Fault-related folds develop above active faults, and as these faults propagate laterally so do the folds they produce. Geomorphic criteria useful in evaluating rates and direction of lateral propagation of active folds in the direction of propagation are: (1) decrease in drainage density and degree of dissection; (2) decrease in elevation of wind gaps; (3) decrease in relief of the topographic profile along the crest; (4) development of characteristic drainage patterns; (5) deformation of progressively younger deposits or landforms; and (6) decrease in rotation and inclination of forelimb. All these criteria are consistent with lateral propagation, but do not prove it. The presence of more than one wind or water gap formed by the same stream, however, is strong evidence of lateral propagation. Rates of lateral propagation of folding may be several times the rate of uplift and fault slip. Lateral propagation of anticlinal folds allows for a new explanation of how drainage may develop across active fold belts. Development of drainage across an active fold belt is probably a function of relatively long structurally controlled drainage diversion parallel to fold axes and development of relatively short antecedent stream reaches, around the nose (plunge panel) of a fold. Water and/or wind gaps form as uplift, drainage diversion, and stream capture associated with fold growth continue.

We present surface evidence and displacement rates for a young, active, low-angle (~20°) reverse thrust fault in close proximity to major population centers in southern California (U.S.A.), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 1,000-10,000 year timescales with maximum slip rates for the central SSCF of 1.9 +1.0/-0.5 mm/yr between ~19-7 ka and minimum slip rates of 1.3 +0.5/-0.3 mm/yr since ~7 ka. Uplift rates for the central SSCF have not varied significantly over the last ~58 ka, with a maximum value of 1.7 +0.9/-0.6 mm/yr for the interval ~58-19 ka, and a minimum value 1.2 +/-0.3 mm/yr since ~7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some point after ~58 ka. The geometry for the SSCF presented here, with a ~20° north-dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45-60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multifault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as the southern California, Highlights • Young faults often undetected but potentially key for seismic hazard assessments. • First geomorphic evidence for the Southern San Cayetano fault (SSCF). • 10 Be dating on offset terraces records Holocene slip rate of 1. 3 +0.5 /-0.3 mm yr-1. • SSCF has major implications for seismic hazard in southern California.

Trenches excavated across the Superstition Mountain fault in the Imperial Valley, California, have exposed evidence for four prehistorical earthquakes preserved in displaced lacustrine stratigraphy associated with ancient Lake Cahuilla. The presence of shoreline peat accumulations along with abundant detrital charcoal allows for high-precision age determination of some stratigraphic units, thereby providing constraints on the timing of three of the palcoearthquakes. These three events occurred within a 480-to 820-year interval during the past 1200 years. The most recent earthquake (event 1) occurred during a fluvial phase of deposition between A.D. 1440-1637, immediately prior to the inundation of the Cahuilla basin at about A.D. 1480 and 1660. A channel margin was offset 2.2 +0.4/-0.15 m in this rupture, suggesting an earthquake with a magnitude & 7. The penultimate event (event 2) also occurred during fluvial deposition after A.D. I280 but before another lakestand at A.D. 1440-1640. Lateral slip could not be resolved for event However, based on juxtaposition of dissimilar units and the amount of deformation producedby this event, it is presumed that this was also a large earthquake. The timing of event 3 is constrained to have occurred between about A.D. 820 and 1280. This event is represented by several fractures and small displacements that rupture up to a distinct stratigraphic level or event horizon. Slip was not resolved for this event. Finally, the timing of event 4 is very poorly constrained to between A.D. 964 and 4670 B.C. Undoubtedly, many events may have occurred during this period. Notably, the past three earthquakes occurred within a period of less than 820 years, and it has been over 350 years since the last earthquake.

Uplifted marine terraces are common landforms in coastal regions where active tectonics are an important component of landscape evolution, such as along the coastal stretches of southern California. The pattern and elevation of shoreline angles on active folds provide information about rates of uplift and fold growth, which is important for defi ning tectonic models. A particularly impressive succession of marine terraces are developed across the Santa Barbara fold belt (SBFB) in southern California, which comprises an east-west linear zone of active folds and (mostly) blind faults on the coastal piedmont and in the Santa Barbara Channel. The fold belt is characterized by several fl ights of emergent late Pleistocene marine terraces uplifted and preserved on the fl anks of active anticlines. At several locations along the fold belt, the fi rst emergent marine terrace is numerically dated by methods that include uranium-series dating on terrace corals , 14 C dating on terrace shells and detrital charcoal, optically stimulated luminescence of marine terrace sands, and oxygen isotopic signatures (δ 18 O) of mollusks. Individual marine terraces have as many as four ages, using up to three different dating methods, providing confi dence in terrace chronology. Ages of higher terraces are estimated assuming a constant rate of uplift for a particular fl ight. Of the 31 terraces, 22 formed during a time of falling sea level, with 9 forming at or near marine oxygen isotope stage (MIS) 3 or 5 highstands.Ages and rates of uplift of the fi rst emergent terrace vary systematically from west (younger and higher) to east (older and lower). The fi rst emergent marine terraces in the westernmost SBFB are approximately 45 ka (MIS 3), and the rate of local surface uplift is ~2 m/k.y. In the central part of the belt, fi rst emergent terraces date to 60-70 ka (MIS 5), and uplift rates decrease to ~1.2 m/k.y. First emergent marine terraces preserved in the easternmost fold belt range from 70 ka to 105 ka (MIS 5), with rates of local surface uplift of ~0.5 m/k.y. Lower rates of uplift in the eastern end of the fold belt result from the MIS 5 terrace being tilted down into the Carpinteria syncline. Rates of vertical uplift in the western end of the fold belt are about six times higher than previously reported, suggesting the seismic hazard is also greater.

Resolving the chronology of marine terrace sequences is critical for determining uplift rates along tectonically active coastlines. Unfortunately, lack of suitable dating materials often makes this difficult. We present here oxygen isotopic data from 21 shells of Olivella biplicata from four marine terraces in the Santa Barbara and Ventura area located in southern California, USA. Terraces U-series dated at 47+0.5kaBP at Isla Vista and 70 4-2 ka Be at Santa Barbara City College (SBCC) provide age control for the isotopic data. Shells from the Isla Vista and SBCC terraces yield average values of 1.117%o and 0.627%o, respectively, and shells from a non-U-series dated terrace at Punta Gorda and an undated terrace at Santa Barbara Point yield average values of 1.010%o and 0.751%o, respectively. The data indicate that stable oxygen isotopic signatures preserved in marine terrace molluscs provide a useful tool for correlating undated terraces with those of known age. Furthermore, we are able to correlate samples collected from offset fragments of the Punta Gorda terrace on either side of the Red Mountain fault, demonstrating the utility of this method for correlating terraces across structural features. Using oxygen isotopic data coupled with the U-series dated wave-cut platform at SBCC we calculate a rate of uplift ranging from 0.62 + 0.03 mm/year (where the elevation of the first emergent terrace is 41 m) to 0.54 + 0.05 mm/year (where the elevation of the first emergent terrace is 36 m) for marine terrace flights preserved on the Mesa hills anticline located in the city of Santa Barbara, California.Coastal areas which have experienced uplift during the Late Pleistocene to present are typically characterized by a series of uplifted marine terraces resembling a flight of stairs. Each terrace forms at sea level as a wave-cut platform overlain by a thin veneer of marine sediments which are capped by more extensive terrigenous sediments, and, often, vegetation. The interface between the wave-cut platform and its sea-cliff is called the shoreline angle, and, as it usually lies within one or two vertical metres of the mean sea level (Lajoie et al.