Iain S. Stewart

Glacio-Seismotectonics: Ice Sheets, Crustal Deformation and Seismicity

Abstract The last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity. This growth has been driven largely by the emergence of postglacial rebound models (PGM) constrained by new field observations that incorporate increasingly realistic rheological, mechanical, and glacial parameters. In this paper, we highlight some of these recent field-based investigations and new PGMs, and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation. The emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern Canada and Fennoscandia. However, many of these models neglect important parameters, such as topography, lateral variations in lithospheric strength and tectonic strain built up during glaciation. In glaciated mountainous terrains, glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism. Such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields. In general, the short-term response to ice fluctuations is similar to the Earths response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre-existing stress state. When the ice fluctuations occur on the spatial scale, and magnitude, of the Late Pleistocene glaciation and deglaciation, however, the viscoelastic response of the Earth (especially the mantle) causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex. The regions of greatest ice thickness and the regions marginal to the Late Pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting. The mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused, and continues to cause, measureable crustal deformation at hundreds of kilometres from the ice margins. For both tectonically active and cratonic regions, palaeoseismic investigations of Late Pleistocene and Holocene faults are an important tool in evaluating earthquake hazard. The predicted response of the Earth over this time, however, is very dependent on the model assumed. For most regions, far more carefully designed field observations are needed to constrain the existing rheological and ice models.

Postglacial tectonics of the Scottish glacio-isostatic uplift centre

New evidence combined with a detailed re-evaluation of postglacial fault movements, seismic activity and shoreline sequences suggests that the period of deglaciation and the early Holocene was more seismically active than the mid to Late Holocene. It is proposed that the large-scale lateral displacements formerly proposed can not be justified, rather all postglacial fault movements appear to be limited to metre-scale vertical movements along pre-existing fault lines. In addition, it is argued that the Younger Dryas ice advance may have produced localised crustal redepression but not the more widespread impact formerly proposed. Both tectonic and postglacial rebound stresses, however, may be needed to explain the contemporary seismotectonics of the Scottish Highlands.

Coastal uplift on active normal faults: The Eliki Fault, Greece

The Eliki Fault forms part of a system of major normal fault segments that borders the southern margin of the Gulf of Corinth half graben. Radiocarbon dating of elevated marine fossils reveals broadly uniform Holocene coastal uplift, at a time-averaged rate of 1.5 mm/yr, both along the Eliki Fault and in the transfer zone that separates it from the neighbouring fault segment. Coseismic uplift increments are considered to account for only a minor part of the 6 m of emergence recorded here in the last 3000 years. Reappraisal of shoreline data from the Perachora Peninsula at the eastern end of the Gulf of Corinth indicates a similar, though less rapid (0.7 mm/yr), pattern of uniform Holocene emergence. As these Holocene coastal records embody both coseismic and interseismic deformation they can be used to characterise long-term tectonic strain.

Uplift, deformation and fluid involvement within an active normal fault zone in the Gulf of Corinth, Greece

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-graben in central Greece. The crest of the fault-block lies above sea-level whilst in the hanging-wall a marine basin exists. As a result, the diagenetic and structural characteristics of the fault zone record complex fluid 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-existence of early phreatic and late vadose cements within the fault zone is related to footwall uplift across the water-table and subsequent erosional unroofing. The present-day elevation of phreatic cements to c. 650m above current sea-level, provides a 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.

Postglacial fault movement and palaeoseismicity in western Scotland: a reappraisal of the Kinloch Hourn fault, Kintail

The Kinloch Hourn fault is the most prominent of a number of suspectedpostglacial faults in the western Scottish Highlands. These faults areinterpreted to have been reactivated by repeated large (M > 6)palaeoseismic events following deglaciation 10,000–13,000 years ago.Based on inferred deflections of drainage courses, previous studies of thefault have estimated 160 ± 40 m cumulative left-lateral displacementalong a 14 km long active segment during postglacial times. Reportedsoft-sediment deformation phenomena imply that activity on the KinlochHourn fault has persisted into the late Holocene, with the most recentmovement having been associated with a magnitude 5.5–6.0 surface-faultingevent between 3500 and 2400 years ago. The marked contrast betweensuch palaeoseismic activity and the present-day seismic quiescence ofwestern Scotland has stimulated this critical reappraisal of the KinlochHourn fault.This paper reassesses the key lines of evidence for postglacial fault activityand palaeoseismicty on the Kinloch Hourn fault, combining the analysis of1:15,000-scale air photos, field-based geomorphic mapping andpalaeoenvironmental investigations. Our reappraisal of inferred drainagedeflections across the fault contends that previous reports of significant(102 m) left-lateral slip on the fault during the Holocene arespurious. Instead, incidences of Holocene channel abandonment along thefault line are non-synchronous and probably reflect non-tectonic drainagechanges. The timing of soft-sediment deformation in the vicinity of the faultis revised to an early Holocene date (8990–8580 calendar years BP), whichis in accord with both the palaeoenvironmental history of the site andconsistent with published ages of earthquake-induced liquefactionphenomena documented elsewhere in western Scotland. An alleged recent(post-2400 radiocarbon years BP) ground rupture on the fault isquestioned in the light of uncertainty about both the nature of the faultedsoil deposit and the late Holocene age attributed to it.The study concludes that there is no convincing evidence for postglacialsurface rupture on the Kinloch Hourn fault and speculates that the casefor significant (101–102 m) postglacial movement on otherfaults in western Scotland may be similarly `unproven.

A rough guide to limestone fault scarps

Abstract The surface roughness of limestone fault scarps in central Greece is examined with the aim of relating variations in rock weathering to palaeoseismic activity. The study employs a micro-roughness meter (MRM) and a carpenters profile gauge to collect field measurements of scarp roughness, presenting the results in terms of roughness indices which permit comparison between different height levels of a scarp. The study confirms the increasing degradation of scarps with height, thereby inferring the time-dependent evolution of fault-scarp roughness. In addition, roughness indices are found to discriminate between fault surfaces exposed during the (1981) Corinth earthquakes and those emergent prior to this event.