Glacial Loading Research Articles

Slip rate variations on faults during glacial loading and post-glacial unloading: implications for the viscosity structure of the lithosphere

Abstract: Many active faults in extensional and contractional tectonic settings experienced a slip rate increase after the last glacial period when the volume of nearby glaciers and lakes decreased. This post-glacial slip rate increase is caused by transient stresses that are generated by the unloading-induced rebound and superimposed on the tectonic background stress field. As the latter is different for normal and thrust faults, the response to loading and unloading should depend on the fault type. Here we use finite-element models including a fault in rheologically layered lithosphere to explore the conditions under which both normal and thrust faults experience a post-glacial slip rate increase. The results show that a post-glacial slip rate increase occurs on normal faults if the lower crust is stronger than the lithospheric mantle, whereas thrust faults accelerate if the lower crust is weaker than the lithospheric mantle. These findings imply that the response of faults to mass fluctuations on the Earth's surface may provide constraints on the rheological stratification of the lithosphere. We use our results to make predictions on the viscosity structure of the Basin-and-Range Province and northern Scandinavia, where palaeoseismological data document a pronounced increase in seismicity as a result of post-glacial unloading and rebound.

Neotectonics, seismicity and stress in glaciated regions

The melting of large ice caps over the span of many centuries to several millennia is an important agent of dynamic change within the solid Earth (Fig. 1). It induces significant redistributions of mass on the surface of the Earth, drives sea-level and land elevation changes, and readjusts lithospheric stresses in formerly glaciated areas and adjacent regions, effects manifest as increased seismicity and, eventually, in powerful earthquakes. It was an understanding that was apparent a century ago (De Geer 1888; Kolderup 1913) but has found increased support during the 20th century. In particular, the discovery of the ‘end-glacial' faults of northern Scandinavia (Kujansuu 1964; Lagerback 1979; Olesen 1988) proved definitively that glacial loads and, therefore, climate can act as tectonic drivers over very short geological times. Fig. 1. Simplifed loading history for central Fennoscandia during the last (Weichselian) major glaciation (modified after Talbot 1999). Ice thickness appears above the time axis and consequent crustal depression appears below. The model illustrates how rapid vertical ice loading and unloading changes lead to far slower crustal depression or rebound, and that the scale of crustal response is considerably less than the imposed ice-mass fluctuations, reflecting viscous flow of the …

Evolution of Neogene volcanism and stress patterns in the glaciated West Antarctic Rift, Marie Byrd Land, Antarctica

Abstract: The orientations and ages of polygenetic volcano chains, elongate volcano edifices, and elongate summit calderas in Marie Byrd Land, Antarctica, are analysed to determine stress directions during volcanism and coeval glaciation across the West Antarctic Rift. Initiation of volcanism c . 36–34 Ma and major polygenetic volcanism beginning c . 14 Ma are broadly coeval with establishment of significant ice volumes in West Antarctica. Middle–Late Miocene volcanism occurred primarily along roughly north–south volcano chains, whereas latest Miocene–Pleistocene volcanism had a strong east–west preferred orientation across the spatial extent of the province. Anisotropic stress conditions controlled both phases of volcanism, and a rapid rotation in the maximum horizontal stress direction from north–south to east–west occurred as early as c . 6 Ma. Glacial loading and unloading appears to have facilitated volcanism in Marie Byrd Land, but ice margin reconstructions make it unlikely that stress field rotation is a result of flexural stresses imposed by glacial loading cycles. The east–west orientation of latest Miocene to Pleistocene maximum horizontal stress is parallel to the absolute motion of the Antarctic plate for the past 6 Ma, and the north–south to east–west stress field rotation coincides with changes in Antarctic–Pacific Ridge spreading and Pacific basin tectonic events associated with plate reorganization.

Tectonic significance of present-day stress relief phenomena in formerly glaciated regions

Abstract: Investigations were conducted in several regions of Norway with the purpose of detecting and measuring stress-relief features and to derive from them valuable information on the shallow-crustal stress state. Stress-relief features are induced by blasting and sudden rock unloading in road construction and quarrying operations, and are common in Norway and probably in other regions of Fennoscandia, as they are in NE America. Stress relief at the Earth's surface is diagnostic of anomalously high stress levels at shallow depths in the crust and characterizes the formerly glaciated Baltic and Canadian Precambrian shields. The mean orientation of the maximum horizontal compressive stress axis, as deduced from the orientation of stress-relief features, was found to be NW–SE, consistent with North Atlantic ridge-push forces. We show that our determined stress orientations are in excellent agreement with other kinds of stress data. As a final step, we estimate ridge-push force magnitudes from gravity data in the North Atlantic region. Our computations suggest that strong ridge-push forces characterize the Baltic and Canadian shields. We conclude from this quantitative analysis and our fieldwork results that present-day stress relief, in Fennoscandia and NE America, is mostly triggered by plate-scale ridge-push forces and not by residual glacial loading stresses.

Three‐dimensional numerical modeling of slip rate variations on normal and thrust fault arrays during ice cap growth and melting

Changes in the volumes of ice caps considerably alter the stress state of the lithosphere by generating a transient signal that is added to the tectonic background stress field. These stress field changes, in turn, affect crustal deformation and in particular the slip behavior of existing faults. Here we use three‐dimensional finite element models to investigate how arrays of normal and thrust faults near a growing and subsequently melting ice cap are influenced in their slip evolution. The results show that regardless of fault dip, both types of faults experience a decrease in their slip rate during ice cap advance and an increase in their slip rate during ice cap retreat if they are located beneath the ice cap. In contrast, faults outside the ice cap that are loaded on their footwall or hanging wall only show the opposite pattern: their slip rate increases during glacial loading and decreases during subsequent unloading. If the load is located along strike of the fault; that is, at one of its tips, the slip behavior of normal and thrust faults is different: The normal fault shows a slip rate increase during unloading, the thrust fault during loading. Our results explain the location and timing of deglaciation‐induced paleoearthquakes in Scandinavia and the contrasting slip histories reported from normal faults in the Basin and Range Province, which are located at different positions relative to the former Yellowstone ice cap. More generally, our findings imply that a uniform slip behavior of faults in formerly glaciated regions should not be expected.

Structure and age of volcanic fissures on Mount Morning: A new constraint on Neogene to contemporary stress in the West Antarctic Rift, southern Victoria Land, Antarctica

Antarctica is characterized by a unique combination of active processes, including active crustal deformation, volcanism, and glacial loading and unloading, but little is known about the Neogene to contemporary geodynamic state of the Antarctic plate. This paper presents new data on the structure and timing of volcanism on the Mount Morning shield volcano, with the purpose of defining Pleistocene stress directions concomitant with volcanism and rifting in the southern portion of the Victoria Land rift basin. Elongate vents and vent alignments indicate parasitic volcanism has predominantly occurred along a primary set of NE fissures. Parasitic basaltic cinder cones yield 40 Ar- 39 Ar cooling ages, presented herein, that range from essentially zero to ca. 3.5 Ma, though mostly of Pleistocene age. The systematic NE trend of the fissures on Mount Morning records magmatically induced fracturing of the volcano flanks controlled by a regional N31°E maximum horizontal stress (S H ) that dominated the Mount Morning area during the Pleistocene and, probably, also the Pliocene. Minor volcanism occurred along shorter NW alignments that have no discernible age difference from the NE fissures. The Pleistocene age of the parasitic volcanism suggests that the stress direction documented at Mount Morning represents the contemporary differential stress field in the area. This NE S H direction differs from the N15°W S H direction measured in a borehole at Cape Roberts, 100 km to the north, and the S H orientations appear to track the changing trend of the segmented rift boundary. The exact cause for the variability in the S H directions is uncertain, but the change in S H could record different stress provinces within the rift system, or it could reflect stress reorientation along the major lithospheric boundary of the rift. The new contemporary stress datum at Mount Morning is consistent with a neotectonic normal-fault to strike-slip fault regime within the Terror Rift, which was active in Pliocene-Pleistocene times and could remain active today. This stress regime still dominates the contemporary geodynamic state of this sector of the West Antarctic Rift system.

A more precise radiocarbon age for an 8 m post-glacial sea-level stand at Cow Head, Newfoundland.

Brookes and Stevens (1985) reported a conventional radiocarbon date of 8250 ± 320 BP (d13C = +1.0 per mil; GSC – 3433 [2s error]; Blake (1986), p. 5; normalized to 8634 ± 320 BP) on three paired valves of Hiatella arctica (Linne) occupying cavities bored by them in limestone breccia at 6 m asl, at Cow Head, Newfoundland, on the Gulf of St. Lawrence (49°55’N; 57°48’W). Over the bored rock surface the beach-ridged surface of nearshore gravel relates to a sea-level stand at 8 m. This terrace is prominent locally, eroded across resistant Cambrian– Ordovician carbonates, as at Lower Head, which encloses Shallow Bay northward (Brookes and Stevens 1985, Fig.1). As mentioned in the original paper, two other shell occurrences in coastal cliffs along this shore have similar conventional 14C dates, and are associated with emerged marine landforms with elevations falling on a plane rising gently northwards, from 6 m asl at Western Brook, 10 km south (8340 ± 150 BP, GSC 1763 [no d 13C determined]), through the Cow Head – Lower Head features, to Parsons Pond, 13.5 km north (GSC – 1762, 8650 ± 150 BP [no d 13C determined]). At the latter site, Grant (in Lowdon et al. 1977, p. 3) reported an emerged marine foreland terraced at 13 m, 8 m, and 5 m asl. A feature at closer to 11 m would be expected there in this projection. A more precise AMS radiocarbon age has been obtained on a fragment of one Hiatella shell at 6 m asl on the same bored outcrop at Cow Head, 8960 ± 25 BP (d13C -1.4 per mil) (UCIAMS – 44422 [normalized to 9382 ± 25]. After normalization and subtraction of an estimated marine reservoir age of 530 years, averaged from values given for northern and southwestern Gulf of St. Lawrence in Dyke et al. (2003), this age calibrates to 10 599 ± 35 cal yr BP (Fairbanks et al. 2005). Such a relative sea-level stand, which includes the geomorphic and taphonomic contexts at Cow Head, and the similar context at Western Brook, and possibly at Parsons Pond, over a coast-wise distance of 23.5 km, could be expected to apply more widely along this shore, although there is no sufficiently detailed survey. The sea-level stand may have been due to (i) regional climatic deterioration sufficient to cause significant increase in glacial loading, briefly slowing isostatic uplift to the same rate as ocean-level rise; (ii) briefly increased glacial melting on a regional or wider scale, causing ocean-level to rise at the same rate as isostatic uplift; or (iii) briefly increased seaice activity which eroded the bedrock surface, destroying the Hiatella colony, and increasing the supply of inshore gravel to blanket the shore platform so formed. The calibrated age of the shells at Cow Head, 10599 ± 35 cal yr BP, is at the upper end of the range of calendar ages determined for a climatic cooling in evidence at ten localities in and around the Gulf of St. Lawrence (Brookes and Grant, in press). Three of these contexts represent glacier stillstand or readvance – along the 700 km Quebec North Shore Moraine System (Dubois and Dionne 1985), and in small highland ice caps in the northern Long Range of Newfoundland (Waitt 1981), and in the Cape Breton Highlands (Grant 1994). The age limits of this cooling have yet to be determined, but its effects, nevertheless, would have been felt in increased seaice activity, possibly accelerating coastal erosion (iii, above). Considering the proximity of the retreating margin of the Laurentide Ice Sheet, retardation of isostatic uplift along this coast of the Goldthwait Sea by ice readvance or stillstand (i, above) is a possibility worthy of geophysical attention.

Apatite single‐grain (U‐Th)/He data from Heimefrontfjella, East Antarctica: Indications for differential exhumation related to glacial loading?

Single‐grain (U‐Th)/He ages from two profiles were used to reconstruct the post‐Permian tectonic‐thermal history of basement rocks in Heimefrontfjella, East Antarctica. The (U‐Th)/He ages from one sample collected below the late Carboniferous/Early Permian sedimentary cover rocks indicate Jurassic–Early Cretaceous basement paleotemperatures of ∼40°–60°C due to post‐Permian burial. Combined apatite fission track and (U‐Th)/He analyses from samples of a profile in Sivorgfjella suggest a period of flexural‐related tilting after ∼87 Ma. The timing was further constrained using forward and inverse models of the (U‐Th)/He data. Model results indicate a Cenozoic phase of relatively rapid cooling from ∼40°C to surface temperatures. As the driving mechanism, we propose flexural isostatic rebound due to glacial load during the development of the intracontinental ice sheet in the hinterland of the Heimefrontfjella region.

Glacial isostasy and plate motion

Abstract The influence of glacial-isostatic adjustment (GIA) on the motion of tectonic plates is usually neglected. Employing a recently developed numerical approach, we examine the effect of glacial loading on the motion of the Earth’s tectonic plates where we consider an elastic lithosphere of laterally variable strength and the plates losely connected by low viscous zones. The aim of this paper is to elucidate the physical processes which control the GIA-induced horizontal motion and to assess the impact of finite plate-boundary zones. We show that the present-day motion of tectonic plates induced by GIA is at, or above, the order of accuracy of the plate motions determined by very precise GPS observations. Therefore, its contribution should be considered when interpreting the mechanism controlling plate motion.

Age distribution of fossil landslides in the Tyrol (Austria) and its surrounding areas

Abstract. Some of the largest mass movements in the Alps cluster spatially in the Tyrol (Austria). Fault-related valley deepening and coalescence of brittle discontinuities structurally controlled the progressive failure and the kinematics of several slopes. To evaluate the spatial and temporal landslide distribution, a first comprehensive compilation of dated mass movements in the Eastern Alps has been made. At present, more than 480 different landslides in the Tyrol and its surrounding areas, including some 120 fossil events, are recorded in a GIS-linked geodatabase. These compiled data show a rather continuous temporal distribution of landslide activities, with (i) some peaks of activity in the early Holocene at about 10 500–9400 cal BP and (ii) in the Tyrol a significant increase of deep-seated rockslides in the Subboreal at about 4200–3000 cal BP. The majority of Holocene mass movements were not directly triggered by deglaciation processes, but clearly took a preparation of some 1000 years, after ice withdrawal, until slopes collapsed. In view of this, several processes that may promote rock strength degradation are discussed. After the Late-Glacial, slope stabilities were affected by stress redistribution and by subcritical crack growth. Fracture propagating processes may have been favoured by glacial loading and unloading, by earthquakes and by pore pressure fluctuations. Repeated dynamic loading, even if at subcritical energy levels, initiates brittle fracture propagation and thus substantially promotes slope instabilities. Compiled age dating shows that several landslides in the Tyrol coincide temporally with the progradation of some larger debris flows in the nearby main valleys and, partially, with glacier advances in the Austrian Central Alps, indicating climatic phases of increased water supply. This gives evidence of elevated pore pressures within the intensely fractured rock masses. As a result, deep-seated gravitational slope deformations are induced by complex and polyphase interactions of lithological and structural parameters, morphological changes, subcritical fracture propagation, variable seismic activity and climatically controlled groundwater flows.

Effect of ice sheet growth and melting on the slip evolution of thrust faults

Field investigations suggest that postglacial unloading and rebound led to the formation or re-activation of reverse faults even in continental shields like Scandinavia. Here we use finite-element models including a thrust fault embedded in a rheologically layered lithosphere to investigate its slip evolution during glacial loading and subsequent postglacial unloading. The model results show that the rate of thrusting decreases during the presence of an ice sheet and strongly increases during deglaciation. The magnitude of the slip acceleration is primarily controlled by the thickness of the ice sheet, the viscosity of the lithospheric layers and the long-term shortening rate. In contrast, the width of the ice sheet, the rate of deglaciation or the fault dip have an only minor influence on the slip evolution. In all experiments, the slip rate variations are caused by changes in the differential stress. The modelled deglaciation-induced slip acceleration agrees well with the occurrence of large earthquakes soon after the melting of the Fennoscandian ice sheet, which led to the formation of spectacular fault scarps in particular in the Lapland Fault Province. Furthermore, our model results support the idea that the low level of seismicity in currently glaciated regions like Greenland and Antarctica is caused by the presence of the ice sheets. Based on our models we expect that the decay of the Greenland and Antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions.

Advance and Retreat of Cordilleran Ice Sheets in Washington, U.S.A.

Cordilleran Ice Sheet glaciations show characteristic patterns of advance and retreat, consisting of (1) advance out outwash, (2) glacial scouring, (3) deposition of till, (4) deposition of recessional outwash south of Seattle in the southern Puget Lowland, glaciomarine drift in the northern lowland, and eskers, kames, and small moraines on the Columbia Plateau. Radiocarbon dates show that the Puget and Juan de Fuca lobes advanced and retreated synchronously. The Puget lobe backwasted to Seattle by 13.4-14 ka yrs BP, where the thinning ice floated in seawater northward to Canada by 13 ka yrs BP depositing glaciomarine drift contemporaneously over 18,000 km2. Compelling evidence against the backwasting, calving, terminus model for the origin of the glaciomarine drift includes: 1) abundant 14C dates demonstrate simultaneous deposition of glaciomarine drift over the entire area; 2) stagnant-ice deposits closely related to glaciomarine drift are not consistent with an actively-calving, backwasting terminus; 3) irrefutable evidence for the nonmarine origin of Deming sand shows that Cordilleran ice was absent immediately prior to deposition of the overlying glaciomarine drift. The pattern of events in the northern Puget Lowland includes: 1) glacial loading under 1800 m of ice during the Vashon maximum; 2) rapid glacial thinning and floating of the ice deposited Kulshan glaciomarine drift 12-13 ka yrs BP; 3) emergence and deposition of fluvial Deming sand 11.5 ka yrs BP; 4) resubmer-gence and deposition of Bellingham glaciomarine drift up to -200 m, well beyond global eustatic sea level rise; 5) emergence 10.5-11.5 ka yrs BP and deposition of Sumas outwash on Bellingham glaciomarine drift; 6) Holocene eustatic sea level rise kept pace with isostatic rebound, thus, post-Sumas marine terraces are absent.

Distribution, structure, and formation of Holocene lava shields in Iceland

Lava shields in Iceland are monogenic shield volcanoes mostly composed of primitive basalts, picrite and olivine tholeiite. The Holocene shields are restricted in space and time: in space they are essentially confined to the North and West Volcanic Zones; in time they are essentially confined to the early part of the Holocene. The volcanic zones are characterised by volcanic systems, that is, 5–20 km wide and 40–150 km long swarms of tectonic fractures and volcanic fissures where most of the Holocene volcanotectonic activity takes place. However, most lava shields are not located inside the volcanic systems but rather at their margins or in between systems. In addition to the lavas of the shields being more primitive than those issued from nearby volcanic fissures, the average volume of a shield is an order of a magnitude larger than that of the lava from a fissure. Here we present the results of new field observations of Holocene shields and provide numerical models to explain their location, time of formation, primitive composition, and large volumes. We made models with varying ice-sheet size and thickness (glacial load), the ice resting on a mechanically layered crust, and studied the stress effects that the load would have on a double magma chamber, that is, a small, shallow crustal chamber at 3 km depth and a deep-seated reservoir at 20 km depth (the base of the crust). Such a pair of chambers is typical for volcanic systems and associated central volcanoes (composite volcanoes) in Iceland. For an ice sheet covering an entire volcanic zone or more, that is, 100 km or wider, the ice-induced compressive stress extends to the deep-seated reservoir and into the upper mantle. Consequently, such a loading suppresses magma accumulation in the reservoir and associated volcanism. During the late-glacial period, when the ice sheet is only 20 km wide, the glacial load generates tensile stresses around the deep-seated reservoir, increases its fracture porosity and magma content, and extends the reservoir laterally and vertically into the upper mantle. Consequently, when the lava shields (and, somewhat earlier, the tablemountains) were erupted, much more melt or magma was available to feed a single eruption than during the later part of the Holocene. And because of the greater vertical extent of the reservoir, this magma tended to be hotter and more primitive than that issued in later-formed fissure eruptions. Also, the stress field generated at the end of the glacial and in the early Holocene favoured dyke injections at the marginal parts of, or in between, the volcanic systems, thereby explaining the location of the lava shields.

Slope instability in relation to glacial debuttressing in alpine areas (Upper Durance catchment, southeastern France): Evidence from field data and 10Be cosmic ray exposure ages

The Upper Durance catchment is an area prone to rock-slope failures. Such failures reflect the combination of high relief, litho-structural controls and paraglacial stress release. The aim of this study is to determine the role of deglacial unloading and resulting paraglacial stress release in conditioning or triggering slope failure. Former dimensions of the Durance glacier are reconstructed, then combined with Digital Elevation Model data in a raster Geographic Information System to quantify the spatial pattern of stresses associated with glacial loading at the Last Glacial Maximum. Preliminary calculations suggest that major rock falls and rock avalanches are associated with areas subject to the highest decompression stresses. Focus on two case studies allows the consequences of paraglacial stress release on slope instability to be evaluated. Description of slope failure runout deposits allows reconstruction of the nature of slope failure. Surface exposure dates based on concentration of cosmogenic 10Be allows the timing of both deglaciation and that of post-glacial rock-slope failures to be established. It is shown that rock-slope failures are concentrated on lower valley-side slopes within the area occupied by ice at the Last Glacial Maximum, and that their locations coincide with zones of inferred high glacial loading stress, consistent with interpretation of both bedrock disruption and large-scale rock-slope failures as paraglacial phenomena induced by stress release following deglaciation. Timing of initial rock avalanche runout deposition at one site is consistent with this conclusion, though later instability episodes at the same site may have occurred independent of the influence of paraglacial stress release.

Modelling of mantle postglacial relaxation in axisymmetric geometry with a composite rheology and a glacial load interpolated by adjusted spherical harmonics analysis

Although studies on glacial isostatic adjustment usually assume a purely linear rheology, we have previously shown that mantle relaxation after the melting of Laurentide ice sheet is better described by a composite rheology including a non-linear term. This modelling is, however, based on axially symmetric geometry and glacial forcing derived from ICE-3G and suffers from a certain amount of arbitrariness in the definition of the ice load. In this work we apply adjusted spherical harmonics analysis to interpolate the ice thicknesses of ICE-3G and ICE-1 glaciological models. This filters out the non-axisymmetric components of the ice load by considering only the zonal terms in the spherical harmonics expansion. The resulting load function is used in finite-element simulation of postglacial rebound to compare composite versus purely linear rheology. Our results confirm that composite rheology can explain relative sea level (RSL) data in North America significantly better than a purely linear rheology. The performance of composite rheology suggests that in future investigations, it may be better to use this more physically realistic creep law for modelling mantle deformation induced by glacial forcing.

Habits of a glacier-covered volcano: seismicity patterns and velocity structure of Katla volcano, Iceland

AbstractThe Katla volcano, overlain by the Mýrdalsjökull glacier, is one of the most active and hazardous volcanoes in Iceland. Earthquakes show anomalous magnitude-frequency behaviour and mainly occur in two distinct areas: within the oval caldera and around Goðabunga, a bulge on its western flank. The seismicity differs between the areas; earthquakes in Goðabunga are low frequency and shallow whereas those beneath the caldera occur at greater depths and are volcano-tectonic. The seismicity shows seasonal variations but the rates peak at different times in the two areas. A snow budget model, which gives an estimate of the glacial loading, shows good correlation with seismic activity on an annual scale. Data recorded by the permanent network South Iceland Lowland (SIL), as well as by a temporary network, are used to invert for a 3D seismic velocity model underneath Eyjafjallajökull, Goðabunga and the Katla caldera. The tomography resolves a 15 km wide, aseismic, high-velocity structure at a depth of more than 4 km between the Eyjafjallajökull volcano in the west and the Katla volcano in the east. Anomalously low velocities are observed beneath the Katla caldera, which is interpreted as being a significantly fractured area of anomalously high temperature.

20th Century sea-level change along the eastern US: Unravelling the contributions from steric changes, Greenland ice sheet mass balance and Late Pleistocene glacial loading

We have considered the influence of ocean temperature and salinity changes, mass changes of the Greenland ice sheet (GIS) and the isostatic response of the solid earth to the most recent glacial cycle on 20th century sea-level change along the US east coast with the intention of better understanding the observed signal as well as determining the potential of the tide gauge data for constraining the recent (past 50–100 yr) mass balance of the GIS and earth viscosity structure. Our results show that the signal due to steric changes is large and displays a complex spatial variation which can account for a significant portion of the observed signal. In contrast, that due to changes in the GIS is relatively small and insensitive to the specific geometry of the mass balance model adopted. As a consequence, the tide gauge data alone are not capable of providing useful constraints on either the magnitude or form of recent GIS mass balance. Our inference of mantle viscosity structure based on the tide gauge data was affected dramatically when the steric effect was accounted for: An earth model with an upper mantle viscosity of 8 × 10 19 Pa s and a lower mantle viscosity of 5 × 10 22 Pa s produced the best fit to the steric-corrected data; the optimal fit to the uncorrected data was obtained for upper and lower mantle viscosities of 5 × 10 20 Pa s and 10 22 Pa s, respectively.

When do faults in sedimentary basins leak? Stress and deformation in sedimentary basins; examples from the North Sea and Haltenbanken, offshore Norway

Faults may be barriers or conduits for fluid flow in sedimentary basins. The properties of faults, however, depend on stress conditions and rock properties at the time of deformation and subsequent diagenesis of the fault zone. Several recent publications have suggested that petroleum reservoirs in the North Sea and at Haltenbanken, offshore mid-Norway, have experienced leakage along faults caused by imposed stresses, related to glacial loading during the Quaternary. The Jurassic reservoirs in these areas are, however, bounded by faults produced during the Upper Jurassic rifting, when the sediments were still soft and, for the most part, uncemented. These faults do not represent zones of weakness. Because of strain hardening and later diagenesis in sandstones and cementation in mudstones, the fault zones are commonly stronger than the adjacent rocks. They are therefore not likely to be reactivated tectonically. Furthermore, there is little evidence of glacial deformation in the Quaternary sediments overlying these oil fields. It has been proposed that very large horizontal stresses, inferred to be related to periods of glacial loading, caused shear failure at pore pressures below fracture pressure and subsequent leakage along these shear zones. We argue that this is not a likely mechanism during progressive burial in sedimentary basins. Very high horizontal effective stresses, up to 60 MPa, at about 3 km (1.8 mi) depth, at Haltenbanken would have caused more mechanical compaction and grain crushing than that observed in situ. External stress, i.e., plate-tectonic stress from spreading ridges (ridge push), will be transmitted primarily through the basement and not through the much more compressible overlying sedimentary rocks. During progressive basin subsidence, chemical compaction, i.e., caused by quartz cementation, causes rock shrinkage, which will relax differential stresses. This makes brittle deformation (shear failure), resulting in open fractures less likely to occur at stresses below the fracture pressure. In subsiding sedimentary basins with progressive compaction, horizontal stress will normally not exceed the vertical stress except when there is significant shortening of the underlying basement.

The post-glacial landscape evolution of the North German Basin: morphology, neotectonics and crustal deformation

The recent evolution of the north German Basin (NGB), which is presently a low-seismic area, was partly affected by glacial loading and unloading of the ice masses. Major stresses acting within the NGB are induced by the North-Atlantic ridge push, the ongoing Alpine collision, and the post-glacial rebound of Fennoscandia. Present-day horizontal stresses within the NGB are directed generally NW–SE, but fan and bend north of 52°N towards NNE. Major basement faults are directed NW–SE, minor faults NE–SW and NNE–SSW, and are clearly detectable in geomorphological and satellite lineaments. Furthermore, the drainage pattern and the distribution of lakes in northern Germany follow exactly block boundaries and, hence, mark zones of present-day subsidence. The understanding of the post-glacial morphology and reactivation of faults requires a view into the very heterogeneous crust and upper mantle below the NGB. The re-adjustment of the individual fault blocks during post-glacial relaxation of the lithosphere leads to differential, crust-dependent uplift and, probably, to the formation of Urstrom valleys. The Urstrom valleys and terminal moraines in northern Germany appear to parallel the major tectonic lineaments and lithospheric “block” boundaries. The lithospheric memory is expressed in the post-glacial landscape evolution of the NGB.

Modelling post-glacial rebound with lateral viscosity variations

SUMMARY Observations of isostatic adjustment of the Earth's surface due to glacial loading provide important constraints on mantle viscosity structure. We solve the forward problem of glacial isostatic adjustment in two complementary ways: a spectral method for strictly 1-D (spherically symmetric) earth models, and a finite element method that can accommodate 3-D viscosity structure. We discuss how each method may be augmented in three ways: to accommodate motion of the centre of mass, to implement a gravitationally self-consistent ocean load via the sea level equation and to include the influence of polar wander. With all these effects implemented, the two methods are benchmarked against each other. We also study the influence of lateral viscosity variations upon measurements of post-glacial rebound (PGR) in two ways: first by observing the effect of viscosity perturbations in an idealized model and second by developing a realistic 3-D viscosity model and comparing it with results of related 1-D (spherically symmetric) models. The 3-D viscosity structure is derived starting from seismic tomography models. We conclude from both approaches that PGR observations are sensitive to both the local viscosity structure and to the viscosity structure beneath the loaded region, even if it is removed from where the observations are made. In particular, PGR measurements made at Hudson Bay tend to reflect the local viscosity structure beneath Hudson Bay; PGR measurements made along the east coast of North America, being sensitive to both the local (east coast) viscosity structure as well as the loaded (continental) viscosity structure, are not reproducible with a 1-D viscosity model.