Glacial Tectonics: a Deeper Perspective Robert M

Quaternary Science Reviews 19 (2000) 1391}1398

Glacial tectonics: a deeper perspective Robert M. Thorson* Department of Geology and Geophysics, University of Connecticut, 345 Mansxeld Road (U-45), Storrs, CT 06269, USA

Abstract

The upper 5}10 km of the lithosphere is sensitive to slight changes ((0.1 MPa) in local stress caused by di!erential loading, #uid #ow, the mechanical transfer of strain between faults, and viscoelastic relaxation in the aesthenosphere. Lithospheric stresses induced by mass and #uid transfers associated with Quaternary ice sheets a!ected the tectonic regimes of stable cratons and active plate margins. In the latter case, it is di$cult to di!erentiate glacially induced fault displacements from nonglacial ones, particularly if residual glacial stresses are considered. Glaciotectonics, a sub-subdiscipline within Quaternary geology is historically focussed on reconstructing past glacier regimes and, by de"nition, does not include these e!ects. The term `glacial tectonicsa is hereby suggested for investigations focussed on the past and continuing in#uences of ice sheets on contemporary tectonics. ( 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction e!ects of glacial mass transfers is blurring the distinction between the study of tectonics, per se, and the study of Presently, there is a conceptual shift in the geosciences `glaciotectonicsa, which, historically, has been primarily away from increasing specialization, towards more inte- concerned with deformed glacial deposits. grative problems at global scales, a shift embodied by the In this paper I show how the physical coupling be- phrase `Earth System Sciencea (Kump et al., 1999). Si- tween glaciation and crustal deformation extends far multaneously, technologically driven advances in instru- beyond the decollement between ice and its substrate (i.e. mental and computational techniques are permitting beyond the scope of glaciotectonics), and instead oper- earth scientists to identify and explain synoptic vari- ates up to crustal scales. I begin by reviewing recent ations in topography, seismicity, and ambient crustal research illustrating the sensitivity of the earth's crust to stress that were not recognizable a decade ago. The stress di!erences far smaller than those associated with recent paper by Peltzer et al. (1996) is a case in point; they ice sheets. I then introduce examples of glacially induced, used synthetic aperture radar (SAR) interferometry to crustal scale deformation from a passive cratonic setting explain the disappearance of transient topographic (Fennoscandia) and an active continental margin (Cas- anomalies at the centimeter scale resulting from the 1992 cadian subduction zone). Next, I explore the basic mech- Landers (California, USA) earthquake, anomalies erased anisms of glacially induced tectonics * e!ective vertical by the in-migration of #uids to 4 km depth. As these stress, membrane #exure, and traction at the base of the conceptual and technical trends continue, the seismotec- crust * and how each of these mechanisms is modi"ed tonic e!ects of past #uctuations of Quaternary ice sheets by the contrasting tectonic domains associated with com- are becoming easier to notice and explain. As a result, the pressive, extensional, and transform strain. Finally, I re- role of glaciers in earth deformation is being recognized visit the question of the scope of glaciotectonics. at a range of spatial scales: from deformation within the ice itself to the reactivation of tectonic faults in response to crustal unloading and viscoelastic relaxation of isos- 2. Sensitivity of the Earth's crust tatic anomalies. Broader recognition of the multiple The ambient seismicity in many regions is extremely sensitive to small changes in stress, regardless of cause. * Tel.: 860-486-1396; fax: 860-486-1383. For example: Rydelek and Sacks (1999) recently demon- E-mail address: [email protected] (R.M. Thorson). strated that seemingly trivial changes in the con"ning

0277-3791/00/$- see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 6 8 - 8 1392 R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398 stress (0.1 MPa) are su$cient to nucleate earthquakes; wet and dry years as well as the water level in the King et al. (1994) proved that fault displacement in one reservoir. locality can transfer stress to other faults at continental Earthquake nucleation processes similar to those be- scales; and Pollitz et al. (1998) implicated the aestheno- neath the Koyna reservoir * dry loading, poroelastic sphere in the transfer of stress across the North Paci"c volume changes, #uid transfers * took place on a much Ocean from the Aleutian Islands to California. larger scale in the vicinity of former Quaternary Ice The role of crustal pore pressure in tectonic processes Sheets. Although the direct loading and pore pressure has also been recently clari"ed. For example: Rojstaczer e!ects are most easily understood, the growth and decay and Wolf (1992) documented a tenfold increase in basef- of ice sheets were also associated with crustal #exure and low stream discharge following the 1989 Loma Prieta aesthenospheric relaxation. Although recognized as im- (California, M 7.1) over a radius up to 50 km from the portant for more than century, the tectonic coupling epicenter; Episodic movements on great transform faults between ice sheets (the water component of the litho- (Byerlee, 1993; Sample and Reid, 1998) are toggled by sphere) and the crust (the silicate component) is seldom pore pressure variations; Thrust faults within accretion- considered in geologically based reviews of the seis- ary wedges at continental margins (Bolton et al., 1999) motectonic literature (Yeats et al., 1997; Keller and are regulated by hydro-mechanical (valving) processes Pinter, 1996). which partition an otherwise continuous process (subduction) into discrete episodes of deformation. De- collemonts would be mechanically impossible without 3. Contrasting examples high pore pressures. Arti"cial triggering of earthquakes by human activ- Postglacial fault scarps in northern Fennoscandia are ities, now well established, works primarily through the the dramatic surface expressions of faults penetrating the e!ects of #uid pressure. For example, the injection of Baltic shield to a depth of about 40 km (Arvidsson, 1996). petroleum brines, waste-water, and experimental tracers The most conspicuous, the Parvie Fault in northern have initiated brittle faulting to a depth of 9 km (Jost Sweden, with a vertical displacement of 10 m and a rup- et al., 1998). Conversely, the withdrawals of #uids in#u- ture length of 150 km (Lagerback, 1992; Muir-Wood, ence seismicity to a minimum depth of 4 km (Gomberg 1988) may have generated a deglacial earthquake with an and Wolf, 1999; Segall, 1989). The so-called `delayeda extraordinary magnitude (Mw"8.2). Similar, albeit response of reservoir seismicity has been shown to be smaller, faults are known for the Canadian and Siberian a consequence of the migration of #uid pressure towards shields (e.g. Dyke et al., 1991), in tectonic settings that potential asperities on faults, with rupture occurring requires the `externally imposed failure conditionsa (John- when a threshold pressure is reached. Dry e!ects such as ston, 1996) caused by glacial loading. Although the Par- crustal loading by water-supply reservoirs (Simpson vie Fault, as well as their counterparts on other shields, et al., 1988; Mandal et al., 1998), the extraction of mass was formed during deglacial mass transfers, it remains from large open-pit mines, and underground explosions somewhat active today, as evidenced by continuing are generally less important than #uid e!ects. micro-seismicity. Ironically, modern seismicity has a gla- Stress changes caused by human activities are small cial heritage. with respect to those of ice sheets, present or past, which Building on previous models (Hasegawa and Basham, also precipitate and dampen earthquake processes. For 1989; James and Morgan, 1990; Johnston, 1987; MoK rner, example, Johnston (1987) calculated that the East Ant- 1980; Muir-Wood, 1988; Stewart and Hancock, 1994) arctic Ice Sheet was responsible for a loading stress I applied the idea of crustal-scale glaciotectonic deforma- reaching 59 MPa, although the change in e!ective stress tion to an active plate margin in the Cascadia Subduc- is much lower. Glaciers, as agents of denudation, are also tion Zone, arguing that the tectonic processes in the responsible for permanent mass transfers from highlands fore-arc basin of the Puget Lowland were, and continue to depositional basins, but at a much slower rate than ice to be, compromised by the mass and #uid transfers asso- loads. The closest analogue to a glacially induced seis- ciated with Quaternary glaciation (Thorson, 1996). More micity from #uids was the damaging Koyna (India) speci"cally, I predicted that a much simpler, preglacial earthquake of December 10, 1967 the largest known ambient tectonic regime has become partitioned into reservoir earthquake (Mandal et al., 1998). Its main four discrete time domains, all of which occur sequen- shock was a strike-slip event with a magnitude of 6.2 tially during each glacial cycle: (Simpson et al., 1988) and a focal depth of less than 5 km, but aftershocks extended to at least 15 km depth in E A phase of normal background stress in which the a zone nearly 10 km wide and 30 km long. Carefully direct and delayed e!ects of glacial mass transfers are instrumented variations in aftershock micro-seismicity insigni"cant (normal Coulomb failure). indicate that the state of stress in the crust in Koyna E A phase of mass loading/crustal strengthening in was controlled by inter-annual variability between which the combination of ice thickening and isostatic R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398 1393

out#ow reduced the deviatoric stresses, contributing pre-existing tectonic structures and glacially induced de- to stability. formation, blurs the distinction between tectonics and E A phase of unloading/weakening during early pos- glacially induced tectonics. Strictly speaking, the glacier tglacial time associated with signi"cant fault displace- itself is a tectonic phenomenon. The base of each ice sheet ments caused by #uid overpressuring and enhanced is traction at the base of the crust due to isostatic a rapidly propagating, gravity-driven detachment in#ow. fault, taking place entirely within the lithosphere. The E A recovery phase in which ambient tectonic stresses, loss of ice by melting is analogous to un-roo"ng by supposedly `eraseda during the deglacial phase, re- denudation 2The glacier sole is essentially a slicken- accumulated to background values. sided decollement, smeared with fault gouge. The rigid lower plate (the crust) undergoes elastic #exure and An attempt to test this hypothesis with "eld data (Holo- changing deviatoric stresses caused directly by the cene fault displacements) was encouraging, but incon- load, and indirectly by the transient decay of the isos- clusive. tatic anomaly and excess pore #uid pressure. (Thorson, Since publication of that paper, the evidence for link- 1996, p. 1187) ing glacial unloading with a transient phase of intense deglacial tectonism has been strengthened. Nearly 40 What separates glacial tectonics from `normala tectonics years ago, Easterbrook (1963) published evidence for an is less the physical processes involved or the scope of enigmatic emergence and re-submergence of a crustal their in#uence, but the time scale. block near Deming, in northwestern Puget Lowland, during glaciomarine conditions about 12 ka. Owing to the magnitude of the required oscillation (30}70 m), the 5. Glacial tectonic mechanisms interpretation of these features remains controversial. Recently, Dragovich et al. (1997) linked these vertical Kinematically, there are three principal directions in motion anomalies (as well as a concentration of bedrock which the forces associated with a change in glacier mass landslides) to the leading edge of the upper plate of can in#uence the state of stress of the lithosphere: The a thrust fault exhibiting both Neogene o!set and modern stresses can be imposed: vertically through changes in seismic activity. Emergence was likely caused by at least e!ective stress; horizontally by imparting a shear traction 30 m of localized tectonic uplift during the most rapid at the top of the crust (conventional glaciotectonics) or at phase of deglaciation, an interval when isostatic uplift its base (horizontal component of the viscoelastic re- \ took place at a rate between 10 and 30 cm yr (Dethier sponse in the aesthenosphere); or three dimensionally et al., 1995), and when the rapid northward #ow of with respect to volumetric strain and #exure. All of these a low-viscosity mantle was in phase with the plate con- e!ects, treated separately below, take place simulta- vergence direction (10 }10 poise; James et al., 2000). neously, and can overlap in time (Fig. 1). Tectonically, this uplift can be interpreted as a structural wedge caught between the leading edge of a Quater- 5.1. Ewective vertical stress nary thrust fault and an antithetic reverse fault behind it. The subsequent re-submergence is more enigmatic, but Here we consider three limiting cases. In the simplest may have been caused by normal faulting and backtilting limiting case, pore pressure e!ects are absent, allowing associated with a subsequent phase of crustal extension the growth of an ice sheet (of in"nite width) to associated with continued growth of the uplift dome create a direct, instantaneous, lithostatic increase in the (Muir-Wood, 2000). Curiously, two other elevated vertical stress: anomalies in uplift pro"le * Mt. Vernon (Dethier et al., 1995) and Alki Point (Thorson, 1996), also lie on the *p "o T * gh*, upper plate of thrusts (Johnson et al., 1999). The ampli- ' *p o tudes of the topographic anomalies ( 30 m for Deming where is the change in vertical stress, * is the density on the McCauley Creek Thrust, ca. 22 m for Mt. Vernon of ice, and h* is the ice thickness. This e!ect is equivalent on the Devil's Mountain Fault, and ca. 9 m for Strand to the `rapida reservoir response in induced seismicity A of the Seattle Fault) scale with fomer ice thickness and directly beneath impoundments. Owing to the Poisson with the magnitude and rate of isostatic uplift. e!ect, however, the increased vertical stress also causes a proportional (but lower) increase in the horizontal *p stress, or &, reducing the potential deviatoric (i.e. tec- 4. Glacial tectonics tonic) stress. The importance of loading diminishes with depth as the stress ratio between pre and post-loaded The magnitude of the deglacial uplifts involved in conditions declines. The principal e!ects are elastic and Puget Lowland and the intimate connection between instantaneous. 1394 R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398

Fig. 1. Schematic diagram showing glaciotectonic domains and processes. (a) Three fundamental processes associated with changes in glacier distribution; elastic e!ects, vertical stresses, and shear. (b) Idealized physical model for changes in vertical stress during loading. The change in e!ective k *p stress is determined by changes in pore pressure ( ) and vertical load ( 4), and are reversed and generally more rapid during unloading. Crustal permeability (k) after Manning and Ingebritsen (1999). Change in vertical load with depth is idealized. Top part of the physical model has no scale; the thickness of layers is exaggerated to show di!erential responses.

In a second limiting case, a wet-based ice sheet is with glacier surges (Kamb et al., 1985) likely takes place saturated with #uid water to near its surface, and the in the bedrock, as well as within the ice. water has unlimited access to the base in an isotropic The permeability of the Earth's crust, whether sedi- crust. In this circumstance, the change in e!ective vertical mentary basins, fractured crystalline basement, and vol- *p stress ( #) is zero because the increased #uid pressure canic rocks, decreases systematically with depth, owing *k *p ( ) fully compensates the increased vertical load ( ). to increased con"ning pressure and the closure of frac- Alternatively tures (Fig. 1). Based on studies of metamorphic #uids and heat transport (Manning and Ingebritsen, 1999), the per- *p " " *p !k ` a # 0 ( ). meability (k) in SK units of m varies with crustal depth (z, km) in a simple, log}log relationship spanning at least This limiting case is realistic for ice-loaded shield areas 10 orders of magnitude: well inside the glacial margin, where fractures in strong Log k"!14}3.2 log z. rocks to a depth of several kilometers are open to the surface, and where the high fracture permeability does Log k for sedimentary basins to a depth of 7 km ranges not restrict the #ow of #uids. In these circumstances, from !12 to !16 m\. Volcanic rocks and crystalline changes in the e!ective vertical stresses remain close to basement are typically one-to-two order of magnitudes zero because the #uid pressure instantaneously mirrors less permeable. Although conceptually straightforward, the changes in load. Changes in horizontal stresses de"ning the boundary between meteoric and connate through the Poisson e!ect remain important. water is di$cult in actual practice. The relationship be- In cases where the potentiometric surface of #uid water tween permeability and depth is least reliable in the within the ice sheet falls to a level appreciably below the shallow crust owing to greater variation in horizontal glacier surface, then the e!ective vertical stress is raised stress and to lithological heterogeneity. by the proportional decrease in #uid pressure. Converse- At some depth, fracture aperture is small enough to ly, during glacier surges, the potentiometric pressure limit the permeability, forcing glacially induced pressure from meltwater (o"1.0 g/cm) may exceed the added changes to take place gradually. The depth at which this vertical load of the ice (o"0.9 g/cm), causing a transi- transition takes place is not known, but likely ranges ent reduction in the e!ective vertical stress during a time widely from about 0.5 km to more than several km. This of rapid mass transfer. Thus, micro-seismicity associated time-dependent migration of #uids in the shallow crust R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398 1395

(Peltzer et al., 1996) is similar to the `delayeda e!ect for interface in strata where the permeability is limiting. In reservoir seismicity (Simpson et al., 1988) and injection this circumstance, the frictional bond between the glacier experiments (Odonne et al., 1999), which takes place over and its bed (whether rock or unconsolidated sediment) is time scales of weeks to years. In the glacially induced stronger than in deeper materials, even that of jointed version of this scenario, changes in #uid pressure of bedrock. The traction associated with the advancing ice `meteoric watera in the open, but low-permeability crust, can thus be transferred downward into the crust, with will lag each instantaneous increment of load. This will displacement along faults, both brittle or ductile, taking cause an instantaneous rise in the e!ective vertical stress place to a depth of up to several hundred meters, and that will decay as the pressure re-equilibrates. Non-linear involving intact slabs of the lithosphere at the kilometer changes in the permeability may occur during re-equili- scale (Aber, 1988). The depth and behavior of such e!ects bration, possibly through the geochemical e!ects (stress- are controlled primarily by the strain rates being applied. corrosion) of in"ltrating water (Kisslinger, 1976). Thrusting at the ice margin, which leads to the familiar push At greater depth, however, where poroelastic e!ects moraines and imbricate ridges, is conceptually similar to the (Segall, 1989) predominate, connate #uid plays a critical behavior at accretionary wedges of subduction zones. role in supporting the crust. A glacially induced increase in A second type of horizonally}directed shear takes place *p vertical stress ( )willbefullycompensatedbyaninstan- at the base of the rigid crust through the e!ect of horizon- taneous rise in #uid pressure, one that will gradually bleed tal #ow within the asthenosphere (James and Morgan, o! through #uid out-migration. This will cause a transient 1990). Traditionally, studies of glacioisostasy have fo- phase during which an initially weakened crust will cussedontheverticalcomponentofmotionandoncrustal strengthen, an e!ect opposite that in the next-higher layer, #exure (MoK rner, 1980), but the horizontal #ow between where the initially strengthened crust will weaken. At even the forebulge and the subglacial depression, can be quite greater depths, #uids are not free to migrate upward, the rapid, up to a meter per year. Recent geodetic investigations proportional change in hydrostatic pressure diminishes, in the western US con"rm that viscoelastic relaxation in the and anisotropy plays a critical, and largely unknown, role aesthenosphere after major earthquakes (Rydelek and in modulating the e!ects of the ice load. Sacks, 1999) raises the strain along other, distant faults. Near the edge of the glacier, however, the e!ects will be Similarly, horizontal isostatic motions during loading and more complicated. Disregarding #uid e!ects, the di!eren- unloading must enhance or diminish the horizontal compo- tial weighting on either side of a glacier margin will be felt nents of regional stress (Muir-Wood, 2000). immediately as the crust is elastically compressed below the load, with no compression beyond it. When #uid 5.3. Three-dimensional yexure e!ects are considered, the di!erential stresses are greater because the glacier imparts a new hydraulic gradient on Mechanical loading is has been reviewed by Turcotte the system. Seepage pressures, particularly near the ice and Schubert (1982). Slab loads, line loads, point loads, margin, may also in#uence the ambient stress. Membrane end loads, and embedded loads all create #exures of the #exure of the crust during isostatic compensation will lithosphere, the size and shape of which are governed cause radial compression and extension beneath the ice largely by the e!ective rigidity of the crust, which, is load and in the forebulge domain, respectively, deforma- sensitive to the third power of membrane thickness. The tion that will a!ect crustal permeability. physics of the process are independent of the composi- tion of the load, whether glacier ice, crystallized lava, or 5.2. Horizontal traction; top and bottom a sedimentary accumulation. Hence, the familiar `moat- forebulgea pro"les of volcanic islands, foreland basins, Drag forces at the glacier}bed interface are controlled and glacial margins are similar. Importantly, membrane by the e!ective overburden stress (ice thickness reduced by #exure includes zones of convexity where the crust is #uid pressure), the viscoelastic properties of the ice, and radially dilated, and zones of concavity where it is the frictional properties (Mohr}Coulomb) of the bound- radially compressed. Radial and tangential extension on ary. Three limiting cases can be visualized. In the "rst case, the forebulge, and radial compression beneath the ice thick ice is frozen to a #at, crystalline bed; horizontally sheets took place during the loading hemicycle; the re- #owing ice and the stationary bed form an ideal simple verse took place, and with higher strain rates, during shear couple limited only by the slow rate of ice deforma- deglaciation, a process that continues today. tion;inthiscase,theshearontherockistrivial.Inthe second case, the ice moves continuously above a very 6. Complications weak, deforming bed, which fully compensates the shear stresses. As before, negligible shear is propagated down- 6.1. Ambient crustal stress ward to the lithosphere. In the third case the ice is frozen (orfrictionallybonded)toitsbednearthemargin,with In the previous discussion, I made the simplifying groundwater out#ow taking place beneath the ice}bed assumption that the ambient lithospheric stress was 1396 R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398 isotropic. Lithospheric stress, however, varies continu- E The creation of a forebulge may contribute to radial ously and heterogeneously in response to plate motion, jointing, which may increase crustal permeability, topographic loading, exhumation, and the mass transfer which may decrease the pore pressure beneath the ice of water during ice ages. Generally speaking, the state of margin, which may steepen the ice pro"le, which may crustal stress can be classi"ed into one of three categories: cause even greater #exure, etc. In a compressive, or thrust regime, the principal stress E Aesthenospheric out#ow caused by ice loading may p p ( ) is horizontal and the minimum stress ( ) is vertical. enhance or diminish stresses of tectonic origin, de- Increased e!ective vertical stresses stabilizes the system pending on the ambient direction of plate convergence. by reducing the deviatoric stress. Conversely, a reduction For example, deglaciation of the Cordilleran Ice Sheet in e!ective vertical stress pushes the system towards near Kodiak Island in southern Alaska may have instability. Fluid injection often produces earthquakes enhanced and reduced the rate of plate convergence on with focal solutions consistent with at least a component the southern and northern #ank of the former ice of reverse faulting (Jost et al., 1998) because the e!ect of sheet, respectively. #uid injection is to raise the pore pressure, which reduces E The rate of de-weighting caused by ablation may ex- the e!ective vertical stress. In an extensional, normal ceed the rate at which excess pore pressure can drain p p fault regime, is vertical and is horizontal. Increas- towards the surface, leaving the crust weaker than it ing the e!ective vertical stress helps destabilize the sys- would be under normal conditions. In thrust and tem, whereas reducing the vertical stress stabilizes it. strike-slip domains, this may lead to a deglacial inter- Fluid extraction (Gomberg and Wolf, 1999) increases val of heightened seismicity. vertical stress by reducing the pore pressure, and is thus E Fault displacement triggered by a rise in pore pressure often associated with normal faulting (Amelung et al., may be a self-limiting phenomenon through the e!ect p p 1999). In a strike slip regime, where and are both of stress dilatancy, in which crustal porosity is in- horizontal, an increase in the e!ective stress stabilizes the creased through microfracturing. system through the Poisson e!ect; pore pressure is espe- E Above the onset of poroelastic e!ects the crust (exclud- cially important in regulating strike slip faults. ing normal fault regimes) is instantaneously Although there are broad tectonic domains where the strengthened by vertical loading, then weakened back ambient stress is relatively uniform, local stresses are to initial conditions by the in-migration of #uids. The often highly heterogeneous at a variety of scales (e.g. opposite happens where poroelastic e!ects predomi- Mueller et al., 1999). Additionally, most faults have an nate; the crust is weakened, then strengthened. Both of oblique component of motion involving two of the these e!ects can take place simultaneously in di!erent idealized regimes above. superimposed crustal layers, concentrating stress in subhorizontal boundaries between these zones. 6.2. Glacial history E The permeability of the shallow crust is highly vari- able, hence the e!ects of changing glacial loads will be The tectonic e!ects of Quaternary ice sheets are unique felt di!erentially; delayed pressure e!ects will continue because their e!ective loads can change rapidly, more to in#uence local stresses until re-equilibration, long than several orders of magnitude faster than the pro- after the ice has disappeared. cesses of mountain building and erosion. Although the stresses are lower than for lithospheric forces, the stress gradients and strain rates can be signi"cantly higher, 7. Conclusion causing brittle rupture where ductile failure might have otherwise occurred. Also, unloading is usually much The tectonic setting of passive continental margins is more rapid than loading, reversing many of the e!ects usually characterized by a thick granitic crust, greater described above. Importantly, the e!ects of loading and uniformity in ambient stress, and low strain rates unloading work in opposite directions, allowing the pos- (10\ yr\). In such a setting * for example in Fennos- sibility for glaciers to trigger episodes of rupture regard- candia (Arvidsson, 1996), the British Isles (Sissons and less of ambient stress. They push and pull. Cornish, 1982), and the North American Craton (Adams and Bell, 1991; Andrews, 1991; Oliver et al., 1970) * gla- 6.3. Coupled ewects cially induced stresses are superimposed on the regional "eld, but may actually be the dominant factor controlling Given the three basic directions of applied stress, the seismicity and crustal motion at the time scale of three fundamental stress domains, the heterogeneity of local 10}10 yr (MoK rner, 1991). In contrast, the tectonic set- stress within the crust at a variety of scales, and the asym- ting of active continental margins is characterized by high metric history of ice sheet loading and unloading, glaciotec- strain rates (10\ yr\), crustal-scale anisotropy, and tonic stresses will be complex. Additionally, the e!ects the dynamic e!ects of plate #exure and buoyancy. In this can either amplify or diminish one another. For example: type of setting * for example, in Cascadia (Mathews R.M. Thorson / Quaternary Science Reviews 19 (2000) 1391}1398 1397 et al., 1970; Clague, 1983; Thorson, 1996), glacially in- In: Slemmons, D.B., Engdahl, E.R., Zoback, M.D., Blackwell, duced stresses and subduction work simultaneously at D.D. (Eds.), Neotectonics of North America. Geological Society Quaternary time scales. In the former case of passive of America Decade Map Volume, Boulder, Colorado, pp. 473}486. margins, ice sheets modify an existing tectonic regime. In Arvidsson, R., 1996. Fennoscandian earthquakes: Whole crustal rup- the latter case, they must be considered part of it. turing related to postglacial rebound. Science 274, 744}746. Glaciotectonics, as conventionally de"ned, `2in- Bolton, A.J., Clennell, M.B., Maltman, A.J., 1999. Nonlinear stress volves structural deformations of the upper horizon of dependence of permeability: a mechanism for episodic #uid #ow in the lithosphere caused by glacial stressesa (Van der accretionary wedges. Geology 27, 239}242. Byerlee, J.D., 1993. Model for episodic #ow of high-pressure water in Wateren, 1995). This de"nition speci"cally excludes de- fault zones before earthquakes. Geology 21, 303}306. formation within the ice itself, and crustal-scale e!ects Clague, J.J., 1983. Glacio-isostatic e!ects of the Cordilleran ice sheet, because the primary research objective of glaciotectonics, British Colombia. In: Smith, D.E., Dawson, A.G. (Eds.), Shorelines is the reconstruction of past glacier regimes from "eld and Isostasy. Academic Press, London, pp. 285}319. observations, especially those associated with deforma- Dethier, D.P., Pessl Jr., F., Keuler, R.F., Balzarini, M.A., Pevear, D.R., 1995. Late Wisconsinan glaciomarine deposition and isostatic re- tion of unconsolidated materials at and below the glacier bound, northern Puget Lowland, Washington. Geological Society sole, and with marginal thrusting at scales of of America Bulletin 107, 1288}1303. 10\}10 m. The academic territory of glaciotectonics is Dragovich., J.D., Zollweg, J.E., Qamar, A.I., Norman, D.K., 1997. The clearly de"ned as subdiscipline within Quaternary geol- Macaulay Creek Thrust, the 1990 5.2-magnitude Deming Earth- ogy. I recommend continued use of the term in this quake, and Quaternary geologic anomalies in the Deming Area, western Whatcom County, Washington * Cause and E!ects? narrowly restricted sense. A new term, however is needed, Washington Geology 25, 15}26. one that encompasses the deformation in the shallow Dyke, A.S., Morris, T.F., Green, D.E.C., 1991. Postglacial tectonic and crust caused by ice}bed traction, and the crustal-scale sea level history of the central Canadian Arctic. Geological Survey deformation described in this review. For now I suggest of Canada Bulletin 397, 56 pp. the following candidate terms, arranged in order of in- Easterbrook, D.J., 1963. Late Pleistocene glacial events and relative sea-level changes in the northern Puget Lowland, Washington. creased simplicity: glacio-seismotectonics, glacio-seismic- Geological Society of America Bulletin 74, 1465}1483. ity, glacially induced tectonics, glacial tectonics, or Gomberg, J., Wolf, L., 1999. Possible cause for an improbable earth- simply tectonics. Although tempted to adopt the last quake: the 1997 Mw 4.9 southern Alabama earthquake and hydro- candidate for its simplicity, the term `glacial tectonicsa carbon recovery. Geology 27, 367}370. seems an acceptable compromise between focus and Hasegawa, H.S., Basham, P.W., 1989. Spatial correlation between seismicity and postglacial rebound in eastern Canada. In: Gregersen, S., breadth. The actual practice of glacial tectonics has Basham, P.W. (Eds.), Earthquakes of North Atlantic Passive Mar- a long pedigree. I recommend adoption of this term to gins. 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