The conundrums of the postglacial tectonic response of the Fennoscandian and Canadian Shields

Bungum, Hilmar; Eldholm, Olav

doi:10.1016/j.earscirev.2022.104146

Cited by 6 publications

(3 citation statements)

References 231 publications

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“…Moreover, their model did not account for transient stresses due to the continuing glacial isostatic adjustment of the craton in this area of North America. The data presented here may provide some quantitative constraints that could be incorporated into the rebound modeling (Bungum & Eldholm, 2022;Peltier et al, 2022;Steffen et al, 2021).…”

Section: Stress Field Along the Boreholementioning

confidence: 99%

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

Wang

Schmitt

Chan³

2023

JGR Solid Earth

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Geophysical logs collected from a deep borehole drilled into the North American Craton in Northeastern Alberta shed valuable light on the state of stress in stable cratons. Observed breakout (BO) azimuths rotate between three depth intervals, ranging from 100° at 1,650–2,000 m to 173° at 2,000–2,210 m, and finally to 145° at the bottom. No apparent fractures that might have disturbed the stress field were found. Two potential causes of these rotating BOs can be either a heterogeneous stress field or elastic and strength anisotropy of the formation. The latter interpretation is favored since observed BO azimuths are strongly controlled by rock metamorphic textures, as validated by their close correlations with dip directions of foliation planes and polarization directions from dipole sonic logs. Monte Carlo realizations further demonstrate that anisotropic metamorphic rocks subjected to a uniform horizontal stress direction could result in the observed azimuth‐rotating BOs. Stress magnitudes inferred from this analysis, which incorporates rock anisotropy and weak foliation failure planes, suggest a normal faulting regime and a maximum horizontal compressive direction consistent with that in the overlying Western Canadian Sedimentary Basin (NE‐SW) and the motion of the North American plate. The inferred stress magnitudes are low and Mohr‐Coulomb analyses demonstrate that the formation is not near the critical condition for slip on weak planes. However, more detailed investigations should be conducted since Monte Carlo calculations indicate that analyses from BO widths, particularly when a conventional Kirsch‐based formula is employed, are highly nonunique, allowing for large variations in potential stress states.

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Section: Stress Field Along the Boreholementioning

confidence: 99%

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

Wang

Schmitt

Chan³

2023

JGR Solid Earth

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“…In areas affected by glacial isostatic adjustment, observations from Europe (Brandes et al, 2018;Sandersen and Jørgensen, 2022;Steffen et al, 2021;Štěpančíková et al, 2022) and North America (Adams, 1989;Brooks and Adams, 2020;Bungum and Eldholm, 2022) demonstrated that tectonic fault zones can be reactivated, resulting in glacially induced faults (Bungum and Eldholm, 2022;Steffen et al, 2014;Stewart et al, 2000). In northern Germany, glacially induced faults were found to be oriented parallel to the margins of the Pleistocene ice sheets, thus documenting the link between neotectonic activities and ice sheet loading (Brandes et al, 2015;Müller et al, 2021).…”

Section: Introduction and Rationalementioning

confidence: 96%

Physical modeling of ice-sheet-induced salt movements using the example of northern Germany

Hardt,

Dooley,

Hudec

2023

Preprint

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Abstract. Salt structures and their surroundings can play an important role in the energy transition related to a number of storage and energy applications. Thus, it is important to assess the current and future stability of salt bodies in their specific geological settings. We investigate the influence of glacial loading and unloading on subsurface salt structures using physical models, based on the example of the Scandinavian Ice Sheet in northern Germany. Apparent spatial correlations between subsurface salt structures in northern Germany and Weichselian ice marginal positions have been observed before and the topic is a matter of ongoing debate. Recently described geomorphological features – termed surface cracks – have been interpreted as a direct result of ice sheet induced salt movement resulting in surface expansion. The spatial clustering and orientation of these surface cracks was so far not well understood, owing to only a limited number of available studies dealing with the related salt tectonic processes. Thus, we use four increasingly complex physical models to test the basic loading and unloading principle, to analyze flow patterns within the source-layer salt and within salt structures, and to examine the influence of the shape and orientation of salt structures with respect to a lobate ice margin in a three-dimensional laboratory environment. Three salt structures of the northern German basin were selected as examples that were replicated in the laboratory. Salt structures were initially grown by differential loading, and buried before loading. The ice load was simulated by a weight that was temporarily placed on a portion of the surface of the models. The replicated salt structures were either completely covered by the load, partly covered by the load, or were situated outside the load extent. In all scenarios, a dynamic response of the system to the load could be observed: while the load was applied, the structures outside the load margin started to rise, with a decreasing tendency with distance from the load margin and, at the same time, the structures under load subsided. After the load was removed, a flow reversal set in and previously loaded structures started to rise, whereas the structures outside the former load margin began to subside. The vertical displacements during the unloading stage were not as strong as during the load stage, and thus the system did not return to its pre-glaciation status. Modeled salt domes that were located at distance from the load margin showed a comparably weak reaction. A more extreme response was shown by modeled salt pillows whose margins varied from sub- parallel to sub-perpendicular to the load margin, and were partly covered by the load. Under these conditions, the structures showed a strong reaction in terms of strain and vertical displacement. The observed strain patterns at the surface were influenced by the shape of the load margin and the shape of the salt structure at depth, resulting in complex deformation patterns. These physical modeling results provide more evidence for a possible interplay between ice sheets and subsurface salt structures, highlighting the significance of three-dimensional effects in dynamic geological settings. Our results lead to a better understanding of spatial patterns of the surface cracks that were mapped at the surface above salt structures.

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“…A common feature of these studies is the demonstration that GIA increases fault activity and seismicity shortly after the main deglaciation phase in many regions formerly glaciated during the Last Glacial Maximum (LGM) (e.g., Wu et al, 1999;Stewart et al, 2000;Muir-Wood, 2000;Hetzel and Hampel, 2005;Steffen et al, 2014;Lund, 2015). In contrast, the potential effects of GIA stress perturbations on present-day faulting and seismicity in and near formerly glaciated regions remain debated (e.g., Bungum et al, 2010;Bungum and Eldholm, 2022;Brandes et al, 2015).…”

mentioning

confidence: 99%

Glacial-isostatic-adjustment strain rate–stress paradox in the Western Alps and impact on active faults and seismicity

Grosset,

Mazzotti,

Vernant

2023

Solid Earth

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Abstract. In many regions formerly glaciated during the Last Glacial Maximum (LGM), glacial isostatic adjustment (GIA) explains most of the measured uplift and deformation rates. GIA is also proposed as a key process contributing to fault activity and seismicity shortly after the LGM and potentially up to the present day. Here, we study the impact of GIA on present-day fault activity and seismicity in the Western Alps. We show that, in the upper crust, GIA induces horizontal compressive stress perturbations associated with horizontal extension rates. The latter agree with the observed geodetic strain rates and with the seismicity deformation patterns. Yet, in nearly all cases, the GIA stress perturbations tend to either inhibit fault slip or promote fault slip with the wrong mechanism compared to the seismicity deformation style. Thus, although GIA from the LGM explains a major part of the Western Alp geodetic strain rates, it does not drive or promote the observed seismicity (which must be driven by other processes). This apparent strain rate–stress paradox results from the gradual diminution over time of the finite shortening induced in the upper crust by the Würm ice cap load. A direct corollary of our results is that seismicity and seismic-hazard studies in the Western Alps cannot directly integrate geodetic velocities and strain rates but instead require detailed modeling of the GIA transient impact.

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The conundrums of the postglacial tectonic response of the Fennoscandian and Canadian Shields

Cited by 6 publications

References 231 publications

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

Heterogeneity Versus Anisotropy and the State of Stress in Stable Cratons: Observations From a Deep Borehole in Northeastern Alberta, Canada

Physical modeling of ice-sheet-induced salt movements using the example of northern Germany

Glacial-isostatic-adjustment strain rate–stress paradox in the Western Alps and impact on active faults and seismicity

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