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Journal of the Geological Society, London, Vol. 152, 1995, pp. 883-893, 12 figs. Printed in Northern

Drainage development and supply within , examples from the Sperchios basin, central

P. P. ELIET & R. L. GAWTHORPE Department of Geology, The University, Manchester M13 9PL, UK

Abstract: Drainage systems juxtaposed with modern depositional environments within the Sperchios allow detailed analysis of the relationships between drainage catchments. sediment supply and /fandelta geometries. Prominent footwall escarpments within the rift arecreated by E-W-trending normal faults, and variations in the topography of these escarpments are related to segmentation and bedrock lithology. Drainage catchments within the rift are classified on the basis of morphology,area, bedrock lithology andrelationship to faultgeometries. Five drainage domains can be identified within the Sperchios rift: (i) footwall, (ii) hanging wall, (iii) transfer zone, (iv) axial and (v) domains. Bedrock lithology has a major control on the size of catchments in all drainage domains. In addition, topographic lows associated with transfer zones along the border fault are sites of major drainage catchments and act as conduits for sediment supply to the rift. Variations in drainage catchments, the resultant sediment supply and accommodation development along the rift produce a range of stratigraphic architectures. Along fault segments, low sediment supply and high subsidence rates lead to aggradational sequence sets, whereas progradational sequence sets develop at transfer zones where rates of are high and the rate of subsidence is low.

Keywords: Greece, drainage, sedimentation, stratigraphy, rift zones.

Sediment supply is considered to be one of the main Pliocene, and is currentlytaking place at a rate of controls onthe stratigraphy of basin fills. Despite this, 10-20 mm a-' (e.g. Billiris et al. 1989). The majority of the controls on sediment supply, and spatialand temporal majornormal faults bound thesouthern sides of the variations in sediment supply have received little attention sedimentary basins (e.g. Gulf of Corinth and Sperchios in sequencestratigraphic studies (see, however, Schlager basins; Fig. 1). The normalfaults cut the pre-extension, 1993; Gawthorpe et al. 1994). One way of determining the NNW-SSE, structuraltrend of the Hellenide massif spatial variations in sediment supply is through the analysis (Pe-Piper & Piper 1984), and in places the formation of the of drainagecatchments in the hinterland of sedimentary extensional fault systems may have utilized structures which basins. The relationshipbetween catchment areaand formedduring the Hellenidethrusting (Jackson & sediment supply is well established (Bull 1962; Denny 1965; McKenzie 1983). Hooke 1968; Rockwell et al. 1988; Harvey 1989; Milliman The Sperchios basin, the most northerly of the central 1992; Leeder 1993), with larger catchments supplying larger Greece rift basins, is approximately 100 km long and 30 km volumes of sediment given constant climatic conditions and wide. The basin-boundingnormal fault zone is associated bedrock lithology. with over 2000m of relief, from the Maliakos Gulf in the This paper examines the drainagecatchments and hanging wall to the summits of such as Mount Iti associated depositional systems in one active rift basin, the (2152m) in the footwall (Fig. 2). The axis of the Sperchios Sperchios basin in (Fig. 1). The factors which extensional basin is parallel to the Gulf of Corinth (Fig. 1) control the location, morphology and size of the catchments andcontains major present day fluvio-deltaic and alluvial are examined, paying particular attention to fault geometry fandepositional systems. Topographyindicates thatthe and bedrock lithology. The relationships between is an asymmetric half and gravity studies catchments,depositional systems and faultgeometries (Apostolopoulos 1993) suggest over 2.5km of sediment is within the Sperchios rift provide a basis for understanding preserved at the centre of the basin. Faulting is thought to variations in sediment supply within rift basins and have have been active throughout the and implications forthe sequencestratigraphic architecture earthquakes linked to fault activity have been recorded from within rifts. Thus, this study of drainage catchments in an thearea near the town of Atalanti,to the east of the active rift has directapplication to the interpretation of Sperchios rift (Roberts & Jackson 1991). Our estimates of ancient extensional basins and to the exploration for subtle the subsidence based on high resolution seismic data from syn-rift plays in the subsurface. the Maliakos Gulf suggest subsidencerates exceeding 1.8 m ka-' along the border fault zone. Geological setting The structure of central Greece is dominated by a series of The Sperchios basin roughly ENE-WSW-trending extensional faults which have The Sperchios basin lies within the Mediterranean climatic created a series of half graben and asymmetric graben (Fig. zone, with wet, warm winters and a three monthdrought 1). Extension startedaround 5 Ma, during the early over the summer. Mean temperatures forJuly reach 28 "C in 883

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Fig. 1. Tectono-sedimentary setting of central Greece, showing major faults and Neogene sedimentary basins. Inset shows the structural setting of central Greece.

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Fig. 2. (a) The topography and structure of the Sperchios basin. The Sperchios border fault lies to the south of the basin and is composed of five major segments. Segments are broken up by major transfer zones around which footwall topography is greatly reduced. (b) Topographic section across the rift through a transfer zone. (c) Topographic section across the rift through the centre of a fault segment.

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the town of which is situated in the centre of the developed along the footwall scarp on the southern side of basin (for location see Fig. 3). The natural vegetation of the the basin (Fig. 3). The Sperchios flows axially west to rift margin consists of maquis and oleander scrub, though east along the basin. Its position is strongly influenced by there is intense cultivation along the alluvial , which is alluvial fan systems and by subsidence associated with the divided into mainly cotton inland and paddy fields towards border fault (Fig. 3); for example, large alluvial fans the delta front. emanating from the footwall divert the Sperchios away from the footwall scarp (Fig. 3). In the east of the study area the Topography and Sperchios river discharges into the Maliakos Gulf, where it has constructed a series of bird’s foot delta lobes (Fig. 3). The topography of the Sperchios basin is closely linked to The location of the activedelta lobe is the result of an the active border fault along southern margin of the basin. in 1880 in the vicinity of (Philippson Elevationsalong theborder fault zoneare consistently 1950; Maroukian & Lagios 1987; Fig. 3). Evidence provided above 900m, with the limestoneescarpments of Knimis, from palaeo-shoreline maps (Kraft et al. 1987) indicates that Kallidromo and Iti dominating the topography (Fig. 2). The the Sperchios delta has prograded over 10 km since 480 BC. continuity of the footwall escarpment is broken by topographic lows, located at approximately 20 km intervals. At the western end of the basin, elevationsreach 1400m Bedrock lithology along the Timfristos range. In contrast, the hanging-wall Bedrock lithology plays a key role in drainagecatchment dip-slopealong thenorthern margin of the basin is development and consequentlyexerts a majorcontrol on characterized by a 600 m which climbs to 1400 m in the sediment supply. The exposedbedrock of the Sperchios Othrysrange to the east. The itself basin can be sub-divided into three major zones: a western comprises a wide alluvial/delta plain which passes eastwards pre-rift clastic zone, a northern hanging-wall ophiolite and into a partially enclosed marine gulf, the Maliakos Gulf limestonezone, and a southern footwall uplandzone, (Fig. 3). dominated by limestoneescarpments and unconsolidated Systematic variations in topography and slope‘ gradients Neogene basin (Fig. 4). Bedrock composition occuralong the length of the Sperchios basin, on both clearly influences thenature of the footwall scarp footwall and hanging-wall slopes (Fig. 2). Topographic topography. For example, the Sperkhiassegment, domin- cross-sections illustratevariations along thestrike of the ated by and conglomerates, has a maximum Sperchios border fault. Section 1 (Fig. 2b),across an elevation of 1400m, whereas the Kompotadessegment, intra-basinal transfer zone, shows low elevations (c500 m) which is located on more resistant Mesozoic limestone, has a on both footwall and hanging-wall slopes. In contrast, within maximum elevation of 2100 m (Figs 2 and 4). thecentral portions of the faultsegments, topography reaches maximum elevations of almost 2000m (section 2; Fig. 2c). Structure Large, low-angle alluvial fans, generally with areas of The border fault zone of the Sperchios basin has a major >l0 km2, characterizedeposition along the hanging-wall control on the topography within boththe basin andthe dip-slope (Fig. 3). These hanging-wall fans often coalesce to surrounding area. The fault zone is segmented on a variety form a broad hanging-wall alluvial fan bajada which of scales, with each major fault segment being broken up contrasts with the small alluvial ‘fans, often<2 km’, into numerouscomponent fault strands (sensu Roberts. &

fig. 3. Geomorphology of the Sperchios basin. The rift is dominated by the footwall escarpment to the south of the rift and contrasting dip-slope of the northern hanging wall. Transfer zone alluvial fans are large (>25 km2) and influence the position of the axial system within the rift. The Sperchios river flows eastward into the partially enclosed Maliakos Gulf where several bird’s foot delta lobes have developed.

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Fig. 4. Simplified geological map of the Sperchios basin. There are three major lithological zones within the basin: pre-rift clastic deposits to the west, ophiolite and limestone to the north, and limestone with Neogene unconsolidated sediments to the south. Based on the 1:500,000 seismo-tectonic map of Greece, with seismogeological data (IGME 1989).

Gawthorpe 1994). The main fault segments identified within drainagecatchment characteristics, basin structureand the Sperchios rift arethe Sperkhias,Kompotades, bedrock lithology. Thermopylae, Kammena Vourla and Arkitsa segments (Fig. 2). There is a strong between topography and structure Drainage networks alongeach individual segment;maximum elevations are situatedat the centre of faultsegments with topography Sperchios drainage is strongly influenced by the relief and decreasing away fromthe centre of the segmenttowards tectonicgradients produced by normal faulting, andboth fault tips. One example of this strike-variation in topography axial (E-W-trending) and lateral(N-S-trending) drainage is along theKompotades segment (Fig. 2), where the systems are present (Fig. 5). Axial drainage is dominated by topography decreases from Mount Iti, at 2152 m, to 589 m the Sperchios river and is fed by a dendritic network which and 374 m at the eastern and western segment boundaries drains the Timfristos range at the western end of the basin respectively. (Figs 2 and 5). Hanging-wall catchments are large, generally Migration of the locus of faulting (i.e. position of the exceeding 10 km2, and drain a relatively even dip-slope with border fault) has affected the south-eastern portion of the minorantithetic faulting. In contrast, the catchment Sperchios basin. Inactivefaults which bound the southern architecture of the footwall drainage system is strongly margin of the Neogene Renginion basin (Fig. 2) are now influenced by the geometry and spatial arrangement of the being uplifted in the footwall of active normal faults which normal faults and bedrock lithology variations. define the present-day coastline. The uplift of Neogene Drainagethroughout the basin is predominantly sediments within the Renginion basin provides a localized dendritic,although excellent examples of trellis drainage area of relatively easily eroded bedrock compared to older networks are found to the southeast of the active rift, within basement lithologies, such as Mesozoic limestone. the uplifted Neogene Renginion basin (Figs 5 and 6). Within this uplifted basin extension has caused blocks of sediment to back-tilt into the Pliocene-Pleistocene border fault to the south. These back-tiltedsurfaces have linearsoutherly Drainage networks and domains draining streamsor dipstreams which combine to form Topographyand tilting influence both slopelength and strike parallel tothe tilt block scarp. This gradient, and have a major influence on drainage catchment combination of dipand strikestreams forms the trellis size, orientationand networktype (e.g. Leeder network which is characteristic of regions wheretectonic 1991). This section of the paper first describes the nature of influence on drainage is high (Ollier 1981). the drainage networks that supply sediment to the Sperchios In contrast tothe lateral and axial drainage systems, basin, and secondly investigates the relationshipbetween discrete areas of drainage (A on Fig. 5) drain internally.

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Fig. 5. Drainage networks of the Sperchios basin. Trellis, centripetal and dendritic drainage networks dominate the basin’s drainage. Ephemeral and permanent streams are highly organized along the footwall, with large drainage networks (e.g. Bistritsa River) entering the Sperchios through transfer zones. Re-direction of the Sperchios occurs, at points 1 and 2, where the Sperchios course is altered by the transfer zone alluvial fans. Based on the Hellenic Army Geographical Service topographic sheets; Pelusyia, Stilk, Lumiu, Sperkhius and Kurpenision, (150 OOO).

Fig. 6. Drainage domains of the Sperchios basin. The drainage networks of the Sperchios basin have been divided into five drainage domains based on their relationship to structure and bedrock lithology (sensu Leeder et al. 1991).

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These largekarstic dolines and poljes are found on the The Sperkhias fault segment (Fig. 6) is dominated by hanging wall and on the island of Evvia to the east of the pre-rift clastic deposits. Along this segment, the mean dra- rift. inage catchment area is 6.8 km2 (n = 14), with the drainage catchments ranging in size from 1 km2 to 21 km2 (Fig. 7). In contrast, catchments developed along the Kompotades fault Drainage domains segment (Fig. 6) are mainly developed on Mesozoic lime- Drainage domains are characterized by a suite of drainage stone, although some of the larger catchments reach pre-rift catchments possessing similar characteristics such as size, clastic deposits further back intothe footwall. The mean areaand length, andshare a similar tectonic and/or drainage catchment area for the Kompotades fault segment lithological substrate (Leeder et al. 1991). On this basis, five is 5.4 km’ (n = ll), ranging in size from 1.6 km2 to 11.3 km’. dominant drainagedomains are defined forthe Sperchios The lower maximum drainage catchment size, compared to rift: (i) footwall, (ii) hanging wall, (iii) transferzone, (iv) the Sperkhias fault segment, is a reflection of these bedrock axial and (v) karstic domains (Fig. 6). changes. The relatively small size of the catchments de- veloped on Mesozoic limestonebedrock is also clearly Footwall drainage domain. This drainage domain consists of shown by the Kammena Vourla segment (Fig. 6), which has catchments developed along the footwall scarp of individual 25 individual catchments, with an mean area of 2.2 km2, and fault segments. Characteristically, the catchments of footwall a range from 0.3 km’ to 12.6 km’ (Fig. 7). Catchments along drainage domains are small, steep and short, with a mean the eastern part of the Thermopylae fault segment (Fig. 6) area of 6.25 km2 (n = 59). Variations in bedrock, which are also developed on Mesozoic limestone, and correspond- control the ease with which the scarp is eroded, influence ingly have relatively small catchment areas, with a mean of the ultimate size of the footwall drainage catchments. 1.74 km2 (n = 5) and a range from 1 km2 to 3.7 km2 (Fig. 7).

a) Footwall drainage Sperkhias segment 35 segmentThermopylae drains sandstone6 &- drains Neogene E YM and sedlment m $ 25 -$ 25 limestone g 20 5 15 0 g) 10 m a.z 0 west Relanveposition alongsegment east west Relative position segmentalong east

Kompotades segment Kammena Vourla segment &- drains limestone, drains limestone 8 conglomerate 25

6 20 E

west Relabve position segmentalong east

b) Hanging-wall drainage

50 Fig. 7. Footwall and hanging-wall dra- 40 inage catchment areas. (a) Catchment 30 areas and relative position along each 20 footwall segment illustrated on Fig. 6. 10 The size and number of drainage 0 catchments along the footwall is a west Retalivehangingwall psiiton a!mg dip slope east function of the variations in bedrock lithology. (b) Catchment areas along the Pre-rift clasticsPre-rift Ophiolite Triassiclimestones hanging-wall dip-slope as a function of 0 bedrock lithology and relative position Neogeneunconsolidated sediment Schist along the axis of the basin.

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However along the western half of this fault segment the In detail, the transfer zone is not a simple catchments are mainly developed on unconsolidatedNeo- relay ramp, but contains a series of fault strands which help gene sediments. Here the mean drainage catchment size is to accommodate the displacementbetween the adjacent 19.6 km2 (n = 6) with a range between 8.4 km2 and 33 km’ faultsegments. These fault strandsform prominent (Fig. 7). These catchments,developed on unconsolidated escarpments which are drained by their own small footwall and thus relatively easily eroded Neogene sediments, are the catchments. However, the main effect of the fault strands largest of the footwall drainage domain. hasbeen to divide the transferzone drainage intothree componentcatchments which supply the Gorgopotamos, Transfer zone drainage domain. Comparedto footwall Chanorrema and Asopos (Fig. 8). catchments, transfer zone catchments are of lower gradient, larger size (10-300 km’), and can extend for several tens of Hanging-wall drainage domain. Drainagecatchments de- kilometres behind the footwall scarp. From the nine transfer veloped on the gently dipping hanging-wall dip-slope of the zone catchments along the length of the Sperchios border rift are generally larger than their footwall counterparts. The fault zone, the mean catchment area is 87.5 km2. meandrainage catchment area along the hanging wall is Bedrock lithology also acts as an important control on 10.9 km2 (n = 74) with areas ranging from 0.3 km2 to 76 km’. catchment area in transfer zones. The Bistritsa transfer zone The high diversity of catchment forms and areas along has a single large catchment, located between the Sperkhias the hanging wall reflects along-strike variations in bedrock andKompotades segments in the west of the study area lithology, dip of the hanging wall and position of antithetic (Fig. 6). This catchment has an area of 300 km’ and drains fault segments. Figure 7b summarizes the variation in pre-rift clastic deposits. In contrast, several catchments are catchment area with respect to relative position along the developed in the Gorgopotamos transfer zone between the hanging-wall dip-slope and dominant bedrock lithology (i.e. Kompotadesand Thermopylaesegments (Figs 6 & 8). where the dominant lithology covers >SO% of the Individually, these are smaller (40km2) and drain a range catchment area). Within the Sperchios basin the hanging- of bedrock lithologies including Mesozoic litnestone and wall drainagedomain displays a relatively well ordered schist as well aspre-rift clastic deposits. Despitetheir nature with drainagecatchments anorder of magnitude smaller size the catchmentsdraining theGorgopotamos larger than their adjoining catchments occurring approxim- transferzone are still considerablylarger than those that atelyevery ten catchments, orat approximately 12 km drain the footwall of the adjacent fault segments (Fig. 8). intervalsalong the hanging wall. This spacing of major

Fig. 8. Detailed drainage catchment analysis acrossthe Gorgopotamos transfer zone. The transfer zone is divided by two fault strands, and this division is exploited by three drainage networks; the Gorgopotamos, Chanorrema and Asopos rivers. The transfer zone fans developed from these rivers coalesce to form a fan area of 37 km2 (A). The exploitation of the transfer zone by drainage is shown by the area of the three transfer zone catchments compared to the adjacent footwall catchments (inset).

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catchment outlet position appearsto beindependent of Along theborder faultzone large catchments are clearly antithetic faults on the hanging-wall and bedrock lithology linked to the position of transfer zones separating individual (Fig. 7b). fault segments. In addition, clusters of relatively large catchments (e.g. A in Fig. 9a) are associated with bedrock Axial drainage domain. The area of the catchment at the that is more easily eroded compared to the adjacent areas. western end of the Sperchios basin that specifically supplies On the hanging-wall dip-slope the position and spacing of sediment to the Sperchios river is 228 km2 (Fig. 6). This area majordrainage outlets are not directly associated with is smaller than the largest of the lateral feeder systems that antithetic fault segments (Fig. 9b). develop through transfer zones, such as the Bistritsa transfer zone. The limited size of the axial domain is dueto the Hellenide thrust sheetsto the west of the basin which Antecedence and capture restrict westward extension of the Sperchios drainage net- Antecedent drainage,in terms of the rift setting under works. In general axial drainage catchments are often larger discussion, comprises thosedrainage networks that existed thanthe Sperchiosexample as there is thepotential to prior to normal faulting. Antecedent drainage systems may exploit the lower topography associated with the end of the maintain their course despite the generation of new slope border fault zone. For example, the axial drainage domains gradients and may include anomalously large catchments within rifts in western are often equal in area to the with major outlets cutting through the fault scarp (Zernitz total lateral drainage of the basin (e.g. the Biiyiik Menderes 1932) often producing dramatic gorges. If a stream can not graben). keep pace with the new slopes generated by faulting, drainagedeviation or drainagereversal may occur (e.g. Karst drainage domain. Karstdrainage (Fig. 6) andthe Leeder et al. 1991; Seger & Alexander 1993). This associated centripetal(internal) drainage developed on commonly occurs in the footwall of fault segments when a Mesozoic limestoneexposures is limited to discrete areas stream course cannot keep pace with uplift and the streamis within the basin, where dolines/poljes develop. The major diverted,often contouring a fault segment to emerge controlon this drainage domain is solutionweathering; through a transfer zone. however, the distribution of exposed Mesozoic limestone is Antecedent catchments are identified by theirlarge partlya function of uplift and subsidence associated with catchment areas along thecentre of fault segments normal faulting. combined with incision along the footwall scarp. Within the Sperchios rift, identifying cases of antecedence is further Drainage development and sediment supply compounded as theupper reaches of someantecedent streams (e.g. X on Fig. lO), developed centrally along a fault Both footwall and hanging-wall drainagecatchments in segment, have been partially captured (C) by stream excess of 25 km2 exhibit aregular spacing of their outlet networks of adjacent catchments. FanA along the streams along the length of the Sperchios basin (Fig. 9). Kompotades faultsegment (Fig. lO),is 21 km2 and anomalously large compared to its catchment (catchment X, R f24& R 124& Fig. 10) which is 11 km2. The upper reaches of catchment X, and also catchment Y draining the Sperkhias fault segment, Footwall and transfer zone have been captured and flow redirected through the Bistritsa catchments >25 kn? transfer zone (T). The alluvial fans (A and B) developed from boththese captured catchments are now 'fossilized', being well vegetated(maquis scrub) and incised. The Sperchios river is eroding fan A and has created a 2.5 m cliff along the banks of the river, testifying to the present low sediment supply to this fan. This compares markedly with the Bistritsa transferzone fan which, because of its high sediment supply, is diverting the Sperchios river away from the border fault zone. Hanging-wall catchments >25 kn?

n R Implications for stratigraphy within rifts Figure 11 summarizes the distribution of catchments around the Sperchios rift andthe location of associated syn-rift depositional systems. In addition, examples of drainage reversal are included based on examples from the Gulf of o 10 M 30 40 H) 60 70 80 W8l DI.Ume along basin 0x1sL (km) Corinth (e.g. Seger & Alexander 1993). Zones of high Fig. 9. Schematic representation of outlet spacing of major sediment supply are located at discrete points along the rift catchments along the axis of the Sperchios basin. (a) Footwall and and are related to the drainagecatchment architecture of transfer zone catchments along the border fault zone defining the the rift margins. Along the border fault zone, high sediment southern margin of the basin. 'A' indicates catchments where supply is associated with the largecatchments in transfer lithology is the main factor controlling catchment area. Note that zones. Antecedent catchments may also create localised spacing of footwall outlets is dominated by the position of transfer areas of high sediment supply along the rift. In contrast to zones. (b) Hanging-wall catchments along the northern dip-slope to the border fault zone, structural control on the development the basin. Note the regular spacing, at approximately 12 km of largecatchments (and hence the location of high intervals, of outlets along the hanging wall. sedimentsupply) onthe hanging-wall dip-slope is not as

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Fig. 10. Transfer drainage, antecedence and capture associated with the Bistritsa transfer zone. The Bistritsa river exploits the transfer zone at T. Catchments X and Y are antecedent to the Sperkhias and Kompotades fault segments; their upper reaches have been captured (C) by the Bistritsa transfer zone drainage system.

pronounced. In addition tothese structural controls on accommodation development at transfer zones compared to drainageand sediment supply, variationsbedrockin the centre of fault segments. However, the large catchments lithology play animportant role in controllingcatchment associated with transferzones result in high rates of size and thus sediment supply in all drainage domains. This sediment supply (Figs 8 and 10) and this, combined with the rather 'static' view of drainage-sediment supply relationships low rates of accommodation development, leads to highly in rift basins is complicated by temporal changes in drainage progradational sequence sets (Figs 3 and 12). This complex networks and catchments as they evolve and capture takes interplaybetween sediment supply and accommodation place. development will lead to varying stratigraphic architectures The variationsin subsidence and uplift aroundfault along the length of the rift zone. segments which contrql slope gradients and lengths also control relative sea-level change and accommodation development within rifts (Gawthorpe et al. 1994). Together, Conclusions the variations in sediment supply, as discussed in this paper, Drainage development within the Sperchios rift is strongly and variations in accommodationdevelopment around rift influenced by slopegradients imposed by deformation basins, controlthe stratigraphy of the basin fill. The around normal fault segments. Five major drainage domains combined effect of variationsin both sediment supply can be identified within the Sperchios rift, each domain has (relatedto catchmentcharacteristics) and subsidence are common characteristics (e.g. morphology, area) which share summarized in Fig. 12. Along the footwall scarp, high fault similar tectonic and/or lithological substrate. Transfer zones slip rates lead to high rates of accommodation development, are characterised by large, low gradient catchments which yet sediment supply is generally low because of the small actas the mainconduits for sedimentsupply along the catchments developed in the footwall of fault segments (Fig. border faultzone. In addition, high sediment supply may 11). As a result, nearthe centre of faultsegments, the also be associated with antecedent catchmentsalong the stratigraphy close tothe footwall scarp is composed of footwall of the basin, but these are unrelated tothe aggradationalsequence sets, which display limited pro- structure of theborder fault zone. In contrast, footwall gradation away fromthe footwall (Fig. 12a). However, catchmentsdeveloped along fault segments are typically antecedent catchments may deliver greater volumes of small andare associated with low sediment supply. The sediment at specific point along the footwall giving rise to drainage outlet positions of large hanging-wall catchments abnormally large, progradational depositional systems (e.g. (>25 km') occur at regular intervals along the axis of the fan A on Fig. 10). Sperchios basin. Thisorganisation of drainage outlet In contrast to fault segments, transfer zones aregenerally position has developed largely independently of lithological characterized by gentlebasin topography (e.g. Fig. 2b) controls and appears to beunrelated to antithetic faults. which results from low slip rates near fault tips and in the The spatial variations in sediment supply estimated from transfer zone. These low slip rates result in lower rates of analysis of drainage networks and catchmentsfrom the

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H

R

C

Fig. 11. Model for the distribution of drainage catchments and associated syn-rift sediments within a coastal rift basin based on the Sperchios basin. The model shows the relationship between drainage catchment areas and fan areas within extensional settings. Drainage within half-graben is dominated by large transfer zone drainage catchments. Transfer zone streams may capture partial areas of antecedent drainage catchments, which existed prior to fault propagation. Variations in drainage catchments are also strongly influenced by bedrock lithology, for example, the difference in area of the footwall catchments associated with the two fault segments.

a) Fault segment centre FOOTWALL SOURCED -small depositional systems

*high sl~prates high footwall elevation *low hanging-wall elevation HANGINGWALL SOURCED *large drainage Catchments extend up the hanglng-wallslope *broad depositional systems Fig. 12. Effect of spatial variations in -type 1 sequences drainage catchments and resultant sedi- retrogradailonal to aggradatinal sequence sei - ment supply on stratigraphy. Low TRANSFER ZONE SOURCED sediment flux from footwall catchments, b) Transfer zone *large depositionalsystems -type 1 sequences together with high rates of hanging-wall subsidence, results in aggradational sequence sets restricted close to the footwall escarpment along fault seg-

,I, ments. In contrast, transfer zone se- * large drainage catchments _, .high sediment supply quence sets are highly progradational low slip rates I, ,- due to high sediment flux sourced from *low lootwall elevations 100-3~m I *high hanging-wall elevations transfer zone drainage catchments and 2-5 km limited subsidence at fault tips.

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Received 27 July 1994; revised typescript accepted 19 January 1995.

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