Intrabasinal Faults Research Articles

Salt tectonics in salt-bearing rift basins: Progradational loading vs extension

Confined salt flow in narrow (<100 km) and deep (ca. 10 km) rift basins with an initially thick (>3 km) layer of evaporites, results in a complex structural style consisting of vertical and closely-spaced diapirs surrounded by thick (6–10 km) and narrow minibasins with intricate internal patterns recording salt withdrawal. These basins are usually explained as the result of thick-skinned extension, while other important factors such as sediment loading and subsalt basin configuration are not commonly addressed. We use sandbox models, 3D reconstruction of the models’ final stage, and structural restoration to evaluate the impact of sediment loading, thick-skinned extension, and different basin configurations on the suprasalt structural style and the spatial and temporal distribution of salt structures in these basins. Models simulating sediment loading display expulsion rollovers and younger salt structures in the progradation direction. In contrast, thick-skinned extension models display stacked depocenters near basin-boundary faults and opposite timing of diapir growth. Along strike changes in basin configuration and subsalt faults impact salt flow and salt structures as follows: (1) Faults perpendicularly facing the sediment progradation contribute to the formation of salt diapirs, (2) Oblique faults favour salt flow along strike, (3) Fault intersections cause salt inflation and salt diapirs, and (4) Basin confinement by graben narrowing or intrabasinal faults produces along-strike differences in the timing of salt mobilization. The sandbox models show close resemblance and are directly comparable to seismic sections through the Nordkapp Basin (Norwegian Barents Sea) and other salt-bearing rift basins with similar configuration.

Climatically driven displacement on the Eglington fault, Las Vegas, Nevada, USA

Abstract The Eglington fault is one of several intrabasinal faults in the Las Vegas Valley, Nevada, USA, and is the only one recognized as a source for significant earthquakes. Its broad warp displaces Late Pleistocene spring deposits of the Las Vegas Formation, which record hydrologic fluctuations that occurred in response to millennial- and submillennial-scale climate oscillations throughout the late Quaternary. The sediments allow us to constrain the timing of displacement on the Eglington fault and identify hydrologic changes that are temporally coincident with that event. The fault deforms deposits that represent widespread marshes that filled the valley between ca. 31.7 and 27.6 ka. These marshes desiccated abruptly in response to warming and groundwater lowering during Dansgaard-Oeschger (D-O) events 4 and 3, resulting in the formation of a pervasive, hard carbonate cap by 27.0 ka. Vertical offset by as much as 4.2 m occurred after the cap hardened, and most likely after younger marshes desiccated irreversibly due to a sudden depression of the water table during D-O event 2, beginning at 23.3 ka. The timing of displacement is further constrained to before 19.5 ka as evidenced by undeformed spring deposits that are inset into the incised topography of the warp. Coulomb stress calculations validate the hypothesis that the substantial groundwater decline during D-O event 2 unclamped the fault through unloading of vertical stress of the water column. The synchroneity of this abrupt hydrologic change and displacement of the Eglington fault suggests that climatically modulated tectonics operated in the Las Vegas Valley during the late Quaternary.

Segmentation and increasing activity in the Neogene-Quaternary Teruel Basin rift (Spain) revealed by morphotectonic approach

The NNW-SSE trending Teruel Basin rift is the largest Late Miocene-Quaternary extensional intracontinental structure located within the central-eastern Iberian Chain (Spain). The structural and morphotectonic study carried out in the central-northern part of this half graben basin (north of Teruel city) has allowed us to analyse rift segmentation, deformation partitioning and rift evolution. Results are based on vertical displacement calculations (fault throw and bending) of the main border and intrabasin fault zones. We use two geomorfological-stratigraphical markers, the Intramiocene Erosion Surface (IES; 11.2 Ma) and the Fundamental Erosion Surface (FES; 3.5 Ma). While the first marker reveals rift initiation under an E-W extension, the late marker records vertical displacements associated to a second, Late Pliocene–Quaternary rifting stage characterized by a nearly multidirectional extension regime with prevailing ENE-WSW trending ó3. Despite the along-axis rift segmentation into three structural domains (northern, central and southern) and the distribution of deformation among border and intrabasin faults in the central and southern domains, a consistent average slip rate (post-IES) of 0.09 mm/a has been calculated on distinct transects across the basin, suggesting a homogeneous crustal-scale extension process in the region. The results also reveal that slip rates during the Late Pliocene-Quaternary (0.12–0.16 mm/a) are higher than the Late Miocene-Early Pliocene (0.05–0.07 mm/a). Slip rate increase is caused by (i) a westward propagation of deformation from the Valencia Through, and (ii) a change in the regional stress field, both enhanced by crustal doming affecting central-eastern Iberia, as well as progressive fault linkage. Throw vs. distance graphs suggest that the main faults are in a transient stage towards coalescence, less advanced within the southern domain. Regional Late Pliocene-Quaternary uplift, concomitant with increasing slip rates in the Teruel Basin rift, has caused the basin to rise, so that synrift sedimentation only took place in rapidly subsiding residual basins until the region became exorheic and the basin was incised by the present-day fluvial network.

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

AbstractThe distribution of deformation during the early stages of continental rifting is an important constraint on our understanding of continental breakup. Incipient rifting in East Africa has been considered to be dominated by slip along rift border faults, with a subsequent transition to focused extension on axial segments in thinned crust and/or with active magmatism. Here, we study high‐resolution satellite data of the Zomba Graben in southern Malawi, an amagmatic rift whose topography is dominated by the west‐dipping Zomba fault. We document evidence for five subparallel fault scarps between 13 and 51 km long spaced ~10–15 km apart. The scarps consist of up to five segments between 4 and 18 km long, separated by minima in scarp height and river knickpoints. The maximum height of each fault scarp ranges from 9.5 ± 4.2 m to 35.3 ± 14.6 m, with the highest scarp measured on the intrabasin Chingale Step fault. We estimate that the scarps were formed by multiple earthquakes of up to Mw7.1 and represent a previously unrecognized seismic hazard. Our calculations show that 55 ± 24% of extensional strain is accommodated across intrabasin faults within the ~50 km wide rift. This demonstrates that a significant proportion of displacement can occur on intrabasin faults during early‐stage rifting, even in thick continental lithosphere with no evidence for magmatic fluids.

Impact of a pre‐existing transverse drainage system on active rift stratigraphy: An example from the Corinth Rift, Greece

AbstractModels to explain alluvial system development in rift settings commonly depict fans that are sourced directly from catchments formed in newly uplifted footwalls, which leads to the development of steep‐sided talus‐cone fans in the actively subsiding basin depocentre. The impact of basin evolution on antecedent drainage networks orientated close to perpendicular to a rift axis, and flowing over the developing hangingwall dip slope, remains relatively poorly understood. The aim of this study is to better understand the responses to rift margin uplift and subsequent intrabasinal fault development in determining sedimentation patterns in alluvial deposits of a major antecedent drainage system. Field‐acquired data from a coarse‐grained alluvial syn‐rift succession in the western Gulf of Corinth, Greece (sedimentological logging and mapping) has allowed analysis of the spatial distribution of facies associations, stratigraphic architectural elements and patterns of palaeoflow. During the earliest rifting phase, newly uplifted footwalls redirected a previously established fluvial system with predominantly southward drainage. Footwall uplift on the southern basin margin at an initially relatively slow rate led to the development of an overfilled basin, within which an alluvial fan prograded to the south‐west, south and south‐east over a hangingwall dip slope. Deposition of the alluvial system sourced from the north coincided with the establishment of small‐scale alluvial fans sourced from the newly uplifted footwall in the south. Deposits of non‐cohesive debris flows close to the proposed hangingwall fan apex pass gradationally downstream into predominantly bedload conglomerate deposits indicative of sedimentation via hyperconcentrated flows laden with sand‐ and silt‐grade sediment. Subsequent normal faulting in the hangingwall resulted in the establishment of further barriers to stream drainage, blocking flow routes to the south. This culminated in the termination of sediment supply to the basin depocentre from the north, and the onset of underfilled basin conditions as signified by an associated lacustrine transgression. The evolution of the fluvial system described in this study records transitions between three possible end‐member types of interaction between active rifting and antecedent drainage systems: (a) erosion through an uplifted footwall, (b) drainage diversion away from an uplifted footwall and (c) deposition over the hangingwall dip slope. The orientation of antecedent drainage pathways at a high angle to the trend of a developing rift axis, replete with intrabasinal faulting, exerts a primary control on the timing and location of development of overfilled and underfilled basin states in evolving depocentres.

Uplift induced structurally controlled landscape development: example from fault bounded Jumara and Jara domes in Northern Hill Range, Kachchh, Western India

The Kachchh palaeo-rift graben at the western continental margin of India is currently undergoing active coseismic deformation in response to the periodic release of NE-SW oriented compressive stresses along the E-W trending intra-basinal faults. The seismically active Kachchh Mainland Fault (KMF) is the largest intra-basinal fault that is characterized by a narrow zone comprising a chain of domes and anticlines forming the rugged hilly topography of the Northern Hill Range (NHR) in the upthrown block. The present paper deals with the Jumara and Jara domes which have received considerable attention with respect to the Mesozoic biostratigraphy, however, no study has been carried out on geomorphological and neotectonic aspects. The major geomorphological units in the area include a low relief but rugged structurally controlled topography over the domes, the KMF scarp, the precipitous Jaramara scarp, the back slope over the Jaramara scarp and the rugged hilly topography over the large Ukra intrusive body that shows a complicated structural contact relation with the Mesozoic rocks ranging from concordant to discordant. Our detailed field studies supplemented by geomorphological analysis using DEM, topographical profiles, drainage characteristics and longitudinal profiles reveal the dominant control of sustained uplift along the KMF under compression in the development of the present youthful and structurally controlled landscape. Our study also suggests that the imposing E-W trending Jaramara scarp is formed due to the long-term retreat of the KMF scarp in response to uplift, while the present low relief scarp along the KMF is largely the result of post-Miocene uplift. The incision in Quaternary miliolite deposits in the Jara river gorge, deeply incising nature of the rivers, several large and small knickpoints and scanty occurrence of Quaternary miliolite deposits indicate an uplift of ~25 m during the Late Quaternary time.

Faulting processes during early-stage rifting: seismic and geodetic analysis of the 2009–2010 Northern Malawi earthquake sequence

SUMMARYIn December, 2009, a rare sequence of earthquakes initiated within the weakly extended Western Rift of the East African Rift system in the Karonga province of northern Malawi, providing a unique opportunity to characterize active deformation associated with intrabasinal faults in an early-stage rift. We combine teleseismic and regional seismic recordings of the largest events, InSAR imagery of the primary sequence, and recordings of aftershocks from a temporary (4-month) local network of six seismometers to delineate the extent and geometry of faulting. The locations of ∼1900 aftershocks recorded between January and May 2010 are largely consistent with a west-dipping normal fault directly beneath Karonga as constrained by InSAR and CMT fault solutions. However, a substantial number of epicentres cluster in an east-dipping geometry in the central part of the study area, and additional west-dipping clusters can be discerned near the shore of Lake Malawi, particularly in the southern part of the study area. Given the extensive network of hanging wall faults mapped in the Karonga region on the surface and in seismic reflection images, the distribution of events is strongly suggestive of multiple faults interacting to produce the observed deformation, and the InSAR data permit this but do not require it. We propose that fault interaction contributed to the seismic moment release as a series of Mw 5-to-6 events instead of a normal main shock–aftershock sequence. We find the depth of fault slip during the main shocks constrained by InSAR peaks at less than 6 km, while the majority of recorded aftershocks are deeper than 6 km. This depth discrepancy appears to be robust and may be explained by fault interaction. Structural complexities associated with fault interaction may have limited the extent of coseismic slip during the main shocks, which increased stress deeper than the coseismic slip zone on the primary fault and synthetic faults to the east, causing the energetic aftershock series. There is no evidence of deformation at the Rungwe volcanic province ∼50 km north of the earthquake sequence between 2007 and 2010, consistent with previous interpretations of no significant magmatic contribution during the sequence.

Fault control on depositional systems and sequence stratigraphic architecture in a multiphase, rifted, lacustrine basin: A case study from the paleogene of the central Bohai Bay Basin, northeast China

Abstract The stratigraphic evolution of rift basins is complicated by out-of-phase displacement on boundary and intrabasinal faults and this analysis of Paleogene strata in the central Bohai Bay Basin of eastern China exemplifies this conundrum. The spatial and temporal distribution of depositional systems and sequences in the central Bohai Bay Basin are evaluated by using seismic reflection data in conjunction with drill cores and well logs. Three second-order composite sequence boundaries are identified on seismic profiles on the basis of truncations below and onlap relationships above and by abrupt changes abrupt changes in gamma-ray values on well logs. Composite sequences are comprised of third-order sequences identified as distinctive seismic packages. Variations in thickness and distribution of third-order sequences are consistent with out-of-phase displacement of boundary and intrabasinal faults which resulted in migration of depocenters in space and time. A range of depositional systems are recognized including fan delta, braid delta, river delta, sublacustrine fan, and shallow and deep lacustrine. During rift-climax stages, lowstand fan deltas developed on the hanging wall adjacent boundary faults to the north, braid deltas on the footwall ramps to the south and sublacustrine fans in the central basin. Following transgression, highstand braid deltas prograded from the north and south whereas river deltas during the late stages of basin fill. Both tectonic activity and climate were important controls on accommodation and the balance between sediment supply and accommodation determined the sedimentary architectures. Faulting, uplift and erosion as well as fault-block rotation controlled the formation of sequence boundaries and sequence geometries whereas variations in sediment flux can be attributed to climate changes. High-quality reservoirs are associated with deltaic channel-fill and sublacustrine fan sandstones. Intrabasinal faults provided pathways for oil migration whereas prodeltaic, lacustrine mudstones are source rocks as well as seals. This study demonstrates that the spatial and temporal distribution of deposystems in rift basins, in general, requires an understanding of the history of boundary and intrabasinal faults within these compartmentalized basins.

Sedimentary architecture during Carboniferous rift initiation – the arid Billefjorden Trough, Svalbard

Analysis of the sedimentary architecture of the early Billefjorden Trough, an exceptionally well-exposed Carboniferous half-graben located in Svalbard, Arctic Norway, allows detailed reconstruction of fault development and basin geometry. Sedimentary facies distribution reveals facies variability in an early synrift fill, when an array of meso-scale intra-basinal faults segmented the basin into rotated blocks with differential subsidence. Growth of these faults into larger systems impacted the distribution of accommodation space and the drainage pattern expressed in fluvial and deltaic sandstone-dominated units. An axially back-stepping character of the early synrift facies associations (Stages 1–3) is reflected in aggrading homogenous facies types of continental depositional Stage 1, which evolved into back-stepping facies belts of deltaic to nearshore depositional environment, Stages 2 and 3, due to the opening of a connection to the sea. The drainage pattern and accommodation distribution in the early synrift deposits reflect a relatively symmetrical basin that predates a later prominent half-graben configuration of the basin. Supplementary material: Figures presenting lithostratigraphic columns of the well data (Supplementary material 1), location of logs in Ragnardalen (Supplementary material 2) and geological map used in Figure 2b (Supplementary material 3) are available at https://doi.org/10.6084/m9.figshare.c.4293077

Sedimentary loading–unloading cycles and faulting in intermontane basins: Insights from numerical modeling and field observations in the NW Argentine Andes

The removal, redistribution, and transient storage of sediments in tectonically active mountain belts is thought to exert a first-order control on shallow crustal stresses, fault activity, and hence on the spatiotemporal pattern of regional deformation processes. Accordingly, sediment loading and unloading cycles in intermontane sedimentary basins may inhibit or promote intrabasinal faulting, respectively, but unambiguous evidence for this potential link has been elusive so far. Here we combine 2D numerical experiments that simulate contractional deformation in a broken-foreland setting (i.e., a foreland where shortening is diachronously absorbed by spatially disparate, reverse faults uplifting basement blocks) with field data from intermontane basins in the NW Argentine Andes. Our modeling results suggest that thicker sedimentary fills (>0.7–1.0 km) may suppress basinal faulting processes, while thinner fills (<0.7 km) tend to delay faulting. Conversely, the removal of sedimentary loads via fluvial incision and basin excavation promotes renewed intrabasinal faulting. These results help to better understand the tectono-sedimentary history of intermontane basins that straddle the eastern border of the Andean Plateau in northwestern Argentina. For example, the Santa María and the Humahuaca basins record intrabasinal deformation during or after sediment unloading, while the Quebrada del Toro Basin reflects the suppression of intrabasinal faulting due to loading by coarse conglomerates. We conclude that sedimentary loading and unloading cycles may exert a fundamental control on spatiotemporal deformation patterns in intermontane basins of tectonically active broken forelands.

Stress evolution and structural inheritance controlling an intracontinental extensional basin: The central-northern sector of the Neogene Teruel Basin

The Teruel Basin is a NNE-SSW trending intracontinental extensional basin located in central-eastern Iberia. It is asymmetrically bounded to the east by a major fault zone, but intrabasinal faults with diverse orientation (NNE-SSW to NE-SW, E-W, or NW-SE) also appear. Offsets of the successive sedimentary units and of two planation surfaces reveal that tectonic activity initiated at the border faults, while intrabasinal ones mainly developed in a later stage. Fractures on a map scale show a prevailing N-S strike in Neogene synrift rocks, while a dense network made of four main fracture sets (NE-SW, E-W to ESE-WNW, N-S and NNW-SSE), likely inherited from Mesozoic rifting stages, is observed in pre-rift units. The results of palaeostress analyses indicate an overall predominance of σ3 directions around E-W, although two stress episodes have been distinguished during the Late Miocene-Pleistocene: (i) triaxial extension with σ3 E-W; (ii) almost ‘radial’ extension (σ1 vertical, σ2 ≈ σ3) with a somehow prevailing σ3 ENE-WSW. A scenario in which the evolving extensional stress field was able to gradually activate major basement structures with different orientation, inherited from previous tectonic events, is proposed as responsible for the evolution and overall pattern of both the eastern active margin and central parts of the central-northern sector of the Teruel Basin.

Constraints on Rift Basin Structure and Border Fault Growth in the Northern Malawi Rift From 3‐D Seismic Refraction Imaging

AbstractWe present new constraints on rift basin structure in the northern Malawi Rift from a 3‐D compressional velocity model to investigate border fault geometry, accommodation zone structure, and the role of preexisting structures underpinning this rift system. The velocity model uses observations from the first wide‐angle refraction study conducted using lake‐bottom seismometers in one of the East African great lakes. The Malawi Rift is flanked by basin‐bounding border faults and crosses several significant remnant structures, making it an ideal location to investigate the development of normal faults and their associated basins. The 3‐D velocity model reveals up to ~5 km of synrift sediments, which smoothly transition from eastward thickening against the Livingstone Fault in the North Basin to westward thickening against the Usisya Fault in the Central Basin. Greater than 4 km of sediment are imaged within the accommodation zone pointing to the early development of the border faults. We use new constraints on synrift sediment thickness to construct displacement profiles for both faults. Both faults accommodate large throws (&gt;7 km) consistent with their significant lengths. The dimensions of these faults are close to or larger than the maximum size predicted by models of fault growth. The presence of an intermediate velocity unit with velocities of 3.75–4.5 km/s is interpreted to represent sediment deposits beneath Lake Malawi from prior rifting in the Permo‐Triassic (Karoo) and/or Cretaceous‐Paleogene. The distribution of preexisting basins implied by these sediments may help account for changes in intrabasinal faulting and border fault development between the two basins.

Analyses of Aeromagnetic Data to Delineate Basement Structures and Reveal Buried Igneous Bodies in Kaladgi Basin, Karnataka

ABSTRACT Aeromagnetic data overcome constrain of inadequate exposures and provide signatures of bodies beneath sediment cover. Present work on analysis of aeromagnetic data over western part of Kaladgi basin provided insight into the basement structures and their role in basin evolution. In the study area, the NW-SE and NE-SW are the major trends of magnetic lineament followed by E-W and N-S trends. Archean to Paleoproterozoic basement is manifested by two structural zones, NW-SE trends related to major lineaments within the basement and the NE-SW trends presumed intra-basinal fault systems which controlled the local depressions. The basin configuration deduced from depth to basement show that the Kaladgi basin is an open deep basin and divided into several sub-basins, separated by fault-controlled NE-SW and NW-SE oriented basement ridges. An intriguing find in the western part are the numerous scattered smaller-scale, circular or semicircular, distinct magnetic anomalies of moderate to strong magnetic signal with strong remenance. Analyses coupled with 3D inversions in combination with sub-surface probing reveal in-homogeneities within basement gneisses and supracrustal rocks of the Kaladgi basin, Dharwar craton. 3D inversions of these circular bodies, suggest that they are apophyses of the intrusions or alternatively as younger intrusive stocks. Sub-surface probing by boreholes over circular bodies revealed leucocratic granite with porphyritic texture emplaced as intrusive within the Chitradurga metasediments. This implies that these intrusives are post-Chitradurga schist and pre Badami sediments as they have not affected the latter. However, they can be presumed to be coeval to potassic granites, which intrude the eastern part of the western Dharwar craton in southern India, until geochronological data are available.

Magnetotelluric study to characterize Kachchh Mainland Fault (KMF) and Katrol Hill Fault (KHF) in the western part of Kachchh region of Gujarat, India

The Kachchh Mainland Fault (KMF) is a major E-W trending fault in the Kachchh region of Gujarat extending >150 km from Lakhpat village in the west to the Bhachau town in the east. The Katrol Hill Fault (KHF) is an E-W trending intrabasinal fault located in the central region of Kachchh Basin and the south of KMF. The western parts of both of the faults are characterized, and the sediment thickness has been estimated in the region using a Magnetotelluric (MT) survey at 17 sites along a 55 km long north-south profile with a site spacing of 2–3 km. The analysis reveals that the maximum sediment thickness is 2.3 km (Quaternary, Tertiary, and Mesozoic) in the region, out of which, the Mesozoic sediments feature a maximum thickness of 2 km. The estimated sediment thickness is found consistent with the thickness suggested by a deep borehole (depth approx. 2.5 km) drilled by Oil and Natural Gas Corporation (ONGC) at Nirona (Northern part of the study area). From 2-D inversion of the MT data, three conductive zones are identified from north to south. The first conductive zone is dipping nearly vertical down to 7–8 km depth. It becomes north-dipping below 8 km depth and is inferred as KMF. The second conductive zone is found steeply dipping into the southern limbs near Manjal village (28 km south of Nirona), which is inferred as the KHF. A vertical-dipping (down to 20 km depth) conductive zone has also been observed near Ulat village, located 16 km north of Manjal village and 12 km south of Nirona village. This conductive zone becomes listric north-dipping beyond 20 km depth. It is reported first time by a Geophysical survey in the region.

ALPINE BASEMENT OUTCROPS AT NORTHERN PELOPONNESUS: IMPLICATIONS FOR THE EARLY STAGES IN THE EVOLUTION OF THE CORINTH RIFT

We focus on the central and eastern southern onshore parts of the Corinth Rift (CR), where basement outcrops are few and of limited extent, to study three key locations: a basement inlier east of Xylokastro, a new outcrop of the Tripolis Unit at the western edge of the Xylokastro horst, and a re-mapped and re-evaluated outcrop of the Pindos Unit northwest of Nemea. The results of this study show that there are elements which could help overcome the significant difficulties in our understanding of the structure and evolution of the rift, posed by the lack of basement outcrops in the central and eastern onshore parts of the Rift. This, in turn, calls for a reconsideration of the generally accepted idea that the broader onshore eastern sector of the CR lacks significant basement and intrabasinal faults.

Identifying Ground-water Resources and Intrabasinal Faults in the Hueco Bolson, West Texas, using Airborne Electromagnetic Induction and Magnetic-field Data

We conducted a high-resolution airborne geophysical survey at the eastern margin of the basin-and-range province of the southwestern U.S. to examine the hydrostratigraphy of the Hueco Bolson and identify possible ground-water resources in this arid area. The survey, flown in 2001 over a 372-km2 area east of El Paso, Texas, acquired time-domain electromagnetic induction (TDEM) and magnetic field data. These data were used to map conductivity trends to depths of at least 200 m that are related to lateral and vertical changes in lithology, water content, water chemistry (EM data), basin geometry, and the location of intrabasinal faults (magnetic data). Pre-survey, ground-based TDEM soundings established the achievable exploration depths and demonstrated that the relatively deep groundwater (80 to 120 m) significantly influenced the transient signal and was within the exploration depth of the airborne system. Airborne EM and magnetic field data identified intrabasinal faults that influence basin-fill deposition. Conductivity-depth slices constructed from airborne TDEM data allowed lateral variations in water quality and lithology to be mapped that helped predict ground-water resource quality within these fault-bounded basins filled with alluvial, lacustrine, and eolian sediments.

Evolution of the alluvial fans of the Luo River in the Weihe Basin, central China, controlled by faulting and climate change - A reevaluation of the paleogeographical setting of Dali Man site

The Luo River is located in the southern part of the Chinese Loess Plateau and the northern part of the Weihe Basin, in Central China. In the basin it flows proximal to the site of the Luyang Wetland core, which is an important archive of climate change over the past 1 Myr in this region. In this paper, the contribution of the Luo River to the sedimentary record is analyzed by reconstructing the evolution of this river during the Middle to Late Pleistocene. It is argued that an alluvial fan of the Luo River has contributed to the sedimentary archive until approximately 200–240 ka. From this moment onwards, the fan became incised and terraces began to form. The formation of a new alluvial fan further downstream led to the disconnection of the Luo River from the Luyang Wetland core site. We propose that this series of events was caused by the displacement of an intra-basinal fault and the resultant faulting-forced folding, which caused increased relative subsidence, and thus increased sedimentation rates at the core site. Therefore, a complete sediment record in the ‘Luyang Wetland’ was preserved, despite the disconnection from the Luo River.The chronology of the fans and terraces was established using existing age control (U-series, ESR, OSL, pIRIR290 and magnetic susceptibility correlation), and through correlation of the loess-paleosol cover to marine isotope stages. Based on sedimentological characteristics of the fluvial sequence, we suggest that incision of the Luo River occurred in two steps. Small incisions took place at transitions to interglacials and the main incision phases occur at the transition from an interglacial to glacial climate.Due to the incision, basal parts of the oldest Luo River alluvial fan are exposed, and it is in one of these exposures that the famous Dali Man skull was retrieved. This study shows that the Dali Man did not live on a river terrace as previously thought, but on an aggrading alluvial fan, during wet, glacial conditions.

Tectonic controls on fault zone flow pathways in the Rio Grande rift, New Mexico, USA

We assessed tectonic controls on the spatial and temporal distribution of fault zone flow pathways in the Rio Grande rift (New Mexico, USA) by using fault zone calcite cements as a geochemical record of syntectonic fluid flow. Cement δ 18 O, δ 13 C, and 87 Sr/ 86 Sr values indicate that older, large-displacement master and basin-margin faults were cemented by more isotopically evolved basinal brines than younger intrabasin faults. These data suggest that diagenetic fluids in basin-bounding faults equilibrated predominantly with downdip Paleozoic carbonates. In contrast, intrabasin faults transmitted fluids from shallow stratigraphic sources. This pattern of flow pathways is linked to the systematic distribution of sediments and faults that record rift evolution, which dictated spatial and temporal variations in fault zone architecture and permeability structure. Our results indicate that the depths from which fluids can be transported in active rift basins ultimately depend on both tectonically mediated variations in the grain size of syntectonic sediments entrained in fault damage zones and fault displacement magnitude.

Embryonic core complexes in narrow continental rifts: The importance of low-angle normal faults in the Rio Grande rift of central New Mexico

The Rio Grande rift in central New Mexico provides an excellent location to study the interaction between high-angle and low-angle (15°–35°) normal faults during crustal extension. Here we evaluate the relative importance of low-angle normal faults (LANFs) in the Albuquerque basin of central New Mexico with goals of testing two conflicting models of rift geometry and producing evolutionary models for the northern and southern parts of the basin. Using physiographic relationships, field observations, structural data analysis, and thermal history modeling, we document two brittle LANF systems on salients in adjacent opposite-polarity half-grabens. These fault systems were both active ca. 20–10 Ma and are locations of maximum fault slip as indicated by thickness of sedimentary fill in adjacent sub-basins and highest elevation rift flanks. Average fault dip increases basinward, and outbound faults were abandoned while intrabasinal faults cut Quaternary units, supporting an evolutionary model where master normal faults initiated at a higher dip, were shallowed by isostatic footwall uplift in regions of highest slip, and became inactive while younger normal faults emerged basinward. These geometrical and kinematic observations are predicted by the rolling-hinge model for the formation of LANFs. This mechanism has been widely applied to core complexes in highly extended terranes (e.g., Basin and Range), regions of orogenic collapse, and mid-ocean ridges, and it is shown here to also be applicable to narrow continental rifts of modest (∼35%) extension. Similarities to core complexes include a physiographic expression of domal uplifts, evolution of a master detachment horizon that initiated as a breakaway, and isostatically rotated low-angle normal faults. Although the degree of extension was too low to juxtapose ductile footwall rocks against brittle hanging-wall rocks, if extension had progressed in the Albuquerque basin, eventually a mature metamorphic core complex would have formed, similar to those preserved in the adjacent Basin and Range Province. The Rio Grande rift, therefore, provides a snapshot of the embryonic stages of core complex formation, bridging the gap between mature core complexes and incipient extensional environments.

Regional setting and correlation of exploration results for the Naarst uranium deposit Dornogobi Province, southeast Mongolia

The Naarst uranium deposit lies within an extensive zone of uranium anomalies extending for many kilometres within the Sainshand Basin graben in the southeastern Gobi region in Mongolia. The sedimentary units of the graben filling, Late Cretaceous age, Sainshand-Binshiren (Cenomanian to Santonian) packages seem to be the predominant host for the uranium. Locally, roll front enrichment has occurred, and some resources are hosted and enriched along fault zones where oxidized groundwater has circulated during ongoing intra-basinal fault movement along fault lines cutting the graben basin. This area was extensively explored from the late 1970s to the early 1980s by Soviet-Russian expeditions. During the period of 2007 to 2010, field mapping, carborne radiometric surveying and diamond drilling were carried out over much of the Naarst occurrence to determine the potential of the area for economic uranium mineralization. The best result from the 2008 exploration was from drill hole OR02 at an intersection depth of 175 to 180 m, which consists of sandstone alternations of fine- to coarse-grained sandstone with thin layers of clay stone, grey and occasionally pink, also with clay–mudstone layers, rich in organic material (plant trash) and pyrite disseminations. Soviet exploration results at Naarst were broadly confirmed in terms of the presence of anomalism and uranium mineralization distribution and the occurrence of uranium within particular rock types. There was not, however, the same confirmation of the depth and uranium grade of the mineralized intersections, along those drill sections chosen to test historic and recent results.