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

We 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.     

Tectonic control of fan size: the importance of spatially variable subsidence rates

We study the geophysical controls on the size of alluvial fans. Simple relationships between catchment characteristics, sediment yield, subsidence patterns and fan size are developed. As predicting fan size is essentially a conservation of mass problem, our analysis is general, applying to all types of fan landform. The importance of spatially variable subsidence rates has gone largely unrecognized in previous studies of modern fans. Here we stress that the distribution of subsidence rates in the depositional basin is a primary control on relative fan size. Both free coefficients in the oft-cited power-law correlation of fan area and catchment area can be shown to be set primarily by the tectonic setting, taken to include source area uplift rate and the subsidence distribution in the depositional basin. In the case of a steady-state landscape, relative fan size is shown to be independent of both climate and source lithology; only during times of significant departure from steady state can relative fan size be expected to vary with either climate or source lithology. Transients associated with (1) a sudden increase in rock uplift rate, (2) a sudden change in climate and (3) the unroofing of strata with greatly differing erodibilities may produce variation of relative fan areas with both climate and source lithology. Variation of relative fan size with climate or lithology, however, requires that catchment–fan system response to perturbations away from steady state is sensitive to climate and lithology. Neither the strength of transient system responses nor their sensitivity to climate or lithology are known at present. Furthermore, internal feedbacks can significantly dampen any climatic or lithological effect. Thus theoretical considerations of the importance of climatic and lithological variables are inconclusive, but suggest that climatic and lithological effects are probably of secondary importance to tectonic effects. Field data from an unsteady landscape in Owens Valley, California, support and illustrate theoretical predictions regarding tectonic control of fan size. Field data from Owens Valley allow, but do not prove, a secondary dependence on source lithology. In addition, the Owens Valley field data indicate no relationship between relative fan size and climate. Headward catchment growth and enhanced sediment bypassing of fans during times of increased sediment yield (glacial) are put forward as plausible explanations.     

Stratigraphic inversion of siliciclastic basin fills: a note on the distinction between supply signals resulting from tectonic and climatic forcing

Rates of accommodation and sediment supply are the principal controls on stacking patterns in siliciclastic basin fills. Stratigraphic inversion is aimed at reconstruction of these controls from the detrital record. Efforts to ‘explain’ siliciclastic basin fills have been focused on analysis and numerical modelling of sequence geometry in response to changes in accommodation, whereas comparatively few studies have attempted to address the role of sediment supply. The compositional and textural properties of siliciclastic basin fills are linked with the evolution of drainage basins through the principle of climatic–physiographic control of sediment production and supply. Application of this principle leads to a method of compositional analysis for distinguishing sequences controlled by high-frequency changes in the rate of accommodation from sequences controlled by high-frequency variations in the rate of sediment supply (order of 10 kyr). This method does not require detailed time control. Changes in rate and type of sediment supplied to depositional systems in response to environmental perturbations in drainage basins are explored in greater detail by means of a numerical model of sediment production under various scenarios of climatic and tectonic forcing. Simulation experiments suggest that drainage basins respond differently to high-frequency tectonic and climatic perturbations. Synthetic time series of cyclically forced sediment production display different types of asymmetric variations in grain size, accumulation rate and residence time of sediments in response to tectonic and climatic forcing. The results also highlight the role of vegetation as the principal modulator of climate forcing, and show that the nonlinear response to climate change may frustrate any attempts at providing broad generalizations of the system's responses. The modelling results confirm the usefulness of a combined analysis of sediment composition and sequence geometry, and the mathematically rich behaviour of the system suggests that further development of this approach is likely to increase our ability to reconstruct forcing mechanisms and initial boundary conditions from the detrital record.     

Sediment supply from landslide-dominated catchments: implications for basin-margin fans

The sediment flux from a mountainous catchment can be expressed as a function of a landslide rate constant κ which accounts for the vigour of hillslope erosion. Since the incising drainage network flushes all or a portion of the products of hillslope erosion to a range front where fan deposition takes place, a conservation of solid sediment volume allows the fan area and progradation distance to be calculated. These parameters are related primarily to the discharge of sediment from the catchment and to local tectonic subsidence.
A survey of modern alluvial fans in a wide range of climatic and tectonic settings shows that the effects of climate and bedrock lithology cannot be discriminated in the scatter of data of catchment area vs. fan area. However, by focusing on over 100 fans in the arid and semiarid zone of SW USA, the impact of tectonic subsidence rate is unambiguous. Although further quantitative data on local tectonic subsidence rates are urgently required, our preliminary analysis suggests considerable potential for reconstructing palaeocatchments where basin tectonic subsidence rates can be estimated. The progradation distances of fans from the northern and southern margins of the Middle Devonian Hornelen Basin of Norway, and the western and north-eastern margins of the Mio-Pliocene Ridge Basin, California, allow catchment sizes and denudation rates to be approximated. Although unique solution sets are not possible, an iteration of parameter values allows plausible parameter combinations to be calculated which shed light on the tectonic and sedimentary history of the proximal basin and upland source regions. Model results suggest significant asymmetry in basin subsidence rates, catchment slopes and transport mechanics between the two margins.     

Sedimentary record of tectonic and climatic erosional perturbations in an experimental coupled catchment‐fan system

This article deals with the stratigraphic record of a climatic or tectonic perturbation of an experimental coupled catchment‐fan system. Following Bonnet & Crave's results (2003), which suggest that it is possible to differentiate between climatic or tectonic causes of surface uplift of an erosional topography from the record of sediment flux output, we design a new experimental device to test this proposition in the sedimentary signal. This device allows the study of a coupled erosion–sedimentation system at the laboratory scale for given and changing uplift and rainfall rates. On the basis of experimental results, we propose a methodology to study alluvial fan architecture from large‐scale geometries to stacking pattern and sequence analysis. In these experiments, the erosional perturbation resulting from climate or tectonic forcing induces a typical dynamic in terms of both sediment supply and the ratio between the sediment and water supply, which controls the transport capacity. The four possible forcings (rainfall rate and uplift rate increase or decrease, respectively) then result in unique dynamics of the combined parameters such as the fan slope, apex aggradation, mean sedimentation rate, grain size distribution, bed thickness and frequency and facies stacking. We first analyse large‐scale geometries (onlap, toplap, downlap or truncation) and then fine‐scale sedimentological features (fining, thinning, coarsening, thickening) in order to discriminate the nature of the forcing. This conceptual model could be adapted to real world alluvial fans in order to recognize and separate the driving mechanisms from each other.     

Source‐to‐sink analysis of ancient sedimentary systems using a subsurface case study from the Møre‐Trøndelag area of southern Norway: Part 2 – sediment dispersal and forcing mechanisms

The composition, volume and stratigraphic organisation of submarine fan systems deposited along continental margins are expected to reflect the landscape from which the sediment was derived. During the Late Cretaceous, the Møre‐Trøndelag margin, Norwegian North Sea was dominated by the deposition of deep‐marine fines; the emplacement of 11 sand‐rich submarine fan systems occurred only during a c. 3 Myr period in the Turonian‐Coniacian. The systems were fed by sediment that was routed through submarine canyons incised into the basin margin; the canyons are underlain by angular unconformities and are interpreted to have resulted from tectonically induced changes in slope physiography and erosion by gravity flows. The areal extent of the onshore drainage catchments that supplied sediment to the fans has been estimated based on scaling relationships derived from modern source‐to‐sink systems. The results of our study suggest that the Turonian fans were sourced by drainage catchments that were up to ca.3600 km², extending more than ca.100 km inland from the palaeo‐shoreline. The estimated inboard catchment extent correlates with the innermost structures of a large, long‐lived, basement‐involved, normal fault complex. On the basis of our analysis, we conclude that increased sediment supply to the Turonian fan systems reflects tectonic rejuvenation of the landscape, rather than eustatic sea‐level or climate fluctuations. The duration of fan deposition is thus interpreted to reflect the ‘relaxation time’ of the landscape following tectonic perturbation, and fan system retrogradation and abandonment is interpreted to reflect the eventual depletion of the onshore sediment source. We demonstrate that a better understanding of the stratigraphic variability in deepwater depositional systems can be gained by taking a complete source‐to‐sink view of ancient sediment dispersal systems.     

Impact of recycling and lateral sediment input on grain size fining trends—Implications for reconstructing tectonic and climate forcings in ancient sedimentary systems

Grain size trends in basin stratigraphy are thought to preserve a rich record of the climatic and tectonic controls on landscape evolution. Stratigraphic models assume that over geological timescales, the downstream profile of sediment deposition is in dynamic equilibrium with the spatial distribution of tectonic subsidence in the basin, sea level and the flux and calibre of sediment supplied from mountain catchments. Here, we demonstrate that this approach in modelling stratigraphic responses to environmental change is missing a key ingredient: the dynamic geomorphology of the sediment routing system. For three large alluvial fans in the Iglesia basin, Argentine Andes we measured the grain size of modern river sediment from fan apex to toe and characterise the spatial distribution of differential subsidence for each fan by constructing a 3D model of basin stratigraphy from seismic data. We find, using a self‐similar grain size fining model, that the profile of grain size fining on all three fans cannot be reproduced given the subsidence profile measured and for any sediment supply scenario. However, by adapting the self‐similar model, we demonstrate that the grain size trends on each fan can be effectively reproduced when sediment is not only sourced from a single catchment at the apex of the system, but also laterally, from tributary catchments and through fan surface recycling. Without constraint on the dynamic geomorphology of these large alluvial systems, signals of tectonic and climate forcing in grain size data are masked and would be indecipherable in the geological record. This has significant implications for our ability to make sensitive, quantitative reconstructions of external boundary conditions from the sedimentary record.     

Distinguishing tectonic from climatic controls on range-front sedimentation

Geologic and chronometric studies of alluvial fan sequences in south-central Australia provide insights into the roles of tectonics and climate in continental landscape evolution. The most voluminous alluvial fans in the Flinders Ranges region have developed adjacent to catchments uplifted by Plio-Quaternary reverse faults, implying that young tectonic activity has exerted a first-order control on long-term sediment accumulation rates along the range front. However, optically stimulated luminescence (OSL) dating of alluvial fan sequences indicates that late Quaternary facies changes and intervals of sediment aggradation and dissection are not directly correlated with individual faulting events. Fan sequences record a transition from debris flow deposition and soil formation to clast-supported conglomeritic sedimentation by ∼30 ka. This transition is interpreted to reflect a landscape response to increasing climatic aridity, coupled with large flood events that episodically stripped previously weathered regolith from the landscape. Late Pleistocene to Holocene cycles of fan incision and aggradation post-date the youngest-dated surface ruptures and are interpreted to reflect changes in the frequency and magnitude of large floods. These datasets indicate that tectonic activity controlled long-term sediment supply but climate governed the spatial and temporal patterns of range-front sedimentation. Mild intraplate tectonism appears to have influenced Plio-Quaternary sedimentation patterns across much of the southern Australian continent, including the geometry and extent of alluvial fans and sea-level incursions.     

Response of eolian geomorphic systems to minor climate change: examples from the southern Californian deserts

Eolian processes and landforms are sensitive to changes in atmospheric parameters and surface conditions that affect sediment supply and mobility. The response of eolian geomorphic systems to minor climate change can be examined through process-response models based on a combination of relations between short-term changes in climatic variables and eolian activity and the geologic and geomorphic record of Holocene eolian activity.At both time scales, eolian activity in southern Californian deserts is strongly controlled by variations in precipitation. Wind energy is not a limiting factor in this region. Formation of eolian deposits is a product of climatic changes that increase sediment supply from fluvial and lacustrine sources and may, therefore, be closely tied to periods of channel cutting and geomorphic instability. During intervening periods, eolian deposits migrate away from sediment source areas and are reworked, modified, and degraded. Remobilization of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods of drier climates. The major control on these processes is decadal to annual changes in rainfall that determine vegetation cover and soil moisture content.     

Accelerated sediment delivery to continental margins during post-orogenic rebound of mountain ranges

Changes in sediment flux to continental margins are commonly interpreted in terms of tectonic growth of topography or climatic change. Here, we show that variations in sediment yield from orogenic systems, previously considered as resulting from climate change, drainage reorganisation or mantle processes can be explained by intrinsic mechanisms of mountain belt/foreland basin systems naturally evolving during post-orogenic decay. Numerical modelling indicates an increase of sediment flux leaving the orogenic system synchronous with the cessation of deposition in the foreland basin and the transition from late syn- to post-orogenesis. Experiments highlight the importance of lithospheric flexure that causes the post-orogenic isostatic rebound of the foreland basin. Erosion of the rebounding foreland basin combined with continued sediment flux from the thrust wedge drives an acceleration in sediment outflux towards continental margins. Sediment budget records in natural settings such as the Northern Pyrenees or Western European Alps also indicate accelerated post-orogenic sediment delivery to the Bay of Biscay and Rhône Delta respectively. These intrinsic processes that determine sediment yield to continental margins must be accounted for prior to consideration of additional external tectonic or climatic controls.     

祁连山东段庄浪河流域地貌特征及其构造指示意义

晚新生代以来,青藏高原北东向扩展,致使祁连山地区遭受了强烈的构造隆升,造就了祁连山地区复杂的构造格局和急剧变化的构造地貌,其典型水系流域地貌特征揭示了该地区的新构造活动和地貌演化过程。庄浪河流域位于祁连山东段,作为青藏高原北东向扩展的前缘地区,庄浪河流域的地貌参数对构造活动非常敏感,提取庄浪河流域的地貌信息,有助于揭示祁连山东段庄浪河流域地貌对构造活动的响应,及系统探讨该区地貌发育特征及其所蕴含的构造意义。庄浪河流域内及边缘发育有庄浪河断裂、天祝盆地南缘断裂、疙瘩沟隐伏断裂以及金强河-毛毛山-老虎山断裂。晚新生代以来,这些断裂仍在活动,并且控制着流域内的构造变形、山体隆升和河流水系地貌发育。本研究采用ALOS DEM 12.5 m数据,基于ArcGIS空间分析技术,通过高程条带剖面、河流坡降指标体系（K,SL,SL/K）和Hack剖面、面积-高程积分值（HI）和积分曲线（HC）等方法,对庄浪河流域地貌特征进行了初步分析。结果表明,庄浪河地区地形起伏由北西向南东递减,构造活动存在东西分异的规律;庄浪河流域内部K值、SL、SL/K、HI值西侧高于东侧,Hack剖面西侧相比东侧上凸更明显;H...     

Predicting sediment discharges and erosion rates in deep time—examples from the late Cretaceous North American continent

Depositional stratigraphy represents the only physical archive of palaeo-sediment routing and this limits analysis of ancient source-to-sink systems in both space and time. Here, we use palaeo-digital elevation models (palaeoDEMs; based on high-resolution palaeogeographic reconstructions), HadCM3L general circulation model climate data and the BQART suspended sediment discharge model to demonstrate a predictive, forward approach to palaeo-sediment routing system analysis. To exemplify our approach, we use palaeoDEMs and HadCM3L data to predict the configurations, geometries and climates of large continental catchments in the Cenomanian and Turonian North American continent. Then, we use BQART to estimate suspended sediment discharges and catchment-averaged erosion rates and we map their spatial distributions. We validate our estimates with published geologic constraints from the Cenomanian Dunvegan Formation, Alberta, Canada, and the Turonian Ferron Sandstone, Utah, USA, and find that estimates are consistent or within a factor of two to three. We then evaluate the univariate and multivariate sensitivity of our estimates to a range of uncertainty margins on palaeogeographic and palaeoclimatic boundary conditions; large uncertainty margins (≤50%/±5°C) still recover estimates of suspended sediment discharge within an order of magnitude of published constraints. PalaeoDEMs are therefore suitable as a first-order investigative tool in palaeo-sediment routing system analysis and are particularly useful where stratigraphic records are incomplete. We highlight the potential of this approach to predict the global spatio-temporal response of suspended sediment discharges and catchment-averaged erosion rates to long-period tectonic and climatic forcing in the geologic past.     

Annual sediment budget of a UK mountain torrent

Research into torrent erosion focuses heavily on bedload transport dynamics and debris flow propagation during specific events. As a result, there is limited understanding of the sediment budgets operating in torrent systems over longer timescales. The aim of this study is to construct a sediment budget of the main geomorphological processes operating in a mountain torrent sediment system over a full year.
The study site is Iron Crag which is a small torrent system (catchment area 2.4 ha) situated in the northern Lake District, UK. The site has the characteristic morphology of a torrent: multiple hillslope sediment sources, steep channel, gorges, and a basal alluvial fan. A measurement scheme was designed to monitor process activity, linking the sediment sources and sinks, from December 1998 to December 1999. Over this time period the sediment budget demonstrates that 184 tonnes of sediment was supplied to the alluvial fan (which acted primarily as a sediment sink). Channel (70%) and bank (25%) sources dominated the sediment supply, and surface processes and rockfall on the hillslopes (5%) made only a minor contribution.
Temporal variations in process activity are significant. Surface processes and rockfall display seasonal variations in yield, whilst channel and bank yields are influenced by individual storm events. Site–specific meteorological data are used to explain these observations and freeze–thaw activity and rainfall characteristics are shown to be important controlling factors.     

Relationships between morphological and sedimentological parameters in source‐to‐sink systems: a basis for predicting semi‐quantitative characteristics in subsurface systems

The study of source‐to‐sink systems relates long‐term variations in sediment flux to morphogenic evolution of erosional–depositional systems. These variations are caused by an intricate combination of autogenic and allogenic forcing mechanisms that operate on multiple time scales – from individual transport events to large‐scale filling of basins. In order to achieve a better understanding of how these mechanisms influence morphological characteristics on different scales, 29 submodern source‐to‐sink systems have been investigated. The study is based on measurements of morphological parameters from catchments, shelves and slopes derived from a ∼1 km global digital elevation model dataset, in combination with data on basin floor fans, sediment supply, water discharge and deposition rates derived from published literature. By comparing various morphological and sedimentological parameters within and between individual systems, a number of relationships governing system evolution and behaviour are identified. The results suggest that the amount of low‐gradient floodplain area and river channel gradient are good indicators for catchment storage potential. Catchment area and river channel length is also related to shelf area and shelf width, respectively. Similarly to the floodplain area, these parameters are important for long‐term storage of sediment on the shelf platform. Additionally, the basin floor fan area is correlative to the long‐term deposition rate and the slope length. The slope length thus proves to be a useful parameter linking proximal and distal segments in source‐to‐sink systems. The relationships observed in this study provide insight into segment scale development of source‐to‐sink systems, and an understanding of these relationships in modern systems may result in improved knowledge on internal and external development of source‐to‐sink systems over geological time scales. They also allow for the development of a set of semi‐quantitative guidelines that can be used to predict similar relationships in other systems where data from individual system segments are missing or lacking.     

Axial–transverse fluvial interactions in half-graben: Plio-Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA

Accurate magnetostratigraphic dating of Plio-Pleistocene alluvium in the Palomas half-graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall-sourced Matuyama-age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall-sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends. Southern areas of the Palomas half-graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low-gradient, footwall-sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south-east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone. We conclude generally that facies architecture of axial and transverse elements in half-graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.     

Decoding temporal and spatial patterns of fault uplift using transient river long profiles

We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas > 10 km² and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5–10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n > 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case > 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved.     

Deciphering the origin of cyclical gravel front and shoreline progradation and retrogradation in the stratigraphic record

Nearly all successions of the near‐shore strata exhibit cyclical movements of the shoreline, which have commonly been attributed to cyclical oscillations in relative sea level (combining eustasy and subsidence) or, more rarely, to cyclical variations in sediment supply. It has become accepted that cyclical change in sediment delivery from source catchments may lead to cyclical movement of boundaries such as the gravel front, particularly in the proximal segments of sediment‐routing systems. In order to quantitatively assess how variations in sediment transport as a consequence of change in relative sea‐level and surface run‐off control stratigraphic architecture, we develop a simple numerical model of sediment transport and explore the sensitivity of moving boundaries within the sediment‐routing system to change in upstream (sediment flux, precipitation rate) and downstream (sea level) controls. We find that downstream controls impact the shoreline and sand front, while the upstream controls can impact the whole system depending on the amplitude of change in sediment flux and precipitation rate. The model implies that under certain conditions, the relative movement of the gravel front and shoreline is a diagnostic marker of whether the sediment‐routing system experienced oscillations in sea level or climatic conditions. The model is then used to assess the controls on stratigraphic architecture in a well‐documented palaeo‐sediment‐routing system in the Late Cretaceous Western Interior Seaway of North America. Model results suggest that significant movement of the gravel front is forced by pronounced (±50%) oscillations in precipitation rate. The absence of such movement in gravel front position in the studied strata implies that time‐equivalent movement of the shoreline was driven by relative sea‐level change. We suggest that tracking the relative trajectories of internal boundaries such as the gravel front and shoreline is a powerful tool in constraining the interpretation of stratigraphic sequences.     

Long-term fluvial archives in NW France: response of the Seine and Somme rivers to tectonic movements, climatic variations and sea-level changes

The Seine and the Somme are the two main rivers flowing from northwestern France into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the central deeps of the Channel. The River Seine has a very well developed terrace system recording incision that began at around 1 Ma. The same age is proposed for the beginning of the main incision in the Somme Valley on the basis of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism and ESR datings. The uplift rate deduced from analysis of the Seine and Somme terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocene. The response of the two rivers to climatic variations, uplift and sea-level changes is complex and variable in the different parts of the river courses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the middle Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to be more homogeneous, especially in the middle valley, where the terrace system shows a regular pattern in which incision occurs at the beginning of each glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some strong differences compared with the middle valley: influence of sea-level variations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene are related to differences in the size of the fluvial basins, to the local tectonic characteristics, to the geometry of the platform connected to the lower parts of the valleys and to the hydrodynamic characteristics of each river. Finally, it is shown from these examples that the multidisciplinary study of Pleistocene rivers is a very efficient tool for the investigation of neotectonic activity.     

Quantifying the slip rates, spatial distribution and evolution of active normal faults from geomorphic analysis: Field examples from an oblique-extensional graben, southern Turkey

Quantifying the extent to which geomorphic features can be used to extract tectonic signals is a key challenge in the Earth Sciences. Here we analyse the drainage patterns, geomorphic impact, and long profiles of bedrock rivers that drain across and around normal faults in a regionally significant oblique-extensional graben (Hatay Graben) in southern Turkey that has been mapped geologically, but for which there are poor constraints on the activity, slip rates and Plio–Pleistocene evolution of basin-bounding faults. We show that drainage in the Hatay Graben is strongly asymmetric, and by mapping the distribution of wind gaps, we are able to evaluate how the drainage network has evolved through time. By comparing the presence, size, and distribution of long profile convexities, we demonstrate that the northern margin of the graben is tectonically quiescent, whereas the southern margin is bounded by active faults. Our analysis suggests that rivers crossing these latter faults are undergoing a transient response to ongoing tectonic uplift, and this interpretation is supported by classic signals of transience such as gorge formation and hill slope rejuvenation within the convex reach. Additionally, we show that the height of long profile convexities varies systematically along the strike of the southern margin faults, and we argue that this effect is best explained if fault linkage has led to an increase in slip rate on the faults through time from  0.1 to 0.45 mm/yr. By measuring the average length of the original fault segments, we estimate the slip rate enhancement along the faults, and thus calculate the range of times for which fault acceleration could have occurred, given geological estimates of fault throw. These values are compared with the times and slip rates required to grow the documented long-profile convexities enabling us to quantify both the present-day slip rate on the fault (0.45 ± 0.05 mm/yr) and the timing of fault acceleration (1.4 ± 0.2 Ma). Our results have substantial implications for predicting earthquake hazard in this densely populated area (calculated potential M_w = 6.0–6.6), enable us to constrain the tectonic evolution of the graben through time, and more widely, demonstrate that geomorphic analysis can be used as an effective tool for estimating fault slip rates over time periods > 10⁶ years, even in the absence of direct geodetic constraints.     

Axial and transverse deep-water sediment supply to syn-rift fault terraces: Insights from the West Xylokastro Fault Block,Gulf of Corinth,Greece

Deep-water syn-rift systems develop in partially- or transiently-linked depocentres to form complicated depositional architectures, which are characterised by short transport distances, coarse grain sizes and a wide range of sedimentary processes. Exhumed systems that can help to constrain the tectono-stratigraphic evolution of such systems are rare or complicated by inversion tectonics. Here, we document a mid-Pleistocene deep-water syn-rift system fed by Gilbert-type fan deltas in the hangingwall of a rift margin fault bounding the West Xylokastro Horst block, on the southern margin of the Gulf of Corinth, Greece. Structural and stratigraphic mapping combined with digital outcrop models permit observations along this syn-rift depositional system from hinterland source to deep-water sink. The West Xylokastro Fault hangingwall is filled by two distinct sediment systems; an axial system fed by coarse-grained sediment gravity flows derived from fault-tip Gilbert-type fan deltas and a lateral system dominated by mass transport deposits fed from an evolving fault-scarp apron. Abrupt changes in stratigraphic architecture across the axial system are interpreted to record changes in relative base level, sediment supply and tectonics. Locally, depositional topography and intra-basinal structures controlled sediment dispersal patterns, from bed-scale infilling of local rugose topography above mass transport complexes, to basin-scale confinement from the fault scarp apron. These acted to generate a temporally and spatially variable, heterogeneous stratigraphic architecture throughout the basin-fill. The transition of the locus of sedimentation from a rift margin to a fault terrace through the syn-sedimentary growth of a basinward fault produced regressive surfaces updip, which manifest themselves as channels in the deep-water realm and acted to prograde the system. We present a new conceptual model that recognises coeval axial and transverse systems based on the stratigraphic architecture around the West Xylokastro fault block that emphasizes the lateral and vertical heterogeneity of rift basin-fills with multiple entry points.