Peter L. K. Knuepfer

Estimating ages of late Quaternary stream terraces from analysis of weathering rinds and soils

Rates of growth of rock weathering rinds and rates of changes of soil morphologic and chemical properties that are calibrated at sites of known ages produce powerful tools for estimating ages of geomorphic surfaces, particularly when materials datable by conventional methods are not widespread. Sequences of latest Quaternary stream terraces from the South Island of New Zealand are ideally suited for application of these calibrated-age techniques because climate and source materials for terrace deposits are internally consistent within two regions, West-land and Marlborough. Modal and mean thicknesses of weathering rinds formed in surface cobbles of Torlesse graywacke provide age estimates for latest Pleistocene and Holocene stream terraces in Marlborough with uncertainties estimated at ±5% to ±40%. Modal rind thickness of subsurface cobbles provides age estimates for latest Quaternary Westland sites with uncertainties of ±10% to more than ±50%. Few dated soils are available to constrain rates of soil development in Westland and Marlborough. Furthermore, soils were sampled from only one pedon on each surface. Morphologic data, however, provide least-squares age estimates for Westland sites with uncertainties typically greater than ±100%, and the calibration data can further bracket likely soil ages to less than 50% uncertainty. The regression data from Marlborough are less reliable, but profile-index ratios help bracket ages to at least ±50%. Distributions of oxalate-extractable Fe and AI and total major-element concentrations allow age estimates for Westland and Marlborough soils with uncertainties estimated at ±15% to ±50%. Despite these uncertainties, the three techniques combine to provide age estimates with the high resolution needed to examine details of fault-slip rates or stream downcutting. The calibration data and weathering rates used herein are applicable only to the New Zealand region studied, although similar calibrated-age techniques should provide high-resolution dating in many other regions. Such techniques are particularly useful when multiple approaches are available to provide independent cross-checks.

A methodology for probabilistic fault displacement hazard analysis (PFDHA)

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Middle-late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan-implications of river response to climate change and active tectonic uplift

Abstract We reconstruct the Holocene river history of the Erhjen River (area: 140 km 2 ) by correlating river terraces aided by 28 radiocarbon dates. Multiple terraces developed in the lower Erhjen River since the middle Holocene; they converge downstream to the Coastal Plain. The rates of channel incision into bedrock calculated from these terraces at Yuehshihchieh are 7–8 mm/year during ca. 5.7–2.5 ka, 5 cm/year during 1.5–1.3 ka, and 1 cm/year since 1 ka; the last is close to the average incision rate since middle Holocene. Meanwhile, only a single but wide middle–late Holocene paleo-floodplain was developed in the upper Erhjen River; it was completely abandoned only after 0.8 ka, likely following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these dated terraces, we identify a doming structure or anticline within the basin, which results in a tilt rate of 10 −6 to 10 −7 per year in the lower Erhjen River valley. The major terraces here had different initial long profiles, which implies that a critical (graded) long profile may not be a prerequisite for formation of a wide erosional terrace surface. Instead, we propose that these terraces were initiated by a series of catastrophic rainfall events, probably climatic-related, which brought a large amount of bedload from hillslopes to prevent the channel from incising when valley widening was facilitated by high-discharge runoff. We find that not only terrace-surface formation but also channel incision can be strongly controlled by climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. Apparently, the climate and its effects on the landscape cannot be regarded as constant during the Holocene even in a humid tropical area like Taiwan. However, such a fluctuation of climate could only be recorded in a setting where rivers have a high tendency to incise so that multiple terraces can be created. The lower Erhjen River that is characterized by active tectonic tilting is an example of this setting.

Late Quaternary fault segmentation from analysis of scarp morphology

A thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments. Paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events. Systematic mapping of fault scarps and degradation-equation modeling of relative scarp ages on the Lemhi fault, a range-bounding normal fault in east-central Idaho, provide a method to assess rupture extent and boundaries between rupture segments. The youngest latest Quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault. The boundaries between rupture segments differ in characteristics, although most are at intersections of the currently active range-bounding fault and mid-Tertiary normal faults. Several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or (probably) in the previous cycle. The Lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone.