Nathan D. Brown

Active thrust sheet deformation over multiple rupture cycles: A quantitative basis for relating terrace folds to fault slip rates

Many recent thrust fault earthquakes have involved coseismic surface faulting and folding, revealing the multifaceted nature of active thrust sheet deformation. We integrate records of surface deformation, subsurface structure and geochronology to investigate active surface deformation over multiple rupture cycles across the Southern Junggar Thrust (SJT) in the southern Junggar basin, NW China. Fluvial terrace geometries – extracted from a 1-m digital elevation model – reveal records of surface faulting across a prominent fault scarp. In addition, terraces exhibit progressive folding across fold scarps. Fault and fold scarps are spatially coincident with a surface-emergent SJT splay and subsurface fault bends along the SJT, respectively, constrained by seismic reflection data. We quantify the magnitude of fault slip at depth implied by fold scarps along Holocene-aged terraces. Our method yields results consistent with independent estimates of slip implied by fault scarp relief for the same terraces. Four late Quaternary terrace records are less continuous, preserved only as fold scarps that suggest folding kinematics involving a component of limb rotation. We develop a new method for quantifying fault slip at depth from terrace folds using a mechanical forward modeling approach. Our analysis yields quantitative relations between fold dip and fault slip, allowing us to quantify SJT fault slip from terrace folds from ~250 ka- present. SJT fault slip rate has decelerated from ~7.0 mm/yr in the Late Quaternary to ~1.3 mm/yr throughout the Holocene. These results provide new insight into the kinematics of fault-bend folding for natural structures and define new methods to accurately estimate fault slip and slip rates from terrace folds in active thrust sheets.

The age and origin of small offsets at van matre ranch along the san andreas fault in the Carrizo Plain, California

To better understand the relationship between geomorphology and fault slip, we investigated the origins of topographic depressions previously interpreted as beheaded channels representing small offsets at Van Matre Ranch (VMR) along the San Andreas fault, Carrizo Plain, California. We excavated four fault-parallel trenches (T1–T4) across depressions and sampled for single-grain postinfrared infraredstimulated luminescence (p-IR IRSL) age estimates of channel fill. Only T2 sediments are young enough (0:38 0:06 ka) to be associated with a nearby drainage (sourced ∼12 m southeast [SE]), providing a short-term slip rate of 31:6 9= − 6:6 mm=yr. The age of the T2 channel fill falls within the uncertainty ranges of the penultimate through fourth event back as dated at Bidart Fan ∼12 km northwest (NW). Handexcavated exposures at nearby T1 indicate that the T2 channel sediments have experienced at least two earthquake events and that the T1 beheaded gully is a fosse between two small offset alluvial fans (∼10 m radius). Reconstructing the alluvial fan apex shows that offset at this location in the 1857 Mw 7.8 Fort Tejon earthquake was ∼4 m. Therefore, offset in the penultimate earthquake is < ∼8 m at the VMR site because we cannot discount that T2 channel sediments experienced four earthquakes. Interestingly, buried channel ages are older at other trenches (4.26–8.12 ka), indicating distant, larger drainage basin sources SE of the study area. Our results indicate that for the Carrizo Plain, (a) there may be appreciable high-frequency variation in paleoearthquake offset along strike and in successive earthquakes at a point; (b) beheaded topographic depressions on the downstream side of the fault have the potential to, but do not necessarily, capture drainage basins on the upstream side of the fault with continued slip; and (c) small catchments may not produce channel landforms or deposits as frequently as has been suggested.