Journal of the Geological Society, London, Vol. 152, 1995, pp. 883-893, 12 figs. Printed in Northern Ireland Drainage development and sediment supply within rifts, examples from the Sperchios basin, central Greece 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 rift allow detailed analysis of the relationships between drainage catchments. sediment supply and alluvial fan/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 fault 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) karst 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 sedimentation 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 mountains 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 central Greece (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 and contains 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 drainage basin is an asymmetric half graben 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 Pleistocene 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 month drought 1). Extension startedaround 5 Ma, during the early over the summer. Mean temperatures forJuly reach 28 "C in 883 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/5/883/4890372/gsjgs.152.5.0883.pdf by guest on 01 October 2021 884 P. P. ELIET & R. L. GAWTHORPE Fig. 1. Tectono-sedimentary setting of central Greece, showing major faults and Neogene sedimentary basins. Inset shows the structural setting of central Greece. l 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/5/883/4890372/gsjgs.152.5.0883.pdf by guest on 01 October 2021 DRAINAGESEDIMENTAND SUPPLY WITHIN RIFTS 885 the town of Lamia 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 river 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 plain, 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 geomorphology 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. avulsion in 1880 in the vicinity of Thermopylae (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 ridge which climbs to 1400 m in the sediment supply. The exposedbedrock of the Sperchios Othrysrange to the east. The sedimentary basin 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 sediments (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 sandstones 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