Erosional denudation of the British Columbia Coast Ranges as determined from fission-track ages of detrital zircon from the Tofino basin, Olympic Peninsula, Washington
In this study, we use up-section changes in fission-track (FT) ages of detrital zircons to infer the tectonic evolution of the source region that fed sediments to the Tofino basin, a Tertiary forearc basin in northwestern Washington State and southwestern British Columbia. We have dated 50 grains from each of eight samples from a 6-km-thick section that ranges in age from 40 to 19 Ma. The detrital zircons have not been reset and thus preserve information about the thermal and denudation history of the source region from which the sediments were derived. Each of our eight samples contains a distinct set of grain-age populations or peaks, indicating that the source region contained several FT source terrains. The number of peaks and their lag time (peak age minus depositional age) contain thermochronologic information about these source terrains. In this study, there is little evidence of active volcanism in the source region; thus the FT ages reflect cooling during denudation of the source regions. The oldest sample in our suite, from the upper Eocene Lyre Formation, shows a dominant peak at ca. 40 Ma, coincident with cooling ages of denuded metamorphic rocks of the Leech River Schist on southern Vancouver Island. This interpretation is supported by paleocurrent directions indicating a source in the vicinity of Vancouver Island, the development of a late Eocene unconformity across much of southern Vancouver Island and western Washington, and the predominance of lithic detritus in this part of the section. Samples from strata above the Lyre Formation are feldspathic, and they presumably reflect erosional denudation of a plutonic source terrain. Peaks in samples from the lower part of this feldspathic interval have widely scattered Jurassic to mid-Cretaceous FT ages; young zircons are notably absent. Up-section, peak ages in samples from the upper part of the feldspathic section form a dominant peak that gets progressively younger with time. While the age of the dominant peak becomes younger in younger samples, the lag time for this peak remains constant at ∼40 m.y. This peak can be recognized in a compilation of zircon FT ages from modern exposures of the Canadian Coast Mountains. We infer that zircons in our samples were derived from progressive erosional denudation of the Coast Plutonic Complex, which had, based on our results, a constant rate of denudation of ∼250 m/m.y. The scatter of ages in the older samples from the feldspathic interval may indicate the unroofing of a high-level sedimentary basin (containing the Cretaceous Nanaimo and Burrard Groups) that used to overlie parts of the Coast Plutonic Complex. Our study, along with others, shows that up-section studies of FT ages of detrital zircons can be effective in resolving the long-term denudation history of continental source regions.
A method for evaluating basement exhumation histories from closure age distributions of detrital minerals
We have developed a two-dimensional, thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement. Using this model, we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata. At relatively high mean denudation rates of 0.5 km m.y.-1 sustained over millions of years, magmatic heating events have minimal influence upon the age distributions of detrital minerals such as K-feldspar that are moderately retentive of radiogenic Ar. At lower rates, however, the effects of batholith emplacement may be substantial. We have applied the approach to detrital K-feldspars from forearc strata derived from the deeply denuded Peninsular Ranges batholith (PRB). Agreement of the denudation history deduced from the detrital K-feldspar data with thermochronologic constraints from exposed PRB basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known. (c) 1999 American Geophysical Union.
The micro-mechanical behaviour of artificially cemented sand particles under compression and shear
The mechanical behaviour of cemented sands at macro-scale has been widely studied in the past, while there is still a lack of laboratory test data for the micro-mechanical response. Therefore, a series of uniaxial compression and tangential shear tests on artificially cemented sand particles have been conducted using a microscope to observe their behaviour. First, the consistency of cemented particles with different cementing agents is discussed. Three breakage modes are proposed according to the images taken using a microscope camera. A new parameter named ‘local roundness at contact’ is introduced to emphasise the effect of contact morphology on sample strength. Moreover, the effect of bond thickness on the mechanical response of cemented particles has been investigated. Finally, the shear strength parameters of artificially cemented sand samples have been determined for two different sample diameters.