Blind Fault Research Articles

On the Mechanism of Stress Superposition Inducing Outburst Under the Influence of Blind Fault Instability

On the Mechanism of Stress Superposition Inducing Outburst Under the Influence of Blind Fault Instability

Utilizing a kinematic and mechanical modelling workflow to constrain fracture network characteristics: Application to the Teapot Dome, Wyoming, USA

Predicting natural fracture networks is critical for unconventional hydrocarbon exploration, carbon capture/storage, and geothermal energy projects. Developing an accurate picture of key fracture network attributes such as intensity and orientation can be challenging, with difficulties arising from the limited and incomplete information typically offered to construct quantitative models.In a new process-based workflow we combine geometrical/kinematic and physics-based mechanical forward models. Realistic geometries guide the evolutionary forward model based on kinematic algorithms derived from naturally deformed rocks, while the mechanisms and conditions dictating fracture attributes are modelled directly as a result of key geomechanical properties and underlying physical laws. Collectively this workflow aims to improve understanding of subsurface fracture distributions by representing the inferred strain history and relating this to fracture attributes; this may go some way to addressing any uncertainty regarding the deformation of key stratigraphic intervals and permit construction of more representative fracture network models.The techniques are applied to a real-world data set from the fractured hydrocarbon reservoirs of the Teapot Dome anticline in Wyoming, USA, and specifically a 2D seismic section through the culmination of the structure. The doubly-plunging, basement-cored anticline developed above a blind fault and is modelled as a fault-propagation/trishear evolutionary forward model while tracking the state of strain. The results illustrate the potential of predicting fracture formation throughout the evolution of a structure with high granularity and detail, and in response to the different rock types and their structural position.

Structural geometry of the Urumqi foreland thrust system, northern Tian Shan: Insights into the seismotectonics of the 2013 M 5.1 Urumqi earthquake and deformation pattern in the Urumqi area

In foreland thrust systems, the complex structural geometry creates greater difficulties in constraining the seismotectonics of moderate magnitude earthquakes without pronounced surface ruptures and in assessing future seismic risk. The 2013 M 5.1 Urumqi earthquake was a moderate event that occurred in the Urumqi foreland thrust system. Based on surface investigations, geological mapping, and interpretations of deep seismic reflection profiles, we suggest that the Urumqi foreland thrust belt is a typical active wedge thrust system. Two connected fault segments of the gently south-dipping blind fault ramp and the north-dipping Xishan back-thrust fault merge at a depth of ~11 km and bound the wedge-shaped fault block. Analyses of the focal mechanism and aftershock distribution indicate that the seismogenic fault responsible for the M 5.1 Urumqi event is the south-dipping Xishan fault ramp with a dip angle of ~10–30°, which highlights the potential seismic hazards of the unruptured north-dipping Xishan back-thrust ramp. The four fault branches on the hanging wall of the Xishan fault are interpreted as flexural-slip thrusts. Our results indicate that multiple surface ruptures over a 6 km-wide area may form if a large earthquake occurs along the Xishan fault system.

Sedimentary features and filling process of the Miocene gravity-driven deposits in Ledong area, Yinggehai Basin, South China Sea

The shallow marine channels and fans in the Yinggehai Basin have huge hydrocarbon exploration potential. The depositional features and filling processes of the submarine channel and fan system during 10.5–7.2 ma were studied based on seismic data, cores, well curves, and lab data. (1) Five depositional elements were recognized in the channel system, including PSC, MSC, DSC, levee, and abandonment fill, and the deposits show the typical sedimentary structure of debris flow. The filling process of the channel system was divided into three stages based on deposits supply intensity. The stages were made up of several filling units, and each unit possessed at least one complete channelized depositional succession. (2) The fan system was developed under the large-scale “U” shaped feeding canyon, and its filling process was divided into two stages, the shape and distribution were controlled by shelf break and syn-sedimentary faults in the early stage, and was influenced by the localized changes of paleogeomorphology in late stage. The influence factors for the development of gravity-driven systems were summarized into two levels: regional depositional environment changes and localized distinctive geologic settings. The former one controlled the development of shallow water gravity-driven deposits in the whole basin, mainly including rapid falling of sea-level, moderate subsidence rate, and stable deposits supply. The latter one controlled the morphology and distribution of depositional elements in gravity-driven systems. The pre-existing banded low terrain induced by blind faults controlled the shape of the confined channel, and variable faulting intensity in different segments of the syn-sedimentary fault and localized paleogeomorphology changes influenced the distribution of channels and inner fan in the fan system. The study of deposition features, internal architecture, and controlling factors may provide a reference for further exploration in this area and a deeper understanding of gravity-driven processes in shallow marine.

Seismic hazard assessment of the urban area of Ambato, Ecuador, in deterministic form

Seismic microzonation of the urban area of Ambato, Ecuador, was done in 2018 in a probabilistic and a deterministic manner. This type of calculation is presented in the first part of the article. For this purpose, three geologic faults and three strong-motion equations were considered. For each geologic fault, recurrence periods are determined using two methods. It is seen that a magnitude 6.3 earthquake associated with the blind faults traversing Ambato may occur in 80 to 100 years, and one of magnitude 6.5 in the next 300 years. Geophysical and geotechnical studies of the urban area of the city of Ambato are presented. These permitted the acquisition of curves with the same period of soil vibration and equal speed of the shear wave in the first 30 m, plus the classification of soils of the city. Later, six models of strong soil movements were considered and horizontal acceleration spectra of the soil were obtained in a mesh of points separated every 500 m, for each soil profile. Average spectra were found for soil profiles C, D and E when making comparisons with the spectra found in the 2018 study. Based on the results of the present study and those from 2018, new spectral forms are proposed for the urban area of the city of Ambato (called spectral envelopes) and compared to spectra reported by seismic regulations in force in Ecuador (NEC-15).

The northern Thessaly strong earthquakes of March 3 and 4, 2021, and their neotectonic setting

A sequence of earthquakes occurred on March 3rd and 4th in Northern Thessaly, northern Greece, associated with previously unknown, blind normal faults within the crystalline Palaeozoic basement of the Pelagonian geotectonic zone. Surficial ground deformation, such as liquefaction phenomena in fluvial plains, as well as soil fissures and rock falls, have been mapped. Geological indications of the unmapped seismic fault, i.e., reactivated shear zones, open cracks, etc., have been identified within the bedrock. Based on geological indications, the main fault projection to the surface could be considered a 15 km NW-SE trending structure and average dip of 45o to the NE. The seismic source of the main shock was modelled, and the Coulomb static stress changes calculated for receiver faults similar to the source. The determination of the active tectonic regime of the region by geodetic data and the well-known faults of NE Thessaly plain are also presented, as well as the revised historical and instrumental seismicity. This earthquake raises new concerns and challenges, revising some established views, such as the status of main stress orientations, the orientation of active tectonic structures, the occurrence of a seismogenic fault in a mountainous massif of crystalline rocks without typical geomorphological expression and the role of blind faults in Seismic Hazard Assessment.

Fumarolic Pathways Were Structurally Controlled by a Strike-Slip Fault System Beneath the Bishop Tuff, Bishop, California

The Volcanic Tableland, a plateau at the northern end of Owens Valley, CA, is capped by the rhyolitic Bishop Tuff. It hosts many tectonic and volcanic landforms, including hundreds of fault scarps, large joint sets, and inactive fumarolic mounds and ridges. The 1986 Chalfant Valley earthquake sequence shed light on a blind strike-slip fault system beneath the Bishop Tuff. The spatial relationships of the volcanic and tectonic structures have previously been well documented, however, the mechanisms of formation of structures and their enhancement as fumarolic pathways remain largely unexplored. We collected fault kinematic indicators, joint orientations, and documented fumarolic alterations of microcrystalline quartz in the Bishop Tuff and combined those field observations with fault response modeling to assess whether strike-slip activity played a key role in the development of fumarolic pathways. We found field evidence of dip-slip and strike-slip faulting that are consistent with the overall transtensional regional tectonics. Our modeling indicates that a blind strike-slip fault system would dilate joints in the overlying Bishop Tuff with preferred orientations that match observed orientations of joints along which fumarolic activity occurred. Our results imply that the localization of fumaroles was tectonically controlled and that fault activity in the valley floor likely initiated prior to tuff emplacement.

Multi-dimensional earthquake vulnerability assessment of residential neighborhoods of Mymensingh City, Bangladesh: A spatial multi-criteria analysis based approach

The assessment of the vulnerability of a city to different natural hazards is the prerequisite to achieving sustainable urban resilience. The city of Mymensingh is located in the most earthquake vulnerable zone of Bangladesh and surrounded by multiple fault lines including the Madhupur Blind Fault, Dauki Fault, and Sylhet-Assam Fault lines. The city also achieved the title of the 8th divisional city in 2015, thereby opening the door to unrestrained future development. Therefore, it is of paramount importance to assess the earthquake vulnerabilities of Mymensingh city and this research is envisioned to evaluate this vulnerability by combining 23 parameters derived from four major dimensions-geological, socio-economic, structural and systematic dimensions. The MCDM techniques of Analytical Hierarchy Process and Weighted Linear Combination coupled with GIS-based spatial analysis were applied in this study to assess the residential neighborhood-level earthquake vulnerability. The findings show that out of the 241 residential neighborhoods of Mymensingh city, 51 are observed to be highly vulnerable, while 123 are exposed to medium vulnerability and 67 to low vulnerability. According to the analysis, the topmost factors that make Mymensingh City highly earthquake-vulnerable are the soil type, proximity to the fire station, elderly population levels, Peak Ground Acceleration, child population levels and distance to healthcare facilities. The simplistic, efficient, and reasonably accurate methodology and findings of this research are expected to be helpful for policymakers in low-income countries to prioritize special consideration areas, resource targeting, and deciding on appropriate planning and development control interventions for disaster management.

Coseismic Deformation Mechanisms of the 2021 Ms 6.4 Yangbi Earthquake, Yunnan Province, Using InSAR Observations

At 21:48 on 21 May 2021, an Ms 6.4 earthquake occurred in Yangbi County, Dali Prefecture, Yunnan Province. At present, uncertainty remains regarding the source parameters and deformation mechanism of the Yangbi earthquake. In this study, we determine fault geometry and slip distribution of the earthquake by InSAR analysis. Then, the Coulomb stress loading caused by the Yangbi earthquake is further analyzed. The results show that the moment magnitude of the Yangbi earthquake was Mw 6.14. The slip mainly occurred at depths of 3–13 km, with a maximum slip of approximately 61 cm at a depth of 6.98 km. The Yangbi earthquake was triggered by a blind fault in the NW-SE in the west parallel to the Weixi-Weishan Fault and its seismogenic fault exhibits strike-slip displacement. A large number of aftershocks were distributed along the fault rupture surface where the Coulomb stress increases. As the depth of the crust increases, the area where the Coulomb stress increases in the Yangbi earthquake, decreases. The occurrence of this earthquake also caused a significant increase in the Coulomb stress in the southeastern section of the Weixi-Weishan Faul. We should pay more attention to its seismic hazards.

Geodetic Model of the March 2021 Thessaly Seismic Sequence Inferred from Seismological and InSAR Data

In this work, we propose a geodetic model for the March 2021 Thessaly seismic sequence (TSS). We used the interferometric synthetic aperture radar (InSAR) technique and exploited a dataset of Sentinel-1 images to successfully detect the surface deformation caused by three major events of the sequence and constrain their kinematics, further strengthened by seismic data analysis. Our geodetic inversions are consistent with the activation of distinct blind faults previously unknown in this region: three belonging to the NE-dipping normal fault associated with the Mw 6.3 and Mw 6.0 events, and one belonging to the SW-dipping normal fault associated with the Mw 5.6, the last TSS major event. We performed a Coulomb stress transfer analysis and a 1D pore pressure diffusivity modeling to investigate the space–time evolution of the sequence; our results indicate that the seismic sequence developed in a sort of domino effect. The combination of the lack of historical records of large earthquakes in this area and the absence of mapped surface features produced by past faulting make seismic hazard estimation difficult for this area: InSAR data analysis and modeling have proven to be an extremely useful tool in helping to constrain the rupture characteristics.

Structural Exploration of Aeromagnetic Data over Part of Gwagwalada, Abuja for Potential Mineral Targets Using Derivatives Filters

Aeromagnetic data are consistently used for economic interest targeting and geological mapping. Besides solving problems that are concerned with the basement, the method has become a useful tool in exploring minerals,hydrocarbons occurrence,groundwater investigations, and geothermal potentials. This study analyses aeromagnetic data from the Nigerian Geological Survey Agency acquired at 100 m terrain clearance over a section of Gwagwalada in Abuja. The study area spans longitudes 7.0875° E to 7.1458° E and latitude 8.9625° N to 9.0° N (about 27 km2 ). After a reduction to the equator (RTE) transformation, the data is downward continued by 50 m. Different filters are applied to outline area of alteration associated to mineral deposit. Regional geologic structures trend NE - SW.The application of vertical derivatives (FVD and SVD) to the RTE grid enhanced shallow structures which trend NE - SW. Horizontal gradients along the X- and Y- directions enhance geological contacts attributable to blind faults. The Tilt derivative (TD) accentuated fault lines which trend NE - SW.

Geophysical characterization of a potentially active fault in the Agua Fría micro-graben, Los Azufres, Mexico

In this study, three geophysical techniques were used to identify, localize, and characterize a partly blind fault in the Llano Grande basin within the Agua Fría Graben. This tectonic basin is located in the Los Azufres Volcanic Complex, one of the major silicic volcanic centers in the Trans-Mexican Volcanic Belt. The 1 km wide Agua Fría graben could be considered as an analogous of the larger graben structures bounded by the Morelia-Acambay Fault System. Since it is filled by recent sediments, it represents a challenge for the recognition and characterization of active faults that lack clear surface expression. Newly collected magnetic data led to the identification of lineaments interpreted as structural discontinuities. Ground penetrating radar and seismic refraction surveys were carried out across one of these magnetic lineaments crossing the basin to characterize the nature and geometry of the inferred discontinuity. The ground penetrating radar profiles allowed the identification of buried deformational structures interpreted as the northern segment of the Agua Fría fault. The subsurface reflectors displaced 1 to 1.5 m by the fault indicate that this structure is potentially active. The opening of trenches based on these results makes it possible to confirm the interpretation of the geophysical profiles, to discuss the precision of the data and to validate their use for such studies. On seismic refraction profiles, the deformation zones are related to low P-wave velocity zones. These geophysical studies demonstrate the potential of such techniques to locate faults in the subsurface, partially characterize the width of the fault zone and the associated displacement within the uppermost of the subsurface. Our results may be applied to define ideal sites for paleoseismic excavations which are essential for the identification and description of historical and prehistoric earthquakes, and thus, for the characterization of the local seismic hazard.

Geometric and kinematic analysis of normal faults bordering continental shelves: A 3D seismic case study from the northwest South China Sea

Normal faults border many a continental shelf, but have only been considered to play a subordinate role to basin-controlling faults in previous studies. Their detailed geometries and kinematic histories are still poorly known. In this study, high-quality three dimensional (3D) seismic data are used to investigate distinct families of normal faults bordering the continental shelf of the Qiongdongnan Basin, northwest South China Sea. Sixty-six (66) normal faults are interpreted and found to mostly tip out in the Upper Miocene. Three large-scale faults offsetting Horizon T30 (the base of Pliocene strata), and three other faults terminating beneath this same horizon, were selected and studied in detail. We discriminate between growth and blind faults by analysing upper tip folding geometry and fault displacement distribution using throw-depth (T-z) plots. The dips of the upper parts of faults are almost twice that of their lower parts. A new three-stage model for fault reactivation in Qiongdongnan Basin is therefore proposed based on our results. In the northwest South China Sea, the propagation of normal faults on the continental shelf can be attributed to: (1) the rotation of the South China block and a reversal in the movement of the Ailao Shan-Red River Fault Zone from left-to right-lateral slip, and (2) large-scale slope instability in the Qiongdongnan Basin at ~5.5 Ma. The results presented here outline the structural evolution of the continental shelf of the northwest South China Sea after 5.5 Ma, and how regional-scale structures such as the Ailao Shan-Red River Fault Zone controlled the evolution of normal faults on continental shelves.

Research on Technology and Application of Buried Faults Identification in Urban Underground Space

With theurbanization rate's rising and three-dimensional expansion and development of urban, the identification of underground buried faults has become the key factor of earthquake risk in urban underground space and surface area. As a typical method of detecting blind faults in underground space, shallow seismic prospecting technology plays an important role in judging and avoiding potential risks such as underground faults in the process of urban expansion and site selection. In this paper, shallow seismic prospecting technology is adopted, and optimized processing technologies such as parameter test, tomographic correction, pre-stack denoising, fidelity and consistency processing, correction iteration, migration imaging, and time-depth relationship deduction are adopted. Underground faults are identified and interpreted in the studied urban area, and fault risk assessment is carried out based on fault characteristics, scale, distribution and overlying strata, thus providing suggestions for regional pattern and construction of urban planning.

An Integrated Multiscale Method for the Characterisation of Active Faults in Offshore Areas. The Case of Sant’Eufemia Gulf (Offshore Calabria, Italy)

Diagnostic morphological features (e.g., rectilinear seafloor scarps) and lateral offsets of the Upper Quaternary deposits are used to infer active faults in offshore areas. Although they deform a significant seafloor region, the active faults are not necessarily capable of producing large earthquakes as they correspond to shallow structures formed in response to local stresses. We present a multiscale approach to reconstruct the structural pattern in offshore areas and distinguish between shallow, non-seismogenic, active faults, and deep blind faults, potentially associated with large seismic moment release. The approach is based on the interpretation of marine seismic reflection data and quantitative morphometric analysis of multibeam bathymetry, and tested on the Sant’Eufemia Gulf (southeastern Tyrrhenian Sea). Data highlights the occurrence of three major tectonic events since the Late Miocene. The first extensional or transtensional phase occurred during the Late Miocene. Since the Early Pliocene, a right-lateral transpressional tectonic event caused the positive inversion of deep (>3 km) tectonic features, and the formation of NE-SW faults in the central sector of the gulf. Also, NNE-SSW to NE-SW trending anticlines (e.g., Maida Ridge) developed in the eastern part of the area. Since the Early Pleistocene (Calabrian), shallow (<1.5 km) NNE-SSW oriented structures formed in a left-lateral transtensional regime. The new results integrated with previous literature indicates that the Late Miocene to Recent transpressional/transtensional structures developed in an ∼E-W oriented main displacement zone that extends from the Sant’Eufemia Gulf to the Squillace Basin (Ionian offshore), and likely represents the upper plate response to a tear fault of the lower plate. The quantitative morphometric analysis of the study area and the bathymetric analysis of the Angitola Canyon indicate that NNE-SSW to NE-SW trending anticlines were negatively reactivated during the last tectonic phase. We also suggest that the deep structure below the Maida Ridge may correspond to the seismogenic source of the large magnitude earthquake that struck the western Calabrian region in 1905. The multiscale approach contributes to understanding the tectonic imprint of active faults from different hierarchical orders and the geometry of seismogenic faults developed in a lithospheric strike-slip zone orthogonal to the Calabrian Arc.

Cenozoic structure and tectonics of North subbasins in Beibu Gulf Basin, northern South China Sea

The spatial and temporal interaction of rifting at the northern South China Sea (SCS) margin and shearing along Red River Fault Zone (RRFZ) is crucial to understanding the regional geodynamics. Their obscure relationship has led to controversial models for the origin of the Beibu Gulf basin within the northeast edge of the SCS. Newly released seismic-reflection data and well logs permit a new comprehensive analysis of the tectono-sedimentary history, which provides important constraints on the basin-forming history. Based on the interpretation of 3D multi-channel seismic data, we analyzed the seismic stratigraphy, fault development and tectonic subsidence, with additional confirmation from analogue modelling. The results suggest that several key unconformities separate the rifting history of North subbasins into three distinct phases. The syn-depositional faults and depocenters experienced a tectonic transition from NE- to NNE-orientated during the first two stages. The extensional structures were overprinted by the conjugate strike-slip faulting during stage III (30–23 Ma), as evidenced by the NEE-trending predominant faults and reactivation of NW-trending blind faults. Together with other regional geologic records and plate reconstruction history, we propose an updated view of the region's tectonic history. The NE-trending rift system initiated at the beginning of the Cenozoic. Tectonic reconfiguration at ~40 Ma induced a regional change of stress field to become north-south stretching that caused a tectonic transition of the basin. Influenced by the regional shearing activity of the RRFZ as well as additional north-south stretching, the basin was reworked by widespread strike-slip deformation. We therefore emphasize that while the basin's overall architecture was defined by the development of NE-trending rifts, both reconfiguration of the stress field acting on pre-existing faults and strike-slip reactivation of blind faults led to the basin's current intricate structure.

Fore-arc high and basin evolution offshore northern Sumatra using high-resolution marine geophysical datasets

We present a morphotectonic study of the offshore northern Sumatra fore-arc high and fore-arc basin and an evolutionary model of the area based on high-resolution marine geophysical datasets. We show that the landward slope of the fore-arc high and the western edge of the Aceh fore-arc basin host a set of deeply-rooted, seaward-dipping backthrusts: main backthrust (MBT) and frontal backthrust (FBT), which we call north Sumatra backthrust system (NSBS). The FBT is imaged reaching the seafloor throughout the study area, whereas the MBT is imaged as a blind fault. Many landward-dipping imbricated thrusts (fore-arc high thrusts-FHT) are also observed below the fore-arc high, which along with the system of backthrusts have been responsible for uplifting the fore-arc high, shortening the Aceh Basin. On the landward slope of the fore-arc high, a strike-slip fault zone is imaged (termed West Andaman Fault - WAF - in accordance with previous studies), is readily interpreted as the product of the slip-partitioning at this oblique convergent margin, with a fraction of the arc-parallel slip localizing at the boundary between fore-arc high and fore-arc basin. Our results allow us to establish the evolutionary history of the thrust faults, strike-slip fault zone and the fore-arc basin. We suggest that the major structural features including backthrusts (MBT, FBT), fore-thrusts (FHTs) and the fore-arc high were developed during the Miocene, and that the growth of the fore-arc high accelerated in the Pliocene onwards owing to a sudden increase in sediment volume provided by the Nicobar fan.

Mixed crystalline basement of Junggar basin revealed by wide-angle seismic evidence

A wide-angle seismic reflection / refraction survey along a ~ 600 km long transect through the Junggar basin from Emin to Qitai allows to receive several images near N-S trending blind faults, which are located at the lower part of the upper crust, the middle crust and the lower crust within the basin and cut up the Moho. These faults, with high seismic velocity and without obvious dislocation, are considered as “extensional faults” formed by north-south compression and east-west extension. These deeply rooted faults provide channels via which basic to ultra-basic materials from upper mantle migrate into the crust and mix up with the crustal material causing thin thickness, high seismic velocity, high density and high magnetic intensity after cooling in the crust of the basin.

3‐D Deep Electrical Resistivity Tomography of the Major Basin Related to the 2016 Mw6.5 Central Italy Earthquake Fault

AbstractWe provide the first 3‐D resistivity image of the Pian Grande di Castelluccio basin, the main Quaternary depocenter in the hangingwall of the Mt.Vettore–Mt. Bove normal fault system (VBFS), responsible for the October 30, 2016 Mw6.5 Norcia earthquake (central Italy). The subsurface structure of the basin is poorly known, and its relation with the VBFS remains debated. Using the recent Fullwaver technology, we carried out a high‐resolution 2‐D transect crossing the 2016 coseismic ruptures coupled with an extensive 3‐D survey with the aim of: (a) mapping the subsurface of the basin‐bounding splays of the VBFS and the downdip extent of intrabasin faults; (b) imaging the infill and pre‐Quaternary substratum down to ∼1 km depth. The 2‐D resistivity section highlights under the coseismic ruptures a main dip‐slip fault zone with conjugated splays. The 3‐D resistivity model suggests that the basin consists of two depocenters (∼300 and ∼600 m deep, respectively) filled with silty sands and gravels (resistivity <300 Ωm), bounded and cross‐cut by NNE‐, WNW‐, and NNW‐trending faults with throws of ∼200–400 m. We hypothesize that the NNE‐trending system acted during the early basin development, followed by NNW‐trending and currently active splays of the VBFS that overprint pre‐existing structures and locally control the infill architecture. Moreover, beneath the basin we detect a shallow NW‐dipping blind fault. The latter is likely a hangingwall splay of the adjacent regional Mts. Sibillini Thrust, which may have been partly involved in the rupture process of the Norcia mainshock.

Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse Ranges in California from reconstruction and luminescence dating of the Orcutt Formation

AbstractThe western Transverse Ranges and southern Coast Ranges of California are lithologically similar but have very different styles and rates of Quaternary deformation. The western Transverse Ranges are deformed by west-trending folds and reverse faults with fast rates of Quaternary fault slip (1–11 mm/yr) and uplift (1–7 mm/yr). The southern Coast Ranges, however, are primarily deformed by northwest-trending folds and right-lateral strike-slip faults with much slower slip rates (3 mm/yr or less) and uplift rates (<1 mm/yr). Faults and folds at the boundary between these two structural domains exhibit geometric and kinematic characteristics of both domains, but little is known about the rate of Quaternary deformation along the boundary.We used a late Pleistocene sedimentary deposit, the Orcutt Formation, as a marker to characterize deformation within the boundary zone over the past 120 k.y. The Orcutt Formation is a fluvial deposit in the Santa Maria Basin that formed during regional planation by a broad fluvial system that graded into a shoreline platform at the coast. We used post-infrared–infrared-stimulated luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope stages 5e-a paleo–sea-level highstands and regional depositional events. The deformed Orcutt basal surface closely follows the present-day topography of the Santa Maria Basin and is folded by northwest-trending anticlines that are a combination of fault-propagation and fault-bend-folding controlled by deeper thrust faults. Reconstructions of the Orcutt basal surface and forward modeling of balanced cross sections across the study area allowed us to measure rock uplift rates and fault slip rates. Rock uplift rates at the crests of two major anticlinoria are 0.9–4.9 mm/yr, and the dip-slip rate along the blind fault system that underlies these folds is 5.6–6.7 mm/yr. These rates are similar to those reported from the Ventura area to the southeast and indicate that the relatively high rates of deformation in the western Transverse Ranges are also present along the northern boundary zone. The deformation style and rates are consistent with models that attribute shortening across the Santa Maria Basin to accommodation of clockwise rotation of the western Transverse Ranges and suggest that rotation has continued into late Quaternary time.