Active Rift Setting Research Articles

Tipping the balance: Shifts in sediment production in an active rift setting

Landscapes in actively developing rifts respond to tectonic forcing over a similar time scale to that of fault array evolution (i.e., 105–106 yr). Consequently transient landscapes (i.e., not in topographic steady state) predominate, characterized by focused incision along extensional fault scarps and regional tectonic tilting of surface slopes across strike. Using a field-calibrated numerical model to explore the controls on landscape evolution across the Corinth rift, central Greece, we demonstrate that this tilting, although subtle, leads to a shift in dominant source area as well as a shift toward sediment-starved conditions within the basin. We show, by comparing model runs with and without imposing tectonic forcing, that the impact of active faulting on relief development along the most active Corinth rift margin locally increases erosion rates and footwall incision. However, the overall sediment flux from this margin is reduced because back-tilting lowers erosion rates in catchment headwaters. Conversely, the hanging-wall side of the rift, as it is downwarped, supplies relatively more sediment as rift-directed channel slopes increase even though the relief is decreasing. In summary, we show that tilting plays a key role in controlling the syn-rift sediment flux and, in a counterintuitive way, modifies the relationship between topographic relief and catchment-averaged erosion rates. Our results provide a new perspective on the origin and timing of sediment starvation relative to structural development in rifts.

Source‐to‐sink analysis in an active extensional setting: Holocene erosion and deposition in the Sperchios rift, central Greece

AbstractWe present a source‐to‐sink analysis to explain sediment supply variations and depositional patterns over the Holocene within an active rift setting. We integrate a range of modelling approaches and data types with field observations from the Sperchios rift basin, Central Greece that allow us to analyse and quantify (1) the size and characteristics of sediment source areas, (2) the dynamics of the sediment routing system from upstream fluvial processes to downstream deposition at the coastline, and (3) the depositional architecture and volumes of the Holocene basin fill. We demonstrate that the Sperchios rift comprises a ‘closed’ system over the Holocene and that erosional and depositional volumes are thus balanced. Furthermore, we evaluate key controls in the development of this source‐to‐sink system, including the role of pre‐existing topography, bedrock erodibility and lateral variations in the rate of tectonic uplift/subsidence. We show that tectonic subsidence alone can explain the observed grain size fining along the rift axis resulting in the downstream transition from a braided channel to an extensive meander belt (>15 km long) that feeds the fine‐grained Sperchios delta. Additionally, we quantify the ratios of sediment storage to bypass for the two main footwall‐sourced alluvial fan systems and relate the fan characteristics to the pattern and rates of fault slip. Finally, we show that ≥40% of the sediment that builds the Sperchios delta is supplied by ≤22% of the entire source area and that this can be primarily attributed to a longer‐term (~106 years) transient landscape response to fault segment linkage. Our multidisciplinary approach allows us to quantify the relative importance of multiple factors that control a complex source‐to‐sink system and thus improve our understanding of landscape evolution and stratigraphic development in active extensional tectonic settings.

Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study

Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02–75mg/L. 73% exceed the health limit (1.5mg/L) in both dry and wet seasons. F− concentrations in rivers are lower (0.2–9.2mg/L) than groundwater (0.09 to 43.6mg/L) while saline lake waters have the highest concentrations (0.27–75mg/L). The higher values are confined to elevations below 2000masl. Oxygen (δ18O) and hydrogen (δD) isotopic values range from −6.2 to +5.8‰ and −31.3 to +33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750–6000ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (<1000ppm). Thus, geochemical F− enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management in Naivasha as well as similar active rift setting environments.

The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

This paper presents combined U/Pb, Th/U and Hf isotope analyses on detrital and magmatic zircon grains together with whole-rock geochemical analyses of two basement and eight sedimentary rock samples from the Namuskluft and the Dreigratberg in southern Namibia (Gariep Belt). The sedimentary sections evolved during the Cryogenian on the SW part of the Kalahari Craton and where therefore deposited in an active rift setting during the break-up of Rodinia. Due to insufficient palaeomagnetic data, the position of the Kalahari Craton within Rodinia is still under discussion. There are possibilities to locate Kalahari along the western side of Australia/Mawsonland (Pisarevski et al. in Proterozoic East Gondwana: supercontinent assembly and break-up, Geological Society, London, 2003; Evans in Ancient Orogens and modern analogues. Geological Society, London, 2009; and others) or together with the Congo-Sao Francisco and Rio de la Plata Cratons (Li et al. in Prec Res 45: 203–2014, 2008; Frimmel et al. in Int J Earth Sci (Geol Rundsch) 100: 323–354, 2011; and others). It is sill unclear which craton rifted away from the Kalahari Craton during the Cryogenian. Although Middle to Upper Cryogenian magmatic activity is known for the SE Kalahari Craton (our working area) (Richtersveld Suite, Rosh Pinah Fm), all the presented samples show no U/Pb zircon ages younger than ca. 1.0 Ga and non-older than 2.06 Ga. The obtained U/Pb ages fit very well to the exposed basement of the Kalahari Craton (1.0–1.4 Ga Namaqua Province, 1.7–2.0 Ga Vioolsdrif Granite Suite and Orange River Group) and allow no correlation with a foreign craton such as the Rio de la Plata or Australia/Mawsonland. Lu–Hf isotopic signatures of detrital zircon point to the recycling of mainly Palaeoproterozoic and to a smaller amount of Archean crust in the source areas. eHf(t) signatures range between −24 and +14.8, which relate to TDM model ages between 1.05 and 3.1 Ga. Only few detrital zircon grains derived from magmas generated from Mesoproterozoic crustal material show more juvenile eHf(t) signatures of +14, +8 to +4 with TDM model ages of 1.05–1.6 Ga. During Neoproterozoic deposition, only old cratonic crust with an inherited continental arc signature was available in the source area clearly demonstrated by Hf isotope composition of detrital zircon and geochemical bulk analysis of sedimentary rocks. The granodiorites of the Palaeoproterozoic basement underlying Namuskluft section are ca. 1.9 Ga old and show eHf(t) signatures of −3 to −5.5 with TDM model ages of 2.4–2.7 Ga. These basement rocks demonstrate the extreme uplift and deep erosion of the underlying Kalahari Craton at its western margin before general subsidence during Cryogenian and Ediacaran time. The sedimentary sequence of the two examined sections (Namuskluft and Dreigratberg) proposes the presence of a basin and an increasing subsidence at the SW part of the Kalahari Craton during the Cryogenian. Therefore, we propose the initial formation of an intra-cratonic sag basin during the Lower Cryogenian that evolved later to a rift basin at the cratonic margin due to increasing crustal tension and rifting together with the opening of the Adamastor Ocean. As the zircons of the sedimentary rocks filling this basin show neither rift-related U/Pb ages nor an exotic craton as a possible source area, the only plausible sedimentary transport direction providing the found U/Pb ages would be from the E or the SE, directly from the heart of the Kalahari Craton. Due to subsidence and ongoing sedimentation from E/SE directions, the rift-related magmatic rocks were simply covered by the input of old intra-cratonic material that explains the absence of Neoproterozoic zircon grains in our samples. The geochemical analyses show the erosion of a continental arc and related sedimentary rocks with an overall felsic provenance. The source area was a deeply eroded and incised magmatic arc that evolved on continental crust, without any evidence for a passive margin. All of this can be explained by the erosion of rocks related to the Namaqua Belt, which represents one of the two major peaks of zircon U–Pb ages in all analysed samples. Therefore, the Namaqua Belt was well exposed during the Cryogenian, available to erosion and apart from the also well-exposed Palaeoproterozoic basement of the Kalahari Craton one potential source area for the sedimentary rocks in the investigated areas.

Interaction between Downslope and Alongslope Processes on the Margins of Daihai Lake, North China: Implication for Deltaic Sedimentation Models of Lacustrine Rift Basin

Abstract:Daihai Lake, a modern lacustrine rift basin, located in Inner Mongolia, North China, serves as an important modern analog for understanding deltaic depositional processes in an active rift setting. Two of the deltas (Yuanzigou delta and Bulianghe delta) on the margins of Daihai Lake were surveyed to compare and contrast stacking patterns using aerial photographs, field trenching and sediment sampling. Shallow cores and trench data collected from the margins of Daihai Lake indicate that a variety of depositional processes have been active since Daihai Lake formed. Two 3‐D sedimentation models which employ chronostratigraphic correlation technique were generated. The chronostratigraphic sedimentation models predict and represent the architectures and sand‐body continuity of sediments. Stratigraphical coincidence of the broad sheeted drifts and channel erosion suggests a coupling between downslope and alongslope processes. Distributary mouth bars are prevalent in the front of deltas on steeper slopes due to the dominance of down‐slope flows. On the contrary, the along‐slope currents favor the development of distal bar deposits with sheeted sandbodies on gentle depositional slopes. This study provides an insight into the architecture of complex sedimentary facies associated with highlighting key differences between downslope flows and alongslope currents. The distribution of sand within these deltas is of particular interests, with applications in understanding the architecture of hydrocarbon reservoirs formed in lacustrine rift basin.

Tide- and wave-generated fluid mud deposits in the Tilje Formation (Jurassic), offshore Norway

Research Article| June 01, 2009 Tide- and wave-generated fluid mud deposits in the Tilje Formation (Jurassic), offshore Norway Aitor A. Ichaso; Aitor A. Ichaso 1Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada Search for other works by this author on: GSW Google Scholar Robert W. Dalrymple Robert W. Dalrymple 1Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (6): 539–542. https://doi.org/10.1130/G25481A.1 Article history received: 28 Aug 2008 rev-recd: 16 Jan 2009 accepted: 22 Jan 2009 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Aitor A. Ichaso, Robert W. Dalrymple; Tide- and wave-generated fluid mud deposits in the Tilje Formation (Jurassic), offshore Norway. Geology 2009;; 37 (6): 539–542. doi: https://doi.org/10.1130/G25481A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Tilje Formation (Early Jurassic; 120–300 m thick) consists predominantly of heterolithic deposits and is thought to have accumulated in tide-dominated estuarine and deltaic environments in an active rift setting. Anomalously thick (>0.5 cm) and internally structureless mudstone layers, which are interpreted to represent fluid-mud deposits, are widespread and occur in three different environmental settings: (1) in the basal part of upward-fining tidalfluvial channels where they generate upward-sanding successions: (2) in the deposits of mouth bars and terminal distributary channels where they are associated with the coarsest sands and the least-bioturbated sediments, suggesting deposition during tidally modulated river floods; and (3) in delta-front successions where they immediately overlie thick, wave-generated storm beds, suggesting that these fluid-mud deposits result from wave resuspension of previously deposited mud. These observations provide criteria for the recognition of ancient fluid-muds and for interpreting their origin. The tectonic setting may be responsible for their abundance. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sedimentology of an intra-montane rift-controlled fluvial dominated succession: The Upper Triassic Oukaimeden Sandstone Formation, Central High Atlas, Morocco

Triassic successions in the High Atlas of Marrakech (Morocco) provide excellent outcrop analogues for continental fluvial systems within an intra-montane rift basin and allow the study of facies distribution and controls on deposition. This paper focuses on the analysis of the Oukaimeden Sandstone Formation (F5), a fluvial dominated formation deposited in an active rift setting. Combination of traditional sedimentological field analysis with modern digital data capture techniques (e.g. spectral gamma-ray, LIDAR terrestrial scanner imaging) allows a detailed description and interpretation of the facies. The Oukaimeden Sandstone Formation is composed of channel sandstone bodies alternating with lenticular shaped fluvial bar sandstones and overbank mudstone deposits. Alternating with the fluvial facies, aeolian sandstones and alluvial fan deposits are also observed. Changes in architectural style are used to subdivide the formation into three members. The lower member (Lower Oukaimeden ) was deposited by an ephemeral braided system. The middle member (Middle Oukaimeden) records a change to perennial braided fluvial conditions in response to tectonics combined with a change in climate towards more humid conditions. The upper member (Upper Oukaimeden) is characterized by the return to ephemeral conditions, which combined with the occurrence of aeolian dunes, is interpreted to record increased aridity. The upper part of the member exhibits tidal influence, related to the first marine incursion into the basin. The Oukaimeden Sandstone Formation provide a well documented outcrop example of deposition within an intra-montane setting influenced by a combination of tectonic and climatic controls.

Sr–Nd–Pb isotopic constraints on multiple mantle domains for Mesozoic mafic rocks beneath the South China Block hinterland

The Sr–Nd and Pb isotopic compositions of middle Jurassic to late Cretaceous mafic rocks across the Chenzhou–Linwu Fault within the South China Block are presented to constrain the characteristics and evolution of Mesozoic mantle source. These data, in combination with geochronological results, can be divided into four groups. Group 1 (~ 175–170 Ma) occurs on either side of the Chenzhou–Linwu Fault and has moderately depleted Sr–Nd and radiogenic Pb isotopes, akin to a FOZO-like source and is inferred to have originated from deep asthenospheric mantle with delaminated lower crustal signatures. Group 2 (~ 175–145 Ma) is restricted to the west of the fault and shows a significant Dupal anomaly and a binary mixing array between a component from an enriched mantle source (EM1) with low ε Nd( t) values and 206Pb/ 204Pb, and intermediate 87Sr/ 86Sr ratios and a FOZO-like component characterized by Group 1. Group 3 (~ 175–125 Ma) occurs to the east of the fault and exhibits low ε Nd( t) values and high Sr–Pb isotopic ratios, similar to EM2 component with minor DMM component. The Sr–Nd–Pb isotopic systematics for Group 4 (~ 93–80 Ma) resembles those of SE China Cenozoic basalts, suggestive of DMM-EM2 reservoir in response to the uppermost asthenospheric mantle source. These temporal and spatial variations, together with other geologic and paleomagnetic data, suggest that (1) the multiple mantle reservoirs include FOZO-, DMM-, EM1- and EM2-like components beneath the South China Block; (2) DMM-EM2 sources have dominated the mantle domains beneath SE Asia since late Cretaceous; (3) the Chenzhou–Linwu Fault corresponds with the early Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks within the South China Block. The Mesozoic mafic rocks within the South China Block developed in an active rift setting in response to the delamination of partially over-thickened lithospheric keel and upwelling of asthenosphere rather than a supra-subduction environment.

Extreme oxygen isotope signature of meteoric water in magmatic zircon from metagranite in the Sulu orogen, China: Implications for Neoproterozoic rift magmatism

Unusual 18O depletion, with δ 18O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ 18O zircons to be distinguished between magmatic and metamorphic origins, because the 18O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ 18O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual 18O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ 18O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ 18O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ 18O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U–Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ 18O zircons are interpreted as crystallizing from negative δ 18O magmas due to melting of meteoric-hydrothermally altered negative δ 18O rocks in an active rift setting at about 780 Ma. The variation in zircon δ 18O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu–Hf isotope analyses give positive ε Hf( t) values of 1.6–4.1 and Hf model ages of 1.18–1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ 18O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ 18O zircons may register centers of low δ 18O magmatism during the supercontinental rifting.

The chemical and hydrological evolution of the Mulhouse potash basin (France): Are “marine” ancient evaporites always representative of synchronous seawater chemistry?

Brine reaction processes were the most important factors controlling the major-ion (Mg, Ca, Na, K, SO 4, and Cl) evolution of brines in the Oligocene, Mulhouse basin (France) evaporite basin. The combined analysis of fluid inclusions in primary textures by Cryo-SEM-EDS with sulfate- δ 34S, δ 18O and 87Sr/ 86Sr isotope ratios reveals hydrothermal inputs and recycling of Permian evaporites, particularly during advanced stages of evaporation in the Salt IV member. The lower part of the Salt IV evolved from an originally marine input. The basin was disconnected from direct marine inputs and a series of sub-basins formed in an active rift setting where tectonic variations influenced sub-basin interconnections and chemical signatures of input waters. Sulfate- δ 34S shows Oligocene marine-like signatures at the base of the member. However, enriched sulfate- δ 18O reveals the importance of synchronous re-oxidation processes. As evaporation progressed other non-marine and/or marine-modified inputs from neighbouring basins became more important. This is demonstrated by increases in K concentrations in brine inclusions and Br in halite, sulfate isotopes trends and 87Sr/ 86Sr ratios. The recycling of previously precipitated evaporites of Permian age was increasingly important with evaporation. This supports the connection of the Mulhouse basin to basins situated north of Mulhouse. The brine evolution eventually reached sylvite precipitation. The chemical signature of the resulting brines is not compatible with global seawater chemistry changes. The fast rate of intra and inter basin brine variations as well as the existence of contemporaneous brines with different chemical signatures, supports our interpretation. The existence of diverse non-marine inputs and associated internal chemical changes to the brine preclude the use of trapped-brine inclusions in reconstructing Oligocene seawater chemistry, without previously identifying all inputs. The general hydrological evolution of the Mulhouse basin is explained as a restricted sub-basin with a first marine stage. This gradually changed to a ~ 40% marine source at the beginning of evaporite precipitation, with the rest of inputs non-marine. The general proportion of solutes did not change greatly over evaporite precipitation. However, as the basin restriction increased the originally marine inputs changed to continental or marine-modified inputs from neighbouring basins north of Mulhouse basin.

Coarse-grained, deep-water sedimentation along a border fault margin of Lake Malawi, Africa; seismic stratigraphic analysis

Lake Malawi, Africa, serves as an important modern analog for understanding depositional processes in an active rift setting. This paper explores the depositional processes and stacking patterns of coarse-grained facies across a large fan delta and associated sub-lacustrine fan system offshore of the South Rukuru River using single-channel seismic data coupled with deep-water vibracores and gravity cores. A companion paper (Wells et al. 1999) discusses the details of the depositional processes of the system inferred from core data. Using 2900 km of single-channel seismic data, eight genetic facies were defined based on seismic reflection character, external geometry, accompanying core data, and location within the modern geomorphic system. Five sand-dominated facies include; (1) mouth-of-canyon-fan facies; (2) progradational-fan-delta facies; (3) channel-fill facies; (4) canyon-fill facies; and (5) basin-plain-fill facies. These facies occur as surficial deposits in water depths of more than 600 m, suggesting that coarse-grained sedimentation characterizes certain localities, in even the deepest parts of Lake Malawi. Climatically driven lake-level changes influenced the internal character of these facies, their thickness, and to some extent their location within the South Rukuru study area. However, climate was generally subordinate to tectonism in controlling the temporal and spatial distribution of the facies. Tectonic controls can be categorized as; (1) orientation of the first-order border fault and intrabasinal faults; (2) syndepositional footwall uplift and rotation; and (3) temporal changes in the rate and location of maximum subsidence. These controls affect the location and stacking of the mouth-of-canyon fans, and the presence and orientation of sub-lacustrine canyons and the zone of maximum sediment accumulation, and depositional gradients.

Anoxic-oxic cyclical lacustrine sedimentation in the Miocene Rubielos de Mora Basin, Spain

Summary The Rubielos de Mora Basin is a Miocene half-graben (3 × 10 km) with a sedimentary fill of over 600 m of alluvial and lacustrine sequences. Three main units make up this basin infill: lower (alluvial) and middle and upper lacustrine units. The upper lacustrine unit is mainly formed in the western part of the basin, by cyclical sequences. These sequences consist of organic-poor, non-laminated mudstones and marls cyclically alternating with thinly laminated facies such as sandy mudstones, bioclastic laminae, oil shales, rhythmites (carbonate-clay, varve-like couplets) and marls. Low-Mg calcite and non-stoichiometric (Mg-poor) dolomite are the dominant carbonate minerals in the non-laminated facies, whereas variable amounts of low-Mg calcite, high-Mg calcite, aragonite and non-stoichiometric dolomite have been recorded in most of the laminated facies. Low-Mg calcite is the main carbonate mineral in sandy mudstones and bioclastic laminae which occur at the lower part of laminated intervals in the cycles. Analysis of oil shales reveals a major macrophyte contribution of organic matter, displaying an early diagenetic evolutionary stage. Cyclical sequences record alternating oxic-anoxic bottom conditions in marginal zones of a meromictic lake, due to cyclical changes in lake water volume. This feature is recorded in the cyclical sequences of mineralogical changes and facies transitions. Strong tectonic subsidence (not balanced by sedimentation and taking place in an active rift setting) and palaeoclimate were the most striking features which could favour the establishment of a meromictic lake during this stage of the basin history.