Tectono-sedimentary evolution of transverse extensional faults in a foreland basin: Response to changes in tectonic plate processes

Late Paleocene to Middle Eocene strata in the easternmost part of the Southern Pyrenees, up to 4 km thick, provide information on tectono-sedimentary evolution of faults transversal to the Pyrenean chain. To know how changes in tectonic plate processes control the structural evolution of transverse faults and the synchronous thickness and lithological distribution of sedimentary strata in a foreland basin, field observations, interpretation of 2D seismic lines tied to lithostratigraphic data of exploration wells and gravity modelling constrains were carried out. This resulted in the following two tectono-sedimentary phases in a foreland basin: first phase, dominated by transverse extensional faulting, synchronous with deposition of marine carbonates (ca. 57 to 51 Ma); and second phase, characterized by transverse contractional faulting, coeval to accumulation of marine and transitional siliciclastics (51 to 44 Ma). During the first phase, Iberia and Adria were moving to the east and west respectively. Therefore, lithospheric flexure in the easternmost part of the Iberian plate was developed due to that Sardinia was over-thrusting Iberia. Consequently, activation of E-dipping normal faults was generated giving rise to thick-deep and thin-shallow carbonate platform deposits across the hanging walls and footwalls of the transverse structures. During the second phase, a shearing interaction between Iberia and Sardinia prevailed re-activating the transverse faults as contractional structures generating thin-shelf and thick-submarine fan deposits across the hanging walls and footwalls of the transverse structures. In the transition between the first and second phases, evaporitic conditions dominated in the basin suggesting a tectonic control on basin marine restriction. The results of our study demonstrate how thickness and lithology distribution, controlled by transverse faulting in a compressional regimen, are influenced by phases related to processes affecting motions and interactions between tectonic plates and continental blocks.     

Rift basins and supradetachment basins: intracontinental extensional end-members

Two end-members characterize a continuum of continental extensional tectonism: rift settings and highly extended terrains. These different styles result in and are recorded by different extensional basins. Intracontinental rifts (e.g. East Africa, Lake Baikal) usually occur in thermally equilibrated crust of normal thickness. Rift settings commonly display alkali to tholeiitic magmatism, steeply dipping (45–60°) bounding faults, slip rates <1 mm yr^-1 and low-magnitude extension (10–25%). Total extension typically requires > 25 Myr. The fault and sub-basin geometry which dominates depositional style is a half-graben bounded by a steeply dipping normal fault. Associated basins are deep (6–10 km), and sedimentation is predominantly axial- or hangingwall-derived. Asymmetric subsidence localizes depocentres along the active basin-bounding scarp. Highly extended continental terrains (e.g. Colorado River extensional corridor, the Cyclade Islands) represent a different tectonic end-member. They form in back-arc regions where the crust has undergone dramatic thickening before extension, and usually reactivate recently deformed crust. Volcanism is typically calc-alkalic, and 80–90% of total extension requires much less time (<10 Myr). Bounding faults are commonly active at shallow dips (15–35°); slip rates (commonly > 2 mm yr^-1) and bulk extension (often > 100%) are high. The differences in extension magnitude and rate, volcanism, heat flow, and structural style suggest basin evolution will differ with tectonic setting. Supradetachment basins, or basins formed in highly extended terrains, have predominantly long, transverse drainage networks derived from the breakaway footwall. Depocentres are distal (10–20 km) to the main bounding fault. Basin fill is relatively thin (typically 1–3 km), probably due to rapid uplift of the tectonically and erosionally denuded footwall. Sedimentation rates are high (? 1 m kyr^-1) and interrupted by substantial unconformities. In arid and semi-arid regions, fluvial systems are poorly developed and alluvial fans dominated by mass-wasting (debris-flow, rock-avalanche breccias, glide blocks) represent a significant proportion (30–50%) of basin fill. The key parameters for comparing supradetachment to rift systems are extension rate and amount, which are functions of other factors like crustal thickness, thermal state of the lithosphere and tectonic environment. Changes in these parameters over time appear to result in changes to basin systematics.     

A general inverse method for modelling extensional sedimentary basins

A two-dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is presented and applied. This model is a generalization of a one-dimensional algorithm which minimizes the misfit between predicted and observed patterns of basin subsidence. Our calculations include the effects of two-dimensional conduction and advection of heat as well as flexural rigidity. More importantly, we make no prior assumptions about the duration, number or intensity of rifting periods. Instead, the distribution of strain rate is permitted to vary smoothly through space and time until the subsidence misfit has been minimized. We have applied this inversion algorithm to extensional sedimentary basins in a variety of geological settings. Basin stratigraphy can be accurately fitted and the resultant spatiotemporal distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes. Perhaps surprisingly, the smallest misfits are achieved with flexural rigidities close to zero. Spatiotemporal strain rate distributions will help to constrain the dynamical evolution of thinning continental lithosphere. The strain rate pattern governs the heat-flow history and so two-dimensional inversion can be used to construct accurate maturation models. Finally, our inversion algorithm is a stepping stone towards a generalized three-dimensional implementation.     

Recognition and significance of streamflow-dominated piedmont facies in extensional basins

Alluvial slopes are piedmonts characterized by parallel steam channels rather than alluvial fans. They are common landforms in extensional basins of the south-western United States but have received little attention from geomorphologists or sedimentologists. Persistence of channellized flow across piedmonts, as opposed to sheetflooding due to loss of flow confinement on alluvial-fan surfaces, distinguishes alluvial-slope and alluvial-fan facies. Miocene strata of the Tesuque Formation (Española basin, New Mexico) and Pliocene strata of the St. David Formation (San Pedro Valley, Arizona) provide examples of extensional basin–piedmont successions constructed by discrete gravel and sand bedload channels and aggrading interfluve floodplains and aeolian sand sheets. Distinction of alluvial-fan and alluvial-slope piedmont deposits has several important implications. The contrasting facies geometries associated with the two landforms produce distinctly different aquifer and reservoir properties. It is hypothesized that alluvial slopes are more likely to form than alluvial fans where mountain fronts lack abrupt structural and topographic definition. This circumstance will most likely be met (a) along tectonically inactive and embayed mountain fronts and (b) on the hangingwall ramp side of half grabens.     

Role of crustal anisotropy in modifying the structural and sedimentological evolution of extensional basins: the Gamtoos Basin, South Africa

Through the investigation of crustal heterogeneities, sedimentary basin architecture and seismic stratigraphy, we demonstrate how a crust‐scale anisotropy controls the initiation of rifting and the subsequent structural and sedimentological evolution of the Mesozoic Gamtoos Basin, southern South Africa. The results demonstrate that the >90‐km‐long Gamtoos Fault established its length very early in its syn‐rift phase (within ～5 Ma of rift initiation) before accruing over 6 s (two‐way‐travel time (TWT)), or >12 km, of displacement without any significant subsequent increase in length. In addition, there is no evidence at the resolution of the data of fault segmentation, isolated depocentres nor of intra‐basin faults progressively coalescing during the syn‐rift interval. The early establishment of length resulted in a rapid transition from a terrestrial depositional environment to anoxic, deep marine conditions. The Gamtoos Fault has a 90° bend in the fault trace that we propose is inherited from the underlying structure. Immediately adjacent to the bend the basin‐fill is significantly deformed and a high‐amplitude (>1.7s TWT) monoclinal fold is observed. Previous workers proposed that the fold was a consequence of a complex interplay between compression and extension. Through a restoration of the basin‐fill deformation we produce a model that suggests that the fold is a consequence of the accommodation of extension by the unusual plan‐view trace of the fault. The evolution of the basin does not conform to current fault growth models and it is proposed that its unusual and complex development can be attributed to the underlying crustal‐scale anisotropy, a fact that is likely to be important in other areas in which crustal stretching is superimposed on heterogeneous continental crust.     

The interaction between normal faulting and drainage in active extensional basins, with examples from the western United States and central Greece

Faulting exerts an important control upon drainage development in active extensional basins and thus helps determine the architecture of the sedimentary infill to a synrift basin. Examples of the interaction between faulting and drainage from the western United States and central Greece may be grouped into a relatively small number of classes based upon the structural position of a drainage catchment: footwall, hangingwall, fault offset and axial. Our examples illustrate the diversity of erosional effects that might arise because of variations in the spacing, orientation and segmentation of faults and their interactions. Where basement lithology is similar, footwall catchments are generally smaller, shorter and steeper than those of the hangingwall. Footwall-sourced alluvial fans and fan deltas are: generally smaller in area than those sourced from similar lithologies in the hangingwall. Wide fault offsets often give rise to large drainage catchments in the footwall. The development of axial drainage depends upon the breaching of transverse bedrock ridges by headward stream erosion or by lake overflow. Once breaching has occurred the direction of axial stream flow is controlled by the potential developed between basins of contrasting widths. Fault migration and propagation leads to the uplift, erosion and resedimentation of the sedimentary infill to formerly active basins, leading to the cutting of footwall unconformities. The outward sediment flux from structurally controlled catchments is modulated in an important way by lithology and runoff. The greatest contrasts in basement lithology arise when fault migration and propagation have occurred, such that the sedimentary fill to previously active basins is uplifted, incised and eroded by the establishment of large new drainage systems in the footwalls of younger faults. Drainage patterns in areas where faults interact can shed light on the relative timing of activity and therefore the occurrence of fault migration and propagation. Facies and palaeocurrent trends in ancient grabens may only be correctly interpreted when observations are made on a length scale of 10–20 km, comparable to that of the largest fault segments.     

Tectonic and sedimentological evolution of the Pliocene–Quaternary basins of Zakynthos island, Greece: case study of the transition from compressional to extensional tectonics

Pliocene–Quaternary basins of the Ionian islands evolved in a complex tectonic setting that evolved from a mid to late Cenozoic compressional zone of the northern external Hellenides to the rapidly extending Pliocene–Quaternary basins of the Peloponnese. The northern limit of the Hellenic Trench marks the junction of these two tectonic regimes. A foreland-propagating fold and thrust system in the northern external Hellenides segmented the former Miocene continental margin basin in Zakynthos and permitted diapiric intrusion of Triassic gypsum along thrust ramps. Further inboard, coeval extensional basins developed, with increasing rates of subsidence from the Pliocene to Quaternary, resulting in four principal types of sedimentation: (1) condensed shelf-sedimentation on the flanks of rising anticlines; (2) coarse-grained sedimentation in restricted basins adjacent to evaporitic diapirs rising along thrust ramps; (3) larger basins between fold zones were filled by extrabasinal, prodeltaic mud and sand from the proto-Acheloos river; (4) margins of subsiding Quaternary basins were supplied at sea-level highstands by distal deltaic muds and at lowstands by locally derived coarse clastic sediment.     

Tectonic evolution of sedimentary basins of northern Somalia

Regional seismic reflection profiles tied to lithological and biostratigraphic data from deep exploration wells have been used to determine the structure and evolution of the poorly known basins of northern Somalia. We recognize six major tectonostratigraphic sequences in the seismic profiles: Middle‐Late Jurassic syn‐rift sequences (Adigrat and Bihen Group), ?Cenomanian‐Campanian syn‐rift sequences (Gumburo Group), Campanian‐Maastrichtian syn‐rift sequences (Jesomma Sandstones), Palaeocene post‐rift sequences (Auradu Limestones), Early‐Middle Eocene post‐rift sequences (Taleh Formation) and Oligocene‐Miocene (Daban Group) syn‐rift sequences. Backstripping of well data provides new constraints on the age of rifting, the amount of crustal and mantle extension, and the development of the northern Somalia rifted basins. The tectonic subsidence and uplift history at the wells can be explained by a uniform extension model with three episodes of rifting punctuated by periods of relative tectonic quiescence and thermal subsidence. The first event initiated in the Late Jurassic (~156 Ma) and lasted for ~10 Myr and had a NW‐SE trend. We interpret the rift as a late stage event associated with the break‐up of Gondwana and the separation of Africa and Madagascar. The second event initiated in the Late Cretaceous (~80 Ma) and lasted for ~20–40 Myr. This event probably correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The backstripped tectonic subsidence data can be explained by a multi‐rift extensional model with stretching factor, β, of 1.09–1.14 and 1.05–1.28 for the first and second rifting events, respectively. The model, fails, however, to completely explain the slow subsidence and uplift history of the margin during Early Cretaceous to Late Cretaceous. We attribute this slow subsidence to the combined effect of a sea‐level fall and regional uplift, which caused a major unconformity in northern Somalia. The third and most recent event occurred in the Oligocene (~32 Ma) and lasted for ~10 Myr. This rift developed along the Gulf of Aden and reactivated the Guban, Nogal and Daroor basins, and is related to the opening of the Gulf of Aden. As a result of these events the crust and upper mantle were thinned by up to a factor of two in some basins. In addition, several distinct petroleum systems developed. The principal exploration play is for Mesozoic petroleum systems with the syn‐rift Oligocene‐Miocene as a subordinate objective owing to low maturity and seal problems. The main seals for the different plays are various shales, some of which are also source rocks, but the Early Eocene evaporites of the Taleh formations can also perform a sealing role for Palaeogene or older generated hydrocarbons migrating vertically.     

Structural evolution of African basins: stratigraphic synthesis

The structural and stratigraphic character of African interior sedimentary basins is highly variable, indicating contrasting basin-forming mechanisms and subsequent subsidence histories. A stratigraphic database has been compiled for African interior depositional basins for the purpose of better understanding basin thermal and structural development. Data are recorded in the form of stratal age, lithology, thickness and elevation of top with respect to present sea level. The data are obtained from published structure contour maps, well sections, and outcrop geology and elevation. There are various degrees of data coverage of the basins, proportional to the amount of water and oil drilling activity. Consequently, there is excellent coverage of North African basins such as the Algerian basin and the Sirte basin, while there is little known about the subsurface of the Congo basin. The stratigraphic data are used to reconstruct the depositional history of the basins, while backstripping leads to the quantification of the thermo-tectonic component of basin subsidence. The nature of basement subsidence can provide constraints on lithospheric flexural rigidity. In addition, the depositional and thermo-tectonic history of each basin bears upon the mechanisms of basin formation and subsidence. Virtually all types of basins are represented in interior Africa, including thrust-loaded basins (Algerian), passive-margin rift basins (Algerian, Sirte), modern active rift basins (East African), ancient rift basins (Benue, Abu Gabra), basins caused by uplift of their margins (Congo, Chad, Illumeden) and even basins that may be related to thermal subsidence of hot-spot domes (Algerian, Sirte).     

Forward modelling of porosity and pore pressure evolution in sedimentary basins

The gravitational compaction of sediments is an important process in forward basin modelling. This paper presents a mathematical model for the one-dimensional compaction of an accreting layer of argillaceous sediments. Realistic constitutive laws for the clay compressibility and the clay permeability, based on soil mechanics tests, were incorporated into the model. The governing equations were put in dimensionless form and the extent of abnormal pore fluid pressure development was found to depend on the sedimentation parameter, a dimensionless group representing the ratio of the sediment hydraulic conductivity to the sediment accumulation rate. The effects of clay compressibility were studied and highly colloidal clays such as montmorillonite developed higher overpressures than less compressible materials. The results also showed that overpressuring developed in shales for cases in which the clay permeability did not go to zero in the limit of zero porosity. Linear models based on simplifying assumptions inappropriate for sedimentary basins were found to give significantly different estimates for the conditions leading to overpressuring. Using reasonable parameters, the model adequately reproduced porosity and pore pressure profiles measured in the sand-shale sequences of the South Caspian Sea.     

Basin filling evolution of the central basins of Mallorca since the Pliocene

A new compilation of data from 436 drill cores using decompaction and backstripping techniques was used to reconstruct the basin filling history from the Pliocene until the present day in the Palma, Inca and Sa Pobla Basins on the island of Mallorca (Spain). Calcareous rocks dominate the source area and provide a limited amount of clastic input to the basins that has resulted in an average accumulation rate of between 5 and 20 m/Ma during the last 5.3 Ma. Carbonate sediment production dominated the basin filling history during early‐mid Pliocene, but during the Quaternary, the sedimentation processes in the Palma Basin were probably enhanced by an evolution in the drainage network that increased the sediment supply and the accumulated thickness caused by stream capture. However, the maximum sedimentation rate filling the depocentres of the three basins has been decreasing since the Pliocene, showing that not only the catchment transport efficiency but also the relative sea level have been controlling the sediment accumulation in these carbonate basins. The isopach cross‐sections support the idea that a palaeorelief was generated during the Messinian sea level drop and that heterogeneities were filled in from the Pliocene to the Quaternary. We conclude that the central basins of Mallorca were filled heterogeneously due to tectonic and geomorphic processes that controlled sediment transport and production, resulting in different average sedimentation thicknesses that decreased since the Pliocene as the accommodation space became filled and the relative sea level dropped.     

River response to lateral ground tilting: a synthesis and some implications for the modelling of alluvial architecture in extensional basins

This paper reviews and synthesizes several Holocene field examples of river response to lateral ground tilting. Key aspects of alluvial architecture modelling in extensional basins are addressed, including the nature of gradual lateral migration, the spatial and temporal history of avulsive sequences, and the underlying controls that determine whether a river responds to lateral tilting through gradual migration or avulsion. A new conceptual model for gradual lateral migration is proposed that unifies previously disparate models. Tilt-induced avulsion in several field examples is associated with sequences that move towards and away from the locus of subsidence during active and quiescent tectonic periods, respectively. These avulsion sequences closely correspond to those produced by several 2D and 3D alluvial architecture models. The rate of lateral tilt appears to control the style of channel movement, with gradual migration occurring at low tilt rates, and avulsion at higher rates. This apparent dependence on tilt rate suggests the mode of channel movement, and also the avulsion frequency, may in part be a function of the imposed tectonic regime.     

Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins1

Multiple episodes of extensional tectonism dominated the formation of Mesozoic fault-bounded basins on the Grand Banks of Newfoundland, the Irish Continental Shelf and the central North Sea. A range of structural and stratigraphic responses in the Jeanne d'Arc, Porcupine and Moray Firth basins support widespread synchronous tectonic controls on sedimentation during one of these episodes, the Late Cimmerian. Rifting was preceded by a phase of related tectonism during which subsidence rates began to vary across broad areas but without significant fault block rotation. This Late Cimmerian ‘onset warp’ pattern of subsidence is considered to have been essential in the establishment of restricted anoxic basins from latest Oxfordian through Kimmeridgian (sensu gallico) time and the development of one prolific layer of organic-rich source rocks. The most prominent and widely recognized structural/lithostratigraphic response to Late Cimmerian rifting was the deposition of sediment wedges. Tithonian to early Valanginian strata generally thicken- into northerly trending faults in the Jeanne d'Arc and Porcupine basins, indicating that extensional stress was orientated WNW-ESE across a very broad area. The misalignment of this regional Late Cimmerian extensional stress with local inherited structural fabric may be responsible for transpressional uplift of individual fault blocks in the Outer Moray Firth basin. Sedimentological responses to Late Cimmerian rifting were varied, though a common lithofacies stacking pattern is recognized. Variably thick conglomerates and/or sandstones were widely deposited at the start of rift deformation, while palaeoenvironments ranged from alluvial and braid plain to submarine fan even within individual basins. The relatively coarse basal sediments fine upwards into a second layer of commonly organic-rich shales and mark The widest variations in palaeoenvironments and sediment thicknesses occurred during the last phase of Late Cimmerian rift tectonism, though all three basins show evidence of decreasing water depths, increasing oxygen levels and increasing grain size. This lithofacies stacking pattern of relatively coarse to fine to coarse (reservoir/source/reservoir) and the development of bounding unconformities are largely attributable to progressive changes in rift-controlled subsidence. Rift basin subsidence rates are interpreted to increase from a low at initiation of faulting to a mid-rift peak, followed by slowing subsidence to the end of extension. A number of counteracting crustal mechanisms that may account for progressive variations in rift-induced subsidence are considered.     

Modelling of pore pressure evolution associated with sedimentation and uplift in sedimentary basins

Unconformities, which represent either periods of interruption of sedimentation or, in most cases events characterized by deposition and subsequent erosion, are commonplace geological phenomena in sedimentary basins, and will affect the pore pressure evolution of the basin fill. The effect of unconformities on pore pressure, as well as on sediment compaction and on burial processes is studied using a numerical basin model. For coarse sediments, which are permeable so that their pore pressure always remains nearly hydrostatic, the effects of both pure deposition interruption (hiatus) and deposition-erosion events are negligible for pore pressure evolution. However, for fine-grained sediments, unconformities can modify the pore pressure and the stress state to varying degrees. The results show that the rate of removal of overlying sediments, the permeability of sediments and time play important roles in the pore pressure evolution. In the East Slope of the Ordos Basin (China), in which overpressure has not been detected in deep wells, the modelling results suggest that the large-scale erosion occurring in the Late Cretaceous and in the Tertiary may have removed high overpressure existing in the basin before the erosion.     

The evolution of sedimentary basins on a viscoelastic lithosphere: theory and examples

Summary. A systematic approach is suggested for modelling the development of sedimentary basins. The theory, which partitions basin formation into initiating and isostatic adjustment processes, is applicable to all modes of basin formation if these processes are linear, or can he represented with sufficient accuracy in an incrementally linear form.
The dynamics of regional isostatic adjustment are characterized by the Heaviside space-time Green functions for the response of elastic and viscoelastic (Maxwell) thin plate models of the lithosphere. It is shown, by convolving the Heaviside—Green functions with cylindrical surface loads, that the rate of isostatic adjustment on a viscoelastic lithosphere is a function of the wavelength of the surface load, long wavelengths being compensated most rapidly.
Six archetypal initiating processes for sedimentary basin development are presented. These processes are those responsible for the subsidence of the Earth's surface which creates a depression in which water and sediments collect. Isostatic amplification of subsidence by sediment and water loads is cast in the form of an integral equation with isostatic Heaviside—Green functions as kernel.
Specific examples, the basins that result from a graben initiating process, are compared with the largest scale structure of the North Sea Basin, a basin that is known to be underlain by a graben system. A model, in which a 50-km wide graben subsides exponentially with a time constant of 5 × 10⁷yr during the interval 180–100 Myr bp , is shown to be consistent with the largest scale structure of the North Sea Basin if the underlying lithosphere is viscoelastic with a flexural rigidity of ∼5 × 10²⁵ Nm and relaxation time constant ∼ 10⁶ yr.     

Tectono-sedimentary processes along an active marine/lacustrine half-graben margin: Alkyonides Gulf, E. Gulf of Corinth, Greece

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year^?1. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year^?1 of footwall uplift and hence 0.5–0.6 mm year^?1 of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year^?1. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp ) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.