometry of the fault zone were combined with an assumption of rigid block motion away from the faults and published slip rates for the other major faults in southern California. The model produces internally consistent motions for all of the blocks. Vector sums of the slip rate across the Pacific - North American boundary yield only the relative plate motion if the path includes the western Transverse Ranges. The model solution indicates that the western Transverse Ranges are not part of the San Andreas system but are a left-step in a separate coastal system that currently accommodates about 1/3 of the Pacific - North American plate motion. The southeastern San Bernardino Mountains are being uplifted because of a left step in the arcuate trace of the San Andreas fault. The western San Bernardino Mountains and the eastern San Gabriel Mountains are being uplifted by the deformation associated with the junction of the San Andreas and San Jacinto faults. Because the convergence in this area can be explained by local geometry, it is clear that southern California cannot be part of the Pacific plate, colliding at the plate rate into North America across the Transverse Ranges. Instead, southern California appears to be a sliver between the San Andreas system and the coastal system, and is rotating counterclockwise as it translates northwest, transferring the convergence to the coastal system. The middle to late Quaternary uplift of the Cajon Pass area was the culmination of the uplift of the San Bernardino Mountains that began in the Miocene. Three distinct phases of uplift have been recognized, suggesting a long-term interaction between the strike-slip activity on the San Andreas system and the compressional tectonics of the Transverse Ranges. The San Bernardino Mountains began to take shape following a pervasive earliest Miocene unconformity. Broad, homogeneous basins, separated by mature uplands of moderate to low relief developed across the southwest-draining regional paleoslope. The earliest activity on the San Andreas fault is believed to be associated with this early extensional phase. Late Miocene to early Pliocene, south-directed thrusting uplifted the "proto" San Bernardino Mountains, creating steep, south-facing relief along the San Andreas. During this time the San Gabriel fault was the most (and perhaps only) active trace of the San Andreas system. Thrusting stopped as the San Andreas fault became active again, probably coincident with the beginning of the opening of the Gulf of California, 5 million years ago. Pliocene and earliest Pleistocene sedimentation took place in narrow east-west trending, structurally controlled basins created by the Mio-Pliocene thrusting. Early to middle Pleistocene, north-directed thrusting across a shallow, south-dipping ramp uplifted the broad central plateau of the San Bernardino Mountains, and created the North Frontal fault system. During the middle and late Quaternary, this activity was largely replaced by south-directed thrusting and lateral ramping on steep, north-dipping planes along the San Andreas fault. This activity produced the tremendous relief and regionally-extensive north-dipping structural blocks in the San Gorgonio and Cajon Pass areas, and continues today. The structures and geomorphology of the range reflects its varied history; different parts of the range are as old as late Early Miocene and as young as the Holocene. All three phases of uplift appear to be related to the southern Big Bend in the San Andreas fault system, which has existed since the Miocene. Contemporaneous and alternating periods of thrusting and strike-slip activity has created bedrock "flaps", displaced fault slivers and strand switching that are responsible for the complex geology associated with San Andreas fault through the Transverse Ranges. Recognition of these features with detailed field work will greatly expand our knowledge of the tectonics and seismic hazards associated with the San Andreas system in southern California.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Albee, Arden Leroy
Thesis Committee:
  • Sieh, Kerry E. (chair)
  • Albee, Arden Leroy
  • Allen, Clarence R.
  • Kirschvink, Joseph L.
  • Silver, Leon T.
Defense Date:14 June 1985
Record Number:CaltechETD:etd-08302006-135307
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-08302006-135307
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3283
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:08 Sep 2006
Last Modified:01 Feb 2013 00:21

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