Bursik, Marcus (1989) Late Quaternary volcano-tectonic evolution of the Mono Basin, eastern California. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-03282006-103736
The Mono Basin of eastern California provides an ideal laboratory in which to study the interaction of volcanic and tectonic processes. The late Quaternary geological record of volcanic activity and range-front faulting is relatively complete in the basin. Range-front faults of the Sierra Nevada offset dateable late Pleistocene glacial moraines, thus affording the opportunity to estimate range-front slip rates. The first two chapters concern dating of moraines that are offset by range-front faults.
In Chapter One, I discuss the ages of the glacial moraines of the Mono Basin and their correlation between canyons. I dated the moraines by studying their morphology and the relative weathering of granitic boulders atop their crests, and by use of the clast-sound velocity (CSV) dating technique. The CSV technique consists of measuring the p-wave speed (Vp) in morainal boulders. Vp decreases with age as boulders weather. Clast-sound velocities enabled statistical division of moraines in each canyon into differently weathered deposits. Relative weathering features of boulder surfaces further helped discern age differences between moraines in a single canyon. Finally, CSV, relative weathering and moraine morphology, considered together, allowed correlation of moraines to an established glacial sequence, and therefore, correlation between canyons. Regression of mean Vp against best estimates of glaciation ages within the glacial sequence provided a further check on the validity of the correlations.
Moraines in all major canyons from Lee Vining south were correlative with the standard late Pleistocene sequence of Tioga, Tenaya, Tahoe and Mono Basin deposits. At Lundy Canyon, however, Tahoe and Tenaya moraines are poorly, if at all, preserved. The prominent moraines extending into the basin are probably of Tioga age. Poor preservation of Tenaya and Tahoe deposits may be due to the narrow, steep-sided morphology of Lundy Canyon, and rapid down-dropping on the range-front fault.
In Chapter Two, I discuss the application of a new quantitative dating technique to the moraines of Lee Vining Canyon. At Lee Vining Canyon, I measured cross-sectional profiles of lateral moraines of different ages to determine whether the degree to which they have been degraded could be used as a relative-dating method. Correlation of the degree of moraine degradation against an independent measure of age suggested that relative ages of late Pleistocene lateral moraines can be inferred from moraine profiles.
Analysis of the degradation of moraine profiles with a diffusion model resulted in equations that relate profile width and maximum slope angle to age. In accordance with the diffusion model, the functional relationship between profile width and estimated age was found to be nearly linear for the moraines of Lee Vining Canyon. Fits of model to data were good, despite evidence of transport of material by non-linear diffusive processes along some of the profiles.
Maximum slope angle is inversely proportional to age according to the diffusion model. Regression of mean maximum slope angle against inverse age for the group of moraines from Lee Vining Canyon suggested that the relationship between the two variables is expressed by the diffusion model.
Deviations of model profile shapes from true shapes suggested that in addition to moraine age, initial profile shape and non-diffusive degradation processes are important in controlling the relationship between slope parameters and age over spans of 10(4) years.
In Chapter Three, I use moraine ages determined in Chapter One to estimate slip rates of range-front faults. For Chapter Three, I measured fault-scarp profiles on the dated lateral moraines of the Mono Basin to determine fault slip rates. I compared these data with what can be deduced about the extension rate due to dike intrusion underneath the Mono Craters. I then considered extension rates in the context of regional strain patterns to infer the mode of deformation and strain relief in the Mono Basin during late Quaternary time.
The extension-rate data indicate that dikes are being intruded underneath the Mono Craters in response to crustal stretching, and because of this, are now accommodating elastic strain that was once accommodated by range-front normal faulting. The section of the range front near the craters accommodated as much as 1 mm/yr of extension until 40,000 to 70,000 years ago. For the past 40,000 to 70,000 years, this section of range front has become inactive, even though extension along the range front to north and south has continued at up to 0.9 mm/yr. Dikes have been intruding underneath the Mono Craters for the past 40,000 years. Depending upon the assumptions used to calculate dike intrusion rates, the dikes accommodate 1 mm/yr of tectonic extension that was previously accommodated by range-front faulting.
Consideration of the extension rates in the context of regional tectonic strain patterns suggests that the Mono Craters are forming along one of the extensional boundary structures of a pull-apart basin, the other extensional boundary of which is the deactivated range-front segment.
If the Mono Craters represent an early stage of caldera formation, then their formation within a pull-apart zone may indicate that this is an ideal tectonic environment in which to form certain types of calderas.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Major Option:||Geological and Planetary Sciences|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||9 June 1988|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||31 Mar 2006|
|Last Modified:||26 Dec 2012 02:36|
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