Spatiotemporal Rates of Tectonic Deformation and Landscape Evolution above a Laterally Propagating Thrust Fault: Wheeler Ridge Anticline, California, USA

The Wheeler Ridge anticline, located in the southern San Joaquin Valley of California, USA, is a well-studied and classic example of a laterally growing fault propagation fold. New high-resolution lidar elevation data combined with nine infrared stimulated luminescence (IRSL) ages of discrete geomorphic surfaces that are bounded by prominent transverse wind and river gaps allow for investigation of tectonic topography through time. Luminescence ages from four of the six surfaces yield depositional ages that range from 32 ka to 153 ka, which are broadly consistent with a previously published soil chronosequence. Our graphical modeling indicates an average surface uplift rate of ~2.1 mm/yr and an average along-strike fold propagation rate of ~20 mm/yr. However, our probabilistic modelling and topographic analysis suggest a rate decrease of both uplift and lateral propagation toward the fault tip from ~2.4 to 0.7 mm/yr and from ~49 to 14 mm/yr, respectively. Rate decreases are not progressive but rather occur in punctuated deformational intervals across previously documented structural barriers (tear faults) resulting in a fold that is characterized by discrete segments that exhibit a systematic deformational decrease toward the east. The punctuated tectonic growth of Wheeler Ridge has also locally controlled the topographic evolution of the anticline by effecting the formational timing and position of at least seven wind and river gaps that result from multiple north-flowing antecedent streams that traverse the growing structure. We quantify the timing of wind and river gap formation, based on IRSL results and inferred incision rates, and present a model for the spatiotemporal evolution of transverse drainages and the topographic development of Wheeler Ridge. Our chronology of gap formation broadly correlates with regional Late Pleistocene dry climate intervals suggesting that both tectonics and climate were integral to the geomorphic development of the Wheeler Ridge anticline.

INTERPRETAÇÃO SÍSMICA E MODELAGEM 3D ESTRUTURAL NA BACIA DE BARREIRINHAS (MARANHÃO, BRASIL)

Discoveries of hydrocarbons in the basins of the African Equatorial Margin and Guinea Gulf stimulated the exploratory interest in the basins of the Brazilian Equatorial Margin, for being together before the Continental Drift. This interest emerges because both African and South American equatorial margin are considered analogous. The Barreirinhas Basin is a member of the Brazilian equatorial margin. The objective of this work is to present the results obtained through the seismic interpretation and structural 3D modeling, in the context of gravitational tectonics, in an area covered by 3D seismic data, in the Barreirinhas Basin. The compressional domain of an extensive-compressive system was mapped. In this study, were identified reverse faults, thrust faults and fault-related folding like fault-bend and fault-propagation fold that can be accompanied by backthrust features in deep to ultra-deep waters. The 3D structural model allowed the representation of the geometric variations present in the study area. The new information will be important for the identification and evaluation of structures with greater potential for hydrocarbon accumulations and can help in future studies to characterize the reservoir, contributing to the evolution of knowledge of the equatorial margin, especially in the Barreirinhas Basin.

Geological Structure of the Sylmar Basin: Implications for Slip Distribution Along the Santa Susana/Hospital and Mission Hills Fault System in the San Fernando Valley, CA, U.S.A.

We developed a forward model using the Trishear module in MOVE to better understand the structure of the northwestern San Fernando Valley and the relationship among the Santa Susana, Hospital, Mission Hills and Northridge Hills faults. This study was motivated by the 1971 San Fernando earthquake and previous work that inferred a high slip rate on the Santa Susana fault, which is in apparent contrast to the lack of significant geomorphic expression of the fault in the Sylmar Basin region. We trenched the Mission Hills anticline from the crest to the base of slope and demonstrate that the Mission Hills anticline is an actively growing fault propagation fold. The associated thrust tip is either deeper than 15 m or sufficiently far to the south that the fault was not encountered in large diameter borings, but the minimum structural relief across the Mission Hills fault since the late Pleistocene is on the order of 37 m, suggesting a minimum uplift rate of 0.5 mm/yr. Our work presents a structural analysis that demonstrates how the Santa Susana fault system evolved in time, with the frontal thrust progressively migrating southward to the Mission Hills fault, and farther south to the Northridge Hills blind thrust. The progression of faulting towards the direction of vergence is compatible with the observed thrust front migration in the western Transverse Ranges of California, and other trust belts around the world.

A 2600-year-long paleoseismic record for the Himalayan Main Frontal Thrust (western Bhutan)

In spite of an increasing number of paleoseismic studies carried out over the last decade along the Himalayan Arc, the chronology of historical and prehistorical earthquakes is still poorly constrained. In this paper, we present geomorphologic and paleoseismic studies conducted over a large river-cut exposure along the Main Fontal Thrust in southwestern Bhutan. The Piping site reveals a 30 m high fault-propagation fold deforming late Holocene alluvial deposits. There, we carried out detailed paleoseismic investigations and built a chronological framework on the basis of 22 detrital charcoal samples submitted to radiocarbon dating. Our analysis reveals the occurrence of at least five large and great earthquakes between 485±125 BCE and 1714 CE with an average recurrence interval of 550±211 years. Coseismic slip values for most events reach at least 12 m and suggest associated magnitudes are in the range of Mw 8.5–9. The cumulative deformation yields an average slip rate of 24.9±10.4 mm yr−1 along the Main Frontal Thrust over the last 2600 years, in agreement with geodetic and geomorphological results obtained nearby.

A 2600-yr-long paleoseismic record for the Himalayan Main Frontal Thrust (Western Bhutan)

In spite of an increasing number of paleoseismic studies carried out over the last decade along the Himalayan arc, the chronology of historical and pre-historical earthquakes is still poorly constrained. In this paper, we present geomorphologic and paleoseismic studies conducted over a large river-cut exposure along the Main Fontal Thrust in southwestern Bhutan. The Piping site reveals a 30-m-high fault-propagation fold deforming late Holocene alluvial deposits. There, we carried out detailed paleoseismic investigations and built a chronological framework on the basis of 22 detrital charcoal samples submitted to radiocarbon dating. Our analysis reveals the occurrence of at least five large and great earthquakes between 485 ± 125 BC and AD 1714 with an average recurrence interval of 550 ± 211 yr. Co-seismic slip values for most events reach at least 13 m and suggest associated magnitudes are in the range of Mw 8.5–9. The cumulative deformation yields an average slip rate of 25.3 ± 4 mm/yr along the Main Frontal Thrust, over the last 2600 yr in agreement with geodetic and geomorphological results obtained nearby.

Syncontractional deposition of the Cretaceous Newark Canyon Formation, Diamond Mountains, Nevada: Implications for strain partitioning within the U.S. Cordillera

The timing of deformation and deposition within syntectonic basins provides critical information for understanding the evolution of strain in mountain belts. In the U.S. Cordillera, contractional deformation was partitioned between the Sevier thrust belt in Utah and several structural provinces in the hinterland in Nevada. One hinterland province, the Central Nevada thrust belt (CNTB), accommodated up to ∼15 km of shortening; however, in most places, this deformation can only be bracketed between Permian and Eocene. Cretaceous deposits of the Newark Canyon Formation (NCF), which are sparsely exposed along the length of the CNTB, offer the opportunity to constrain deformation timing. Here, we present mapping and U-Pb zircon geochronology from the NCF in the Diamond Mountains, which demonstrate deposition of the NCF during proximal CNTB deformation. Deposition of the basal NCF member was under way no earlier than ca. 114 Ma, a tuff in the middle part of the section was deposited at ca. 103 Ma, and the youngest member was deposited no earlier than ca. 99 Ma. Intraformational angular unconformities and abrupt along- and across-strike thickness changes indicate that NCF deposition was related to growth of an east-vergent fault-propagation fold. Clast compositions define unroofing of upper Paleozoic sedimentary rocks, which we interpret as the progressive erosion of an anticline ∼10 km to the west. CNTB deformation was contemporaneous with shortening in the Sevier thrust belt, which defines middle Cretaceous strain partitioning between frontal and interior components of the Cordillera. Strain partitioning may have been promoted by renewed underthrusting during a period of high-flux magmatism.

Rekonstruksi Struktur Geologi Kali Lutut dan Sekitarnya, Temanggung, Jawa Tengah

Kali Lutut merupakan lokasi tersingkapnya seri batuan khusus yang disebut Lutut Beds, yang tersusun oleh batupasir dan batugamping dengan mineral kuarsa yang melimpah dan fragmen rombakan berbagai jenis batuan seperti batuan metamorf, vulkanik, batuan dasar, dan karbon. Penelitian ini dilakukan untuk merekonstruksi kondisi struktur geologi di Kali Lutut dan sekitarnya sehingga diketahui karakteristik struktur geologi yang ada. Penelitian ini secara umum didasarkan pada evaluasi data lapangan yang didukung oleh data seismik yang berada di luar daerah penelitian. Data lapangan berupa 62 titik pengamatan diperoleh melalui penyelidikan langsung untuk mengetahui kondisi geologi daerah penelitian dan pengambilan sampel batuan. Berdasarkan data lapangan, analisis petrografi, dan analisis fosil yang dipadukan dengan analisis struktur geologi maka dihasilkan peta geologi daerah penelitian. Penampang geologi dibuat berdasarkan peta geologi didukung oleh data seismik yang berada di luar daerah penelitian, yang dilakukan dengan metode dip-domain untuk menggambarkan kondisi bawah permukaan daerah penelitian. Stratigrafi daerah penelitian terdiri dari Batuan Dasar, Formasi Pelang, Formasi Kerek, Intrusi Andesit, Formasi Penyatan, Formasi Kaligetas, dan Aluvium. Struktur geologi daerah penelitian dikelompokkan menjadi tiga kelompok yaitu kelompok sesar turun, kelompok sesar naik dan lipatan, serta kelompok sesar mendatar. Sesar naik memiliki karakteristik berupa fault-propagation fold yang membentuk imbrication thrust fault berorientasi barat-timur.

Numerical Simulation of Deformation Band Occurrence and the Associated Stress Field during the Growth of a Fault-Propagation Fold

Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orientations during the tectonic evolution of a basin, and thus the corresponding fluid flow patterns in a reservoir. This study aims to predict the main stress orientations and evolution during the growth of a fold by using the limit analysis method. Fourteen different steps have been integrated as 2D cross sections from an early stage to an evolved stage of a schematic and balanced propagation fold. The stress evolution was followed during the time and burial of syn tectonic layers localized in front of the thrust. Numerical simulations were used to predict the occurrence and orientation of deformation bands, i.e., compaction and shear bands, by following the kinematic of a fault-propagation fold. The case study of the Sant-Corneli-Boixols anticline was selected, located in the South Central Pyrenees in the Tremp basin, to constrain the dimension of the starting models (or prototypes) used in our numerical simulations. The predictions of the numerical simulations were compared to field observations of an early occurrence of both pure compaction- and shear-enhanced compaction bands in the syn-tectonic Aren formation located in front of the fold, which are subjected to early layer parallel shortening during the burial history. Stress magnitude and stress ratio variations define the type of deformation band produced. Our results show that the band occurrence depends on the yield envelope of the host material and that a small yield envelope is required for these shallow depths, which can only be explained by the heterogeneity of the host rock facies. In our case, the heterogeneity can be explained by a significant contribution of carbonate bioclasts in the calcarenite rock, which change the mechanical behavior of the whole rock.