54 research outputs found
Axial and transverse deepâwater sediment supply to synârift fault terraces: insights from the West Xylokastro Fault Block, Gulf of Corinth, Greece
Deepâwater synârift systems develop in partiallyâ or transientlyâlinked 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 tectonoâstratigraphic evolution of such systems are rare or complicated by inversion tectonics. Here, we document a midâPleistocene deepâwater synârift system fed by Gilbertâtype 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 synârift depositional system from hinterland source to deepâwater sink. The West Xylokastro Fault hangingwall is filled by two distinct sediment systems; an axial system fed by coarseâgrained sediment gravity flows derived from faultâtip Gilbertâtype fan deltas and a lateral system dominated by mass transport deposits fed from an evolving faultâscarp 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 intraâbasinal structures controlled sediment dispersal patterns, from bedâscale infilling of local rugose topography above mass transport complexes, to basinâscale confinement from the fault scarp apron. These acted to generate a temporally and spatially variable, heterogeneous stratigraphic architecture throughout the basinâfill. The transition of the locus of sedimentation from a rift margin to a fault terrace through the synâsedimentary growth of a basinward fault produced regressive surfaces updip, which manifest themselves as channels in the deepâwater 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 emphasises the lateral and vertical heterogeneity of rift basinâfills with multiple entry points
Basement structure and its influence on the structural configuration of the northern North Sea
The northern North Sea rift basin developed on a heterogeneous crust comprising structures inherited from the Caledonian orogeny and Devonian postorogenic extension. Integrating two-dimensional regional seismic reflection data and information from basement wells, we investigate the prerift structural configuration in the northern North Sea rift. Three seismic facies have been defined below the base rift surface: (1) relatively low-amplitude and low-frequency reflections, interpreted as pre-Caledonian metasediments, Caledonian nappes, and/or Devonian clastic sediments; (2) packages of high-amplitude dipping reflections (>500 ms thick), interpreted as basement shear zones; and (3) medium-amplitude and high-frequency reflections interpreted as less sheared crystalline basement of Proterozoic and Paleozoic (Caledonian) origin. Some zones of Seismic Facies 2 can be linked to onshore Devonian shear zones, whereas others are restricted to the offshore rift area. Interpreted offshore shear zones dip S, ESE, and WNW in contrast to W to NW dipping shear zones onshore West Norway. Our results indicate that Devonian strain and ductile deformation was distributed throughout the Caledonian orogenic belt from central South Norway to the Shetland Platform. Most of the Devonian basins related to this extension are, however, removed by erosion during subsequent exhumation. Basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters, e.g., in the Stord and East Shetland basins. Prerift structures with present-day dips >15° were reactivated, although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction
Quantifying faulting and base level controls on syn-rift sedimentation using stratigraphic architectures of coeval, adjacent Early-Middle Pleistocene fan deltas in Lake Corinth, Greece
Quantification of allogenic controls in rift basinâfills requires analysis of multiple depositional systems because of marked alongâstrike changes in depositional architecture. Here, we compare two coeval EarlyâMiddle Pleistocene synârift fan deltas that sit 6 km apart in the hangingwall of the PirgakiâMamoussia Fault, along the southern margin of the Gulf of Corinth, Greece. The Selinous fan delta is located near the fault tip and the Kerinitis fan delta towards the fault centre. Selinous and Kerinitis have comparable overall aggradational stacking patterns. Selinous comprises 15 cyclic stratal units (ca. 25 m thick), whereas at Kerinitis 11 (ca. 60 m thick) are present. Eight facies associations are identified. Fluvial and shallow water facies dominate the major stratal units in the topset region, with shelfal fineâgrained facies constituting ca. 2 m thick intervals between major topset units and thick conglomeratic foresets building downâdip. It is possible to quantify delta build times (Selinous: 615 kyr; Kerinitis: >450 kyr) and average subsidence and equivalent sedimentation rates (Selinous: 0.65 m/kyr; Kerinitis: >1.77 m/kyr). The presence of sequence boundaries at Selinous, but their absence at Kerinitis, enables sensitivity analysis of the most uncertain variables using a numerical model, âSynâStratâ, supported by an independent unit thickness extrapolation method. Our study has three broad outcomes: (a) the first estimate of lake level change amplitude in Lake Corinth for the EarlyâMiddle Pleistocene (10â15 m), which can aid regional palaeoclimate studies and inform broader climateâsystem models; (b) demonstration of two complementary methods to quantify faulting and base level signals in the stratigraphic recordâforward modelling with SynâStrat and a unit thickness extrapolationâwhich can be applied to other rift basinâfills; and (c) a quantitative approach to the analysis of stacking patterns and key surfaces that could be applied to stratigraphic pinchâout assessment and crossâhole correlations in reservoir analysis
Building up or out? Disparate sequence architectures along an active rift marginâCorinth rift, Greece
Early Pleistocene synrift deltas developed along the southern Corinth rift margin were deposited in a single, dominantly lacustrine depocenter and were subject to the same climate-related base-level and sediment supply cyclicity. Two synrift deltas, just 50 km apart, show markedly different sequence geometry and evolution related to their location along the evolving border fault. In the west, strongly aggradational fan deltas (>600 m thick; 2â4 km radius) deposited in the immediate hanging wall of the active border fault comprise stacked 30â100-m-thick stratal units bounded by flooding surfaces. Each unit evolves from aggradational to progradational with no evidence for abrupt subaerial exposure or fluvial incision. In contrast, in the central rift, the border fault propagated upward into an already deep lacustrine environment, locating rift-margin deltas 15 km into the footwall. The deltas here have a radius of >9 km and comprise northward downstepping and offlapping units, 50â200 m thick, that unconformably overlie older synrift sediments and are themselves incised. The key factors driving the marked variation in sequence stratigraphic architecture are: (1) differential uplift and subsidence related to position with respect to the border fault system, and (2) inherited topography that influenced shoreline position and offshore bathymetry. Our work illustrates that stratal units and their bounding surfaces may have only local (<10 km) extent, highlighting the uncertainty involved in assigning chronostratigraphic significance to systems tracts and in calculating base-level changes from stratigraphy where marked spatial variations in uplift and subsidence occur
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 âĽ40% of the sediment that builds the Sperchios delta is supplied by â¤22% of the entire source area and that this can be primarily attributed to a longer-term (~10 6 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
Rift kinematics preserved in deep-time erosional landscape below the northern North Sea
Our understanding of continental rifting is, in large parts, derived from the stratigraphic record. This record is, however, incomplete as it does not often capture the geomorphic and erosional signal of rifting. New 3D seismic reflection data reveal a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542âkm2, preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted palaeo-rivers showing that the landscape preserved a >2 Myr long period of transient tectonics. We interpret that this landscape records a punctuated uplift of the footwall of a major rift-related normal fault (Vette Fault) at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault (Ăygarden Fault) and burial by post-incision sediments. As such, we show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts
Straight from the source's mouth: Controls on fieldâconstrained sediment export across the entire active Corinth Rift, central Greece
The volume and grainâsize of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grainâsize exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grainâsizes of channel bars and fullâweighted grainâsize distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grainâsize increases westward along the southern coast of the Gulf of Corinth, with the coarseâfraction grainâsizes (84th percentile of weighted grainâsize distribution) ranging from approximately 19 to 91 mm. We find that the median and coarseâfraction of the sieved grainâsize distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grainâsize export is primarily controlled by the input grainâsize within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarseâfraction of the grainâsize distribution are predominantly transported in bedload; however, typical sandâgrade particles are transported as suspended load at bankfull conditions, suggesting disparate sourceâtoâsink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grainâsizeâspecific bedload discharged into the Gulf of Corinth using the grainâsize measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475â16 cm). Our results suggest that the grainâsize of sediment export at the rift scale is particularly sensitive to catchment lithology and fluvial mophodynamics, which complicates our ability to make direct inferences of tectonic and palaeoenvironmental forcing from local stratigraphic characteristics
Strain migration during multiphase extension, Stord Basin, northern North Sea rift
In regions experiencing multiple phases of extension, rift-related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre-existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time-structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β-factor). Our results show that the locations of basin-bounding fault systems were controlled by pre-existing crustal-scale shear zones. Within the basin, Permo-Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E-W extension direction. Rift faults control the locus of syn-RP1 deposition, whilst during the inter-rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β-factor) for the Stord Basin is up to β = 1.55 (¹10%, 55% extension). During the subsequent Middle Jurassic-Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin
Tectono-sedimentary evolution of active extensional basins
dimensional evolution of basin linkage through fault propagation, the evolution of drainage and drainage catchments and the effects of changes in climate and sea/lake level. In particular, the processes of fault propagation, growth, linkage and death are major tectonic controls on basin architecture. Current theoretical and experimental models of fault linkage and the direction of fault growth can be tested using observational evidence from the earliest stages of rift development. Basin linkage by burial or breaching of crossover basement ridges is the dominant process whereby hydrologically closed rifts evolve into open ones. Nontectonic effects arising from climate, sea or lake level change are responsible for major changes in basin-scale sedimentation patterns. Major gaps in our understanding of rift basins remain because of current inadequacies in sediment, fault and landscape dating
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