Transient landscape and stratigraphic responses to drainage integration in the actively extending central Italian Apennines

Drainage networks in continental rifts are generally reported as dynamic features that produce transitions between endorheic and exorheic conditions. While this is of major importance for landscape development, sediment dispersal, and basin stratigraphy, the controls of drainage network evolution across an array of normal fault bounded basins are still not well understood. In this study we use the central Italian Apennines – an area that has been affected by active normal faulting and regional uplift over the last ~3 Myrs – to determine the controls on drainage network evolution and its impact on transient landscape evolution and basin stratigraphy. We compile previously published stratigraphic and fault-related data with new geomorphological constraints for the Aterno River system (~1300 km2), for which a wealth of data has been collected following the destructive L'Aquila earthquake in 2009. We use this compilation to demonstrate how the different basins along the river system were initially isolated during the Early Pleistocene but became fluvially integrated with one another and the Adriatic coast between ca. 1.2 and 0.65 Ma. We conclude that the spatial and temporal pattern of drainage integration is mostly explained by a long-term increase in sediment and water supply relative to basin subsidence due to the Early to Middle Pleistocene climatic transition, the progressive increase in fault-related topography, and the transport of sediment and water down-system as drainage integration occurred. Overall we conclude that rates of sedimentation and basin subsidence in the central Apennines are well-matched, allowing tipping points between over- and under-filled conditions to be easily reached. We also show that consecutive drainage integration events produce discrete waves of river incision and terrace formation, and conclude that drainage integration is of major importance, at least equivalent to tectonics and climate, in controlling transient landscape evolution and rift basin stratigraphy.publishedVersio

The 1912 Iceland earthquake rupture: Growth and development of a nascent transform system

We have mapped in detail surface ruptures of the 1912 magnitude 7.0 strike-slip earthquake in south Iceland. This earthquake ruptured fresh basalt flows that had covered the pre-existing fault. The observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic-scale observations from laboratory experiments. The shear offset distributed across the zone of surface fractures produced by this earthquake is right-lateral and is in the range of 1 to 3 m. Total mapped rupture length is 9 km, but total rupture length is probably at least ∼ 20 km. This interplate earthquake had an exceptionally high ratio of slip to fault length and, by inference, stress drop. The north-south trending rupture of the 1912 earthquake is part of the “bookshelf” faulting in the east-west trending South Iceland Seismic Zone. We ascribe the “bookshelf” faulting in the South Iceland Seismic Zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust.This research was supported by the National Science Foundation, the Icelandic National Power Authority (Landsvirkjun), and the Department of Geological Sciences of Columbia University. Lamont-Dohert Contribution 5036.Peer Reviewe

Drainage integration and sediment dispersal in active continental rifts:A numerical modelling study of the central Italian Apennines

Progressive integration of drainage networks during active crustal extension is observed in continental areas around the globe. This phenomenon is often explained in terms of headward erosion, controlled by the distance to an external base‐level (e.g. the coast). However, conclusive field evidence for the mechanism(s) driving integration is commonly absent as drainage integration events are generally followed by strong erosion. Based on a numerical modelling study of the actively extending central Italian Apennines, we show that overspill mechanisms (basin overfilling and lake overspill) are more likely mechanisms for driving drainage integration in extensional settings and that the balance between sediment supply vs. accommodation creation in fault‐bounded basins is of key importance. In this area drainage integration is evidenced by lake disappearance since the early Pleistocene and the transition from internal (endorheic) to external drainage, i.e. connected to the coast. Using field observations from the central Apennines, we constrain normal faulting and regional surface uplift within the surface process model CASCADE (Braun & Sambridge, 1997, Basin Research, 9, 27) and demonstrate the phenomenon of drainage integration, showing how it leads to the gradual disappearance of lakes and the transition to an interconnected fluvial transport system over time. Our model results show that, in the central Apennines, the relief generated through both regional uplift and fault‐block uplift produces sufficient sediment to fill the extensional basins, enabling overspill and individual basins to eventually become fluvially connected. We discuss field observations that support our findings and throw new light upon previously published interpretations of landscape evolution in this area. We also evaluate the implications of drainage integration for topographic development, regional sediment dispersal and offshore sediment supply. Finally, we discuss the applicability of our results to other continental rifts (including those where regional uplift is absent) and the importance of drainage integration for transient landscape evolution.publishedVersio

Field calibration of sediment flux dependent river incision

Bed erosion and sediment transport are ubiquitous and linked processes in rivers. Erosion can either be modeled as a “detachment limited” function of the shear stress exerted by the flow on the bed, or as a “transport limited” function of the sediment flux capacity of the flow. These two models predict similar channel profiles when erosion rates are constant in space in time, but starkly contrasting behavior in transient settings. Traditionally detachment limited models have been used for bedrock rivers, whereas transport limited models have been used in alluvial settings. In this study we demonstrate that rivers incising into a substrate of loose, but very poorly sorted relict glacial sediment behave in a detachment limited manner. We then develop a methodology by which to both test the appropriate incision model and constrain its form. Specifically we are able to tightly constrain how incision rates vary as a function of the ratio between sediment flux and sediment transport capacity in three rivers responding to deglaciation in the Ladakh Himalaya, northwest India. This represents the first field test of the so-called “tools and cover” effect along individual rivers

Dynamics of long term fluvial response in postglacial catchments of the Ladakh Batholith, Northwest Indian Himalaya

Upland rivers control the large-scale topographic form of mountain belts, allow coupling of climate and tectonics at the earth’s surface and are responsible for large scale redistribution of sediment from source areas to sinks. However, the details of how these rivers behave when perturbed by changes to their boundary conditions are not well understood. I have used a combination of fieldwork, remotely sensed data, mathematical analysis and computer modelling to investigate the response of channels to well constrained changes in the forcings upon them, focussing in particular on the effects of glacial remoulding of the catchments draining the south flank of the Ladakh batholith, northwest Indian Himalaya. The last glacial maximum for these catchments is atypically old (~100 ka), and this allows investigation of the response to glaciation on a timescale not usually available. The geomorphology of the catchments is divided into three distinct domains on the basis of the behaviour of the trunk stream – an upper domain where the channel neither aggrades above or incises into the valley form previously carved by glacial abrasion, a middle domain where the channel incises a gorge down into glacial sediments which mantle the valley floor, and a lower domain where the channel aggrades above this postglacial sediment surface. This landscape provides a framework in which to analyze the processes and timescales of fluvial response to glacial modification. The dimensions of the gorge and the known dates of glacial retreat record a time averaged peak river incision rate of approximately 0.5 mm/y; the timescale for the river long profile to recover to a smooth, concave up form must exceed 1 Ma. These values are comparable with those from similarly sized catchments that have been transiently perturbed by changing tectonics, but have never been quoted for a glacially forced basin-scale response. I have also demonstrated that lowering of the upper reaches of the Ladakh channel long profiles by glacial processes can systematically and nonlinearly perturb the slope-area (concavity) scaling of the channel downstream of the resulting profile convexities, or knickzones. The concavity values are elevated significantly above the expected equilibrium values of 0.3-0.6, with the magnitude controlled by the relative position of the knickzone within the catchment, and thus also by the degree of glacial modification of the fluvial system. This work also documents the existence of very similar trends in measured concavities downstream of long profile convexities in other transiently responding river systems in different tectonoclimatic settings, including those responding to changes in relative channel uplift. This previously unrecognised unity of response across a wide variety of different environments argues that such a trend is an intrinsic property of river response to perturbation. Importantly, it is consistent with the scaling expected from variation in incision efficiency driven by evolving sediment flux downstream of knickzones. The pervasive nature of this altered scaling, and its implications for fluvial erosion laws in perturbed settings, have significant consequences for efforts to interpret past changes in forcings acting on river systems from modern topography. I follow this by examining in detail the channel hydraulics of the Ladakh streams as they incise in response to the glacial perturbation. I present a new framework under which the style of erosion of a natural channel can be characterized as either detachment- or transport-limited based upon comparison of the downstream distribution of shear stress with the resulting magnitude of incision. This framework also allows assessment of the importance of sediment flux driven effects in studied channels. This approach is then used to demonstrate that fluvial erosion and deposition in the Ladakh catchments is best modelled as a sediment flux dependent, thresholded, detachment-limited system. The exceptional quality of the incision record in this landscape enables an unprecedented calibration of the sediment flux function within this incision law for three different trunk streams. The resulting curves are not compatible with the theoretically-derived parabolic form of this relation, instead showing nonzero erosion rates at zero sediment flux, a rapid rise and peak at relative sediment fluxes of less than 0.5 and a quasi exponential decrease in erosional efficiency beyond this. The position of the erosional efficiency peak in relative sediment flux space and the magnitude of the curve are shown to be both variable between the catchments explored and also correlated with absolute sediment flux in the streams.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Processes, rates, and time scales of fluvial response in an ancient postglacial landscape of the northwest Indian Himalaya

Both glacial and fluvial processes are key elements in molding landscapes in high mountain environments—glaciers are highly efficient erosional agents and producers of sediment but are restricted spatially, while rivers can transmit such signals through landscapes and flush this sediment out of mountain belts and into sedimentary basins. However, little research has focused on the manner in which these two agents of landscape change interact, especially on longer time scales. We analyze a suite of catchments that drain the previously glaciated Ladakh Batholith in the northwest Indian Himalaya; which preserve the oldest known moraine succession in this mountain chain. We describe and quantify the rates, processes, and time scales of postglacial recovery of the fluvial system across a previously unstudied time interval of 105–106 yr. We demonstrate that glacial modification of the upper reaches of a catchment can have profound first-order influence on the hydraulic scaling of the channel downstream, where increasing degree of glacial modification systematically and nonlinearly elevates the channel concavities of downstream reaches above the expected value range of 0.3–0.6. We also demonstrate that the response time of these systems as they recover must exceed 500 k.y., which is longer than any previously reported estimate for recovery times from glaciations, but is comparable with estimates from many tectonically perturbed landscapes

Transient landscape and stratigraphic responses to drainage integration in the actively extending central Italian Apennines

Drainage networks in continental rifts are generally reported as dynamic features that produce transitions between endorheic and exorheic conditions. While this is of major importance for landscape development, sediment dispersal, and basin stratigraphy, the controls of drainage network evolution across an array of normal fault bounded basins are still not well understood. In this study we use the central Italian Apennines – an area that has been affected by active normal faulting and regional uplift over the last ~3 Myrs – to determine the controls on drainage network evolution and its impact on transient landscape evolution and basin stratigraphy. We compile previously published stratigraphic and fault-related data with new geomorphological constraints for the Aterno River system (~1300 km2), for which a wealth of data has been collected following the destructive L'Aquila earthquake in 2009. We use this compilation to demonstrate how the different basins along the river system were initially isolated during the Early Pleistocene but became fluvially integrated with one another and the Adriatic coast between ca. 1.2 and 0.65 Ma. We conclude that the spatial and temporal pattern of drainage integration is mostly explained by a long-term increase in sediment and water supply relative to basin subsidence due to the Early to Middle Pleistocene climatic transition, the progressive increase in fault-related topography, and the transport of sediment and water down-system as drainage integration occurred. Overall we conclude that rates of sedimentation and basin subsidence in the central Apennines are well-matched, allowing tipping points between over- and under-filled conditions to be easily reached. We also show that consecutive drainage integration events produce discrete waves of river incision and terrace formation, and conclude that drainage integration is of major importance, at least equivalent to tectonics and climate, in controlling transient landscape evolution and rift basin stratigraphy

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., 10^5–10^6 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