Distribution of extensional strain in space and time: Evidence from the Gulf of Corinth, Greece and other rift systems Lisa McNeill 1, Rebecca Bell 1,2, Jon Bull 1, Tim Henstock 1 1 National Oceanography Centre, University of Southampton, UK 2 Now at GNS Science, Lower Hutt, New Zealand

Examples of variable strain distribution Strain distribution along a rift, e.g., basin segmentation and fault activity with time Strain distribution across the rift, e.g., evidence for localisation of strain with time, symmetry of rifting Relative complexity of rifting in time and space, e.g., number of significant faults and polarity Depth-dependent stretching (crustal, lithospheric) Along-axis rift propagation

The significance of young rift systems - what can they offer? Details of early rifting processes are often masked by later deformation in more mature systems Active systems with high strain rates, simple pre-rift history and a stratigraphic record of recent deformation offer the opportunity to: –Quantify extensional strain across and along the rift and with depth, often at high resolution –Examine rift development and change on short timescales (100,000 ka - 1 Ma) relative to net extension over longer timescales Young rifts may not all proceed to spreading, but distribution of strain within the earliest rift phase is of importance to our understanding of rift initiation, rifting mode and the evolution of rift systems Complimentary to rift systems at continental breakup, e.g., Woodlark basin, Red Sea, Gulf of California Example: The Corinth rift –Young: Initiating ~ 5 Ma, present phase < 2 Ma –Present-day strain rates are high (up to 20 mm/yr) –A “narrow rift”, but within the more complex Aegean “wide rift” or “core complex” type (e.g., Buck, 1991)

Gulf of Corinth, Greece McClusky et al., 1996

Bell et al., in press, Basin Research

B. Moho depth (Sachpazi et al., 2007) Crustal thickness: Decreases W-E (pre-existing crustal thickening of the Hellenide mts. in west). Along rift axis, decreases from >40 km to <30 km eastward (Sachpazi et al., 2007; Zelt et al., 2005) Thickness of brittle crust: ~ 10 km Thermal structure: Limited data (Fytikas and Kolios, 1979) suggest heat flow increasing eastward, up to 80 mW/m 2 around Corinth, but may be affected by arc volcanism Magmatism: Little evidence of rift- related magmatism in imaged upper crust Rate of extension: 5-20 mm/yr, increasing westward (geodetic, e.g., Avallone et al., 2007). Duration of rifting: ~5 Ma, initiation of active rift system ~1-2 Ma. Rift width: ~60-80 km, active rift system ~20-30 km. A. Geodetic strain rates across the Gulf (Clarke et al., 1998; Briole et al., 2000) A B

Methods Quantifying fault slip rates –Marine terrace uplift; Paleoseismology; (Surface exposure dating), Stratigraphic analysis offshore Distribution of syn-rift sediments and geometry of depocentres through time –Seismic stratigraphy; Mapping of onshore sediments Patterns of uplift and subsidence (past and present) Extensional strain across the rift over a range of timescales –Geodetic present day strain; Fault displacement; Crustal thinning Methods largely applied to upper crust. But these patterns of strain distribution and shallow rift development inform us about deformation at depth

Seismic stratigraphy of active rift system S N Bell et al., 2008, GSA Bulletin, Bell et al., in press, Basin Res. McNeill et al., 2005; Lykousis et al., 2007 Cyclical stratigraphy can be identified in youngest syn-rift sediments - correlated to glacio-eustatic cycles Two distinct syn-rift sequences in active offshore basin: A ca. 0.4 Ma B - ca. 0.4 Ma Ma

Bell et al., 2008, GSA Bulletin

Sachpazi et al., 2003; Bell et al., in press Rohais et al., 2007, Ford et al., 2007 (Collier and Jones, 2003) Offshore (active rift): 2 distinct sequences Onshore (syn-rift strata of inactive and active rift): 3 distinct sequences What is the correlation between onshore and offshore sequences? Comparing onshore and offshore syn-rift stratigraphy

Changing rift geometry in space and time

Development of the active rift basin Basement Depth (time) Basin Fill (time) Bell et al., in press Net subsidence and basin fill (over ~2 Ma) appears relatively simple (dominated by a single depocentre controlled by multiple fault segments)

Development of the active rift basin Bell et al., in press On shorter timescales, changes in basin segmentation, relative fault activity and polarity and depocentre development can be seen: Two discrete km length depocentres/segments (opposite polarity) of Unit B coalesce to form a larger km length depocentre/segment during deposition of Unit A (dominated by N-dipping faults)

Development of the modern Corinth rift basin Bell et al., in press Complex rift structure, extension distributed across multiple active faults (see also McNeill et al., 2005): Simple rift structure (1/2 graben, few active faults), structure maintained during rift evolution:

Rift Structure and Evolution (modern and earlier rift system) Contrasting strain distribution and rift development in west and east rift - Influence of initial crustal/lithospheric structure or rheology? Multiple faults active across rift (more complex rift structure) Strong evidence for northward migration and focus of rifting Extension focussed on fewer faults (simpler rift structure) Earlier rift history less well constrained (and debate over current activity of S margin faults) Onshore faults after Collier and Dart (1991), Rohais et al. (2007), Roberts (1996), Ford et al., 2007), Goldsworthy and Jackson (2001)

Extension on Different Timescales Geodetic strain rates show a strong W to E gradient This contrasts with geologic extension patterns over rift history: - long-term basin fill and basement depth of modern rift (maximum in central rift) - estimates of extension during rift history (B above)

Summary of Corinth rift characteristics Syn-rift sediments fill an apparently single depocentre in the active central rift but this results from multiple linked depocentres and changes in fault activity with time (100’s ka timescale) Along-axis differences in distribution of strain: –Complex rifting in west: multiple faults, changes in rift geometry/fault polarity with time. Now the location of greatest present-day extensional strain (GPS) –Simpler asymmetrical rift geometry in central-east rift –What controls this difference? Pre-existing crustal structure, different mode of extension (detachment faulting proposed in west), rift propagation? Rift narrows and rift axis shifts northward with time in western rift (and to some extent elsewhere) - evidence for localisation of strain Little evidence for along-axis rift propagation on 2-5 Ma timescale (synchronous initiation of rifting favoured) Extrapolating geodetic extension rates over rift history (1-5 Ma) does not produce observed patterns of geological extension

Basin/Fault Segmentation and Evolution Example: The significance of linking isolated fault segments and the breaching of an intrabasin high to form one depocentre, E.g., Basin and Range (Anders and Schlische, 1994) Segmentation is observed at a range of spatial scales but how does segmentation in the early stages of rifting relate to that of evolved rifted margins and spreading axes? Rift and fault segments interact and link in early stages of development shifting patterns of displacement, subsidence and sediment accumulation. In general numerous small fault systems will coalesce into a few larger systems along strike (e.g., Cowie et al., 1998)

Complex fault and rift geometry (changing along rift axis) also observed in other rift systems E.g., Gulf of Suez (Bosworth et al., 2005). Polarity of faults, geometry of rift and number of active faults changing along rift system Such detailed studies are more commonly based on rift systems in the early stages of extension and over relatively short timescales How does this complexity evolve into the the structure of mature rift systems? Rift Complexity Bosworth et al., 2005

Strain localisation Strain localisation at fault scale, e.g., Suez rift (Gawthorpe et al., 2003) And at rift/basin scale, e.g., Woodlark Basin (Goodliffe and Taylor, 2007), Red Sea (Cochran, 2005), Taupo rift zone (Berryman and Villamor, 1999) At what stage of the rifting process does this change in strain distribution take place and is it driven by the same process in each case? (In the Corinth rift, rift localisation and northward fault migration may be in part linked to regional processes) 5 km

Depth-dependent stretching commonly observed at rifted margins (e.g., Davis and Kusznir, 2004) Whitmarsh et al. (2001) Davis and Kusznir (2004) suggest that depth- dependent stretching may be limited for small amounts of extension (i.e., early stages of rifting) Extension with Depth Davis and Kusznir, 2004

Conclusions High resolution studies of young rift systems reveal details of strain distribution and rift evolution in space and time In the Corinth rift, major changes in rift geometry occur on timescales of ~0.5 Ma and rift structure varies significantly along axis on a length scale of ~ 50 km For this example (and others), the proposed chronostratigraphy requires testing - IODP drilling and sampling. A proposal has been submitted in order to more fully resolve strain rates and rift evolution. Many of the characteristics and processes observed in young rift systems are similar to those of more mature rift systems (in spite of differences in extensional strain, rifting duration, scale of rift system, magmatism and the effects of evolution to seafloor spreading) However, some parameters differ and others remain unresolved in mature rift systems. A major challenge will be to compare and link these observations between the different stages of rifting, and across a range of spatial and temporal scales, to determine common processes and parameters

Acknowledgements To co-workers, collaborators and for discussion: Carol Cotterill, Richard Collier, Mike Leeder, Aris Stefatos, George Ferentinos, George Papatheoderou, Rob Gawthorpe, Daniela Pantosti, Paolo De Martini, Alfred Hirn, Mary Ford, Brian Taylor, Andy Goodliffe, Maria Sachpazi Studies funded by NERC, Royal Society, University of Southampton

The significance of young rift systems - what can they offer? The details of the early rifting process are often difficult to determine from mature rifted systems (post-breakup) Young active rifts offer a chance to understand this early stage, their importance in influencing final rift structure and mechanisms of rift initiation The Corinth rift is an example of early stage rifting (initiating ~ 5 Ma, present phase < 2 Ma) with a relatively simple pre-rift tectonic history Present-day strain rates are high (up to 20 mm/yr) Near sea level position and sedimentation rates allow upper crustal strain during rifting to be quantified An example of a “narrow rift” in a backarc position, but within the context of the more complex Aegean “wide rift” The rift may eventually fail, but distribution of strain within the earliest rift phase is of importance to our understanding of both the initiation of rifting and the evolution of mature rift systems Complimentary to transitional rift systems, e.g., Woodlark basin, Red Sea, Gulf of California

Bell et al., in press