Uplift, Deformation and Fluid Involvement Within an Active Normal Fault Zone in the Gulf of Corinth, Greece

Journal ofthe Geological Society, London, Vol. 151, 1994, pp. 531-541, 7 figs. Printed in Northern Ireland Uplift, deformation and fluid involvement within an active normal fault zone in the Gulf of Corinth, Greece GERALDROBERTS' & IAIN STEWART' 'Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London WClE 6BT, UK 'Division of Geography and Geology, West London Institute, College of Brunel University, Borough Road, Isleworth TW7 SOU, UK Abstrart: The active Pisia Fault Zone is exposed close to the crest of a rotated carbonate fault-block that forms part of the southern margin to the Gulf of Corinth half-grabenin central Greece. The crest of the fault-block lies above sea-level whilstin the hanging-wall a marine basin exists. As a result, the diagenetic and structural characteristics of the fault zone record complexfluid involvement. Both early phreatic carbonate syn-kinematic cements (characterized by drusy fabrics, baroque dolomites and crack-seal textures) and late vadose carbonate syn-kinematic cements (characterized by flowstones containing cave-collapse debris) exist in the uplifted footwall to the fault. Downward percolation of vadose meteoric waters and the upwelling of pore waters with elevated temperatures produced the diagenetic features observed within the fault zone. The co-existenceof early phreatic and late vadose cementswithin the fault zone is related to footwall uplift across the water-table and subsequent erosionalunroofing. The present-day elevation of phreaticcements to c. 650mabove current sea-level, providesa minimum estimate of footwall uplift along the fault. The wider implication is that temporal changes in deformation and fluid flow may typify fault-block evolution. Recent studies have highlighted the uncertainty associated Diagenesis and footwall uplift in the crests of with predicting diagenetic and deformation features within carbonate fault-blocks the crests of fault-blocks. The poorly-understoodfactors that combine to limit prediction include the sources of pore Figure 1 summarizes the diageneticfeatures common in waters involved in diagenesis (Bjerlykke et al. 1989; Burley fault-controlled sea-cliffs composed of carbonates. It is well & MacQuaker 1992), the drivingmechanisms for fluid established that in such settingsthe water-table, which migration(Sibson et al. 1975; Carter et al. 1990;Sibson separatesthe vadose environment from the phreatic 1990), and the textural features associated with faults and environment, is generally at or slightly above sea-level due fracturesthat mayinfluence porosityand permeability tothe hydrodynamic head(Allan & Mathews 1982; (Knipe 1992). The preservation potential of structures and Humphrey 1988). Thus although after periods of heavy rain, diagenetic features at the crest of fault-block structures is the water-table may be higher than sea-level, out-pourings also uncertain, because uplift above sea-level may result in of freshwater from springs along the cliffswill return the erosion of the crest of the structure (Barr 1987;Yielding water-tableclose tothe level of sea-level. Similarly, the 1990). water-table cannot be belowsea-level as sea-water would This paper focuses on the diagenetic and deformation pour intothe air-filled fractures again restoringthe features within the crest of the carbonate fault-block that water-table close to sea-level. Phreatic cements, precipitated formsthe southern shores of the Gulf of Corinth below the water-table, are characterized by drusy fabrics half-graben, central Greece. Field and petrological observa- and dolomites and can be distinguished easily in the field tions from actively evolving fault zones, such as those in the fromvadose cements,precipitated above the water-table, Gulf of Corinth,permit the spatial relationships between which are commonly asymmetricand form stalactites, structuresand diagenetic features to beexamined more stalagmites and flowstoneswithin karstic cavities. This easily than can be achieved from studies of core material permits field geologists to distinguish deformation features from fault-block crests in the sub-surface. In the first part of developed close to surface from those formed at depth. In the paper, the idealized diagenesis of an active normal fault addition,the fluid sources within thestructure can be within carbonates is discussed. Following a brief outline of constrained. the regionalgeological setting, thepaper focuses onthe Withinactive normalfault zones vadose andphreatic deformation and diagenetic features of the Pisia Fault Zone, diagenetic products may, however,not show the simple an active normal fault zone in the eastern Gulf of Corinth. depth variationshown in Fig. 1. Field observations in Finally, the wider implications of this study in terms of the central Greece haveshown that fault-block crests are not sources of fluid, the driving mechanisms for fluid flow, the stationary, but become uplifted relative to sea-level during nature of faults and fractures and the preservation potential earthquake episodes (Jackson et al. 1982; Vita-Finzi & King of features are discussed for other fault-block crests. 1985). If the level of the water-table stays close to sea-level Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/3/531/4889927/gsjgs.151.3.0531.pdf by guest on 26 September 2021 532 G. ROBERTS & I. STEWART i 1 DEPTH VARIATIONIN j CEMENT GEOMETRIES i DIAGENESIS i FAULT PLANE BOUNDING i j A ZONE OF CARBONATE .I.--_-- i 1) INFILTRATION ZONE i 1 Vertical caves, soils, 1 vegetation, collapse i B- Growth Hiatus breccias. fine-grained i showing speleothems. i dissolution and __-___-I.___-_._.- j 1 internal sediment 2) PERCOLATION ZONE 1 l C- Central hole CaCO, and H,CO, are in 1 i equilibrium, so that little D- Clean calcite i crystals grow dissolution or precipitation i i orthogonal to the occurs. Water descends i Flowstone growth zones through existing cavities. i i No Scale is Abundant speleothems in 1 implied the lower portion. i i .----------------3) CAPILLARY ZONE .li .-------- 4) UPPER LENTICULAR ZONE aSmall crystals on i Sub-horizontal caves, hydraulic i the sides of the i cavity are erosion and corrosion due to i mixing of waters predominate. i overgrown by Some speleothems stch as i larger crystals cave pearls, calcite rafts. i whose growth Internal sediments and collapse i surfaces point i inwards from the INFLUX INTO breccias. Porosity is filled with 1 margins of the WATERSEA THE PHREATIC water resulting in drusy fabrics. cavity. Equant SYSTEM INTERNALSYSTEMSEDIMENT I 5) STAGNANT ZONE 1 crystals fill the 1 centre of the (Waters do not mix with Zone4 cavity. and this zone grades downwards1 -1cm SPELEOTHEMS i I m I into the deep conflate zone) i Fig. 1. Illustration of the depth variationin diagenesis that may be expected within the crestsof carbonate fault-blocks lying closeto sea-level. The diagram is adaptatedfrom Gascoyne & Schwarcz (1982), Scholle et al. (1983) and Tucker& Wright (1990). duringfootwall uplift then, presumably,phreatic cements at least 700 m of syn-rift stratigraphy (Brooks & Ferentinos will be over-grown by vadosecements and subjected to 1984; Higgs 1988). karstification as the rocks emerge above the water-table/sea- At the eastern end of this majorbasin-bounding fault level. With knowledge of eustaticsea-level changes, it is system, along thesouthern shores of the Gulf of possible to assess footwall uplift relativeto thewater- Alkyonides,faulting is expressedonshore as aseries of table/sea-level by examiningoverprinting relationships discontinuous fault strands that delimit the northern flanks between vadose and phreatic cements. Also, it is clear that of thecarbonate-dominated Gerania Mountains (Fig. 3). erosion accompanying footwall upliftwill progressively Some of these fault strands were ruptured during the 1981 strip-off theearliest-formed vadose portions of the Gulf of CorinthEarthquakes (M,<6.7) (Figs 2 & 3) fault-block,thereby unroofing the deeper-seated phreatic (Jackson et al. 1982; Taymaz et al. 1991). Well-located zone,and influencing thepreservation potential of hypocentrallocations for the 1981 earthquakes combined deformationfeatures in theuppermost levels of thefault with field studies of the surface-breaks indicate the geometry zone. of the fault to be moderately-dipping (40-50") and planar down to depths of c. 12 km (Jackson et al. 1982, Jackson & White 1989, Taymaz et al. 1991). The surface expression of Geological background to the Gulf of Corinth fault strands are topographic escarpments several hundred The Gulf of Corinth is a 120 km long, 30 km wide marine metres in height and delimited at their base by limestone basin that lies within the Aegean extensional province. This faultscarps and 1981 groundruptures. The fault zone province, characterized by north-south extension, separates studied in this paper, here termed the Pisia Fault Zone, lies plate convergence and subduction alongthe Hellenic Trench along the base of theescarpment between Pisia and from dextral strike-slip crustal motion along along the North Perachora (Fig. 3). Aegean Trough (Fig. 2) (Kelletat et al. 1976; Le Pichon & Raised beaches,uplifted coastal notches and incised Angelier 1979; Le Pichon 1982;Billiris et al. 1991). drainage basins along the southern margin of the Gulf of Seismological and structural studies indicate that extension Alkyonides indicate substantial uplift of the footwall to this is accommodated by movement on east-westtrending stretch of thebasin-bounding

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