Basin Research - Volume 1, Issue 3, 1988

Abstract Average strain rates are calculated from earthquakes in the period 1908‐81 that occurred in the Aegean Sea extensional region, and in the convergent zone associated with the Hellenic Trench. In spite of large uncertainties resulting from the use of an MS:Mo relationship, seismic N‐S extensional rates in the Aegean are in the region 20–60 mm yr^‐1 whereas seismic shortening rates in the Hellenic Trench are less than about 15 mm yr^‐1. This is surprising because Africa and Eurasia are known to be converging, not separating. This apparent anomaly is caused by most of the convergence in the Hellenic Trench occurring aseismically. By contrast, the seismic extensional rates in the Aegean agree quite well with those expected from other arguments. The present day extensional rates are sufficiently high for McKenzie's instantaneous stretching model to be applicable. There is some evidence that these high extensional rates have operated throughout the last 5 Myr.

Abstract An account is given of the structural setting of the various Neogene sedimentary basins of western Greece. Compressional basins are attributable to foreland loading by the Alpine fold and thrust belt of the Outer Hellenides, and to active subduction in the adjacent western Hellenic arc. Late extensional basins are related to N‐S crustal extension in the Aegean marginal basin and, in western Greece, are superimposed on the earlier compressional structures. The local seismicity provides evidence that the main E‐W‐trending basin‐bounding faults of the extensional basins form a linked system that includes NW‐SE‐ and NE‐SW‐trending transfer zones of transtension. The transfer zones are themselves the sites of small extensional basins.

Abstract The initial stages of rift‐basin evolution are periods of great landform change. Fault scarps are newly created axes of erosion which, along with footwall uplands, act as sediment sources for subsiding hanging wall basins. Scarps formed during neotectonic normal faulting of Mesozoic carbonates in mainland Greece and western Turkey display a varied pattern of degradation related to the history of fault development and variations in fault zone architecture. Alternating zone‐parallel compact breccia sheets and incohesive breccia belts, of contrasting resistance to erosion, underlie scarps. Meso‐scale slip‐plane phenomena, such as corrugations, gutters, comb fractures, and pluck holes, together with geomorphological features, such as subsurface solution pipes, and vegetation result in initial variations in the denudability of erosionally resistant compact breccia sheets. Migration with time of slip‐plane activity within a fault zone into its hanging wall (i.e. intrafault‐zone hanging wall collapse) adds to the structural heterogeneity of fault scarp footwalls. Quaternary talus, whether offset across a fault, banked unconformably against a slip plane, or faulted against a reactivated slip plane, has a dampening effect on degradation. The complexities of fault zone architecture combined with a history of hanging wall collapse lead, in the Aegean region, to non‐uniform degradation and scarps which are commonly stepped and occasionally cavitated.

Abstract The Plio‐Quaternary history of the Gulf of Corinth Basin has been controlled by dominantly north‐south extension. The basin has an asymmetric graben geometry that is, at the present time, controlled by a master fault (the Gulf of Corinth Fault) downthrowing to the north and running offshore from the north Peloponnese coast.

Detailed structural interpretation of single‐channel seismic data collected during RRS ‘Shackleton’ cruise 1/82 combined with onshore structural studies indicates that the basin geometry is not controlled simply by the main Gulf of Corinth Fault. The subsidence history for the uppermost 1 km of sediment can be documented using time‐structure contour maps and isochron maps. These indicate that there is a general narrowing in the size of the basin with time, achieved by fault‐controlled subsidence switching to antithetic faults concentrated towards the basin centre. It can also be demonstrated that growth of sediments into topographic lows is not only controlled by sea bed rupture but also by more passive sea bed flexure over ‘blind’ faults at depth.

The main conclusion of this study is that the 3D geometry of the Gulf of Corinth Basin changes not only spatially but also temporally. Active growth faulting and, therefore, the position of depocentres can switch across the basin and the relative importance of synthetic and antithetic faults controls the geometry of the basin, forming grabens, asymmetric grabens and half‐grabens throughout the basin history.

Abstract Detailed single‐channel continuous seismic reflection profiling data from four gulfs as well as onshore neotectonic investigations have allowed the study of the neotectonic structure of the Hellenic arc along a complete transverse section from its external area in the trench to the internal back‐arc area.

Messiniakos Gulf is an asymmetric NW‐SE structure with considerable tilt towards the NE. It is the direct continuation of the continental slope from the trench to the island arc (Peloponnesus, Crete, Dodekannese). Argolikos Gulf is an almost symmetric NW‐SE graben occupying the northern edge of the Cretan back‐arc basin. Saronikos Gulf is a multi‐complex structure of a NW‐SE graben in the SW (Epidaurus Basin) and alternation of E‐W horsts and grabens in the North. Its neotectonic evolution is characterized by the Plio‐Quaternary volcanic arc activity. Southern Evoikos Gulf is a relatively shallow neotectonic graben in the back‐arc area at the northern prolongation of the Cycladic Platform.

Each of the above neotectonic basins has its own characteristics which are probably due to their geodynamic position in the Hellenic arc. In general, there is a decrease in the neotectonic deformation, the sediment thickness and the sedimentation rates from SW to NE, going from the periphery to the core of the arc.

Abstract During the migration of the back arc extension from central to western Greece the Corinth and Patras grabens are being formed. Orthogonal opening of these graben zones is accomplished by WNW listric normal faults and NNE transfer faults which produce an along‐axis fragmentation. The listric faults show an increase in the dip of the fault plane westwards as well as a decrease in the maximum extension rate from 50% to the east in the Corinth graben, to 10% to the west in the Patras graben. Similarly, towards the west, Plio‐Quaternary deposits become thinner whereas Pliocene sediments thin‐out indicating a westward rift propagation.

As the back arc extension migrates westwards it is interacting or is being superimposed above another orthogonal fault system consisting of NNW and ENE normal faults. These faults have been formed during general uplift behind the orogenic front which has been migrating from western Greece to the Ionian islands. The ENE‐trending Rio graben which belongs to his orthogonal system connects the Patras graben to the Corinth graben and has subsequently been active as a transfer fault between them.

Plio‐Quaternary geodynamic processes in central continental Greece are quite similar to those earlier processes observed in the central Aegean region which reflect the initial stage of continental break‐up behind a migrating orogenic front.