Katherine M. Scharer

Quantification of growth and lateral propagation of the Kashi anticline, southwest Chinese Tian Shan

Received 11 February 2006; revised 16 January 2007; accepted 2 February 2007; published 28 March 2007. [1] The Kashi anticline is a north vergent, asymmetric, doubly plunging detachment fold located in the SW Tian Shan foreland. We combine structural, magnetostratigraphic, and topographic data to define the fold’s lateral propagation, surface uplift, and concomitant exhumation. Two new magnetostratigraphic sections indicate that the fold began growing at � 1.4 Ma and by 1.07 Ma, deformation had propagated eastward � 13 km at an average rate of � 40 km/Myr. Subsequently, propagation rates increased at least twofold, until the fold reached >60 km in length by 0.8 ± 0.3 Ma. Since then, eastward fold propagation slowed to � 15 km/Myr, and the eastern 15–25% of the fold remains buried in the rapidly aggrading foreland. The structure and topography of the emergent fold support interpretations of fold growth in three stages: initial symmetric lateral growth both east and west to a total length of � 30 km followed by, first, rapid and, then, slower eastward lengthening to 72 ± 10 km total length. Shortening rates as high as 1.9� 0.2 +0.3 mm/yr characterize the western part of the fold but decrease toward the east. Significant dissection of the emergent fold does not occur until topographic relief is sufficient (� 200 m) to permit stripping of protective conglomerates from across the fold’s upper surface. As differential rock uplift continues following breaching of the conglomerate, � 75% of the rock raised above local base level is subsequently eroded at rates as high as 2.4 km/Myr. Despite extensive erosion, the modern fold topography mimics spatial patterns of both long-term shortening and variations in rock uplift.

Kinematic models of fluvial terraces over active detachment folds: Constraints on the growth mechanism of the Kashi-Atushi fold system, Chinese Tian Shan

During detachment folding, the relationship between differential uplift and shortening depends on the mechanism of fold growth, such as limb rotation or hinge migration, and may vary over the lifetime of a fold. Thus, neither long-term shortening rates nor the present fold geometry unambiguously constrain the kinematics of fold growth. Where rivers cut through growing anticlines, flights of abandoned fluvial terraces act as passive kinematic markers. As shortening progresses, the terraces become deformed and thereby preserve critical information about the kinematics and evolution of active fold growth. To constrain recent fold growth across three detachment folds in the Kashi-Atushi fold system in the SW Tian Shan, China, we surveyed flights of deformed terraces and compared them with geometric models of successively emplaced horizontal unconformities (terraces) across pregrowth strata deformed by hinge migration, limb rotation, and a combination of the two. Migration of angular hinges and curved hinge zones were also compared. Each kinematic model predicts both a distinct geometry for the deformed terraces and contrasting angular relationships between the terraces and the pregrowth strata. Notably, limb rotation and migration of curved hinge zones result in progressively rotated terraces that cut across pregrowth strata, whereas all limb-lengthening models result in parallelism between pregrowth strata and terrace straths across much of the fold. The Kashi-Atushi terraces show clear evidence of abandoned axial surfaces, concentrated deformation near the core of the folds, and progressive tilting with age. When compared to the model predictions, the folds are likely growing by a combination of limb rotation in the tight cores of the folds and hinge-zone migration of pregrowth strata across the flanks of the folds.

Quasi-periodic recurrence of large earthquakes on the southern San Andreas fault

It has been 153 yr since the last large earthquake on the southern San Andreas fault (California, United States), but the average interseismic interval is only ∼100 yr. If the recurrence of large earthquakes is periodic, rather than random or clustered, the length of this period is notable and would generally increase the risk estimated in probabilistic seismic hazard analyses. Unfortunately, robust characterization of a distribution describing earthquake recurrence on a single fault is limited by the brevity of most earthquake records. Here we use statistical tests on a 3000 yr combined record of 29 ground-rupturing earthquakes from Wrightwood, California. We show that earthquake recurrence there is more regular than expected from a Poisson distribution and is not clustered, leading us to conclude that recurrence is quasi-periodic. The observation of unimodal time dependence is persistent across an observationally based sensitivity analysis that critically examines alternative interpretations of the geologic record. The results support formal forecast efforts that use renewal models to estimate probabilities of future earthquakes on the southern San Andreas fault. Only four intervals (15%) from the record are longer than the present open interval, highlighting the current hazard posed by this fault.

Paleoearthquakes on the Southern San Andreas Fault, Wrightwood, California, 3000 to 1500 b.c.: A New Method for Evaluating Paleoseismic Evidence and Earthquake Horizons

We present evidence of 11–14 earthquakes that occurred between 3000 and 1500 b.c. on the San Andreas fault at the Wrightwood paleoseismic site. Earthquake evidence is presented in a novel form in which we rank (high, moderate, poor, or low) the quality of all evidence of ground deformation, which are called “event indicators.” Event indicator quality reflects our confidence that the morphologic and sedimentologic evidence can be attributable to a ground-deforming earthquake and that the earthquake horizon is accurately identified by the morphology of the feature. In four vertical meters of section exposed in ten trenches, we document 316 event indicators attributable to 32 separate stratigraphic horizons. Each stratigraphic horizon is evaluated based on the sum of rank (Rs), maximum rank (Rm), average rank (Ra), number of observations (Obs), and sum of higher-quality event indicators (Rs >1 ). Of the 32 stratigraphic horizons, 14 contain 83% of the event indicators and are qualified based on the number and quality of event indicators; the remaining 18 do not have satisfactory evidence for further consideration. Eleven of the 14 stratigraphic horizons have sufficient number and quality of event indicators to be qualified as “probable” to “very likely” earthquakes; the remaining three stratigraphic horizons are associated with somewhat ambiguous features and are qualified as “possible” earthquakes. Although no single measurement defines an obvious threshold for designation as an earthquake horizon, Rs, Rm, and Rs >1 correlate best with the interpreted earthquake quality. Earthquake age distributions are determined from radiocarbon ages of peat samples using a Bayesian approach to layer dating. The average recurrence interval for the 10 consecutive and highest-quality earthquakes is 111 (93–131) years and individual intervals are ±50% of the average. With comparison with the previously published 14–15 earthquake record between a.d. 500 and present, we find no evidence to suggest significant variations in the average recurrence rate at Wrightwood during the past 5000 years.

Changing Views of the San Andreas Fault

A combination of high-resolution laser imaging with improved radiocarbon dating techniques is providing new ways to view earthquake behavior. The magnitude 7.0 earthquake that struck Haiti on 12 January 2010 is a reminder of the devastation caused by large earthquakes. Because recurrence of large (M 7–8) earthquakes is rare, on the order of centuries, studying the past behavior of a fault guides future expectations. Paleoseismologists examine the stratigraphic and geomorphic history of deposits and landforms along a fault for evidence of past ruptures. Such observations provide information on when earthquakes happened, what parts of the fault failed, and the size of the earthquakes. The collected geologic data form the backbone of probabilistic seismic hazard analyses (1) used by the insurance and engineering industries and are increasingly used to explore models of lithosphere rheology and fault interaction (2, 3). Because of sparse data, however, inferences about patterns of strain accumulation and release are a common occurrence. On pages 1119 and 1117 of this issue, Zielke et al. (4) and Grant Ludwig et al. (5) present data and interpretations providing an exciting new view that questions fault behavior models that have been applied to the south central San Andreas Fault for decades, highlighting the value of revisiting old problems with new techniques.