Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation?

Rapid rock exhumation in mountain belts is often associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks towards the Earth's surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages (Braun, 2016). Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The N&#8211;S trending Brenner fault defines the western margin of the Tauern Window and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps (Rosenberg & Garcia, 2011; F&#252;genschuh et al., 2012). We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drillcore. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ~8 to ~10&#160;Ma and from ~7 to ~9&#160;Ma, respectively, but differ by only ~1&#160;Myr in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active 19&#177;2 Ma ago and caused 35&#177;10&#160;km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2&#177;0.9&#160;km/Myr and became inactive 8.8&#177;0.4&#160;Ma ago. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.ReferencesBraun, J., 2016, Strong imprint of past orogenic events on the thermochronological record: Tectonophysics, v. 683, p. 325&#8211;332.F&#252;genschuh, B., Mancktelow, N., Schmid, S., 2012, Comment on Rosenberg and Garcia: Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 101, p. 1451&#8211;1455.Rosenberg, C.L., Garcia, S., 2011, Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 100, p. 1129&#8211;1145.Wolff, R., Hetzel, R., Dunkl, I., Anczkiewicz, A.A., Pomella, H. 2020, Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation? Geology, v. 48, doi:10.1130/G46940.1.

Download Full-text

Fast cooling of normal-fault footwalls: Rapid fault slip or thermal relaxation?

Geology ◽

10.1130/g46940.1 ◽

2020 ◽

Vol 48 (4) ◽

pp. 333-337

Author(s):

Reinhard Wolff ◽

Ralf Hetzel ◽

István Dunkl ◽

Aneta A. Anczkiewicz ◽

Hannah Pomella

Keyword(s):

Thermal Relaxation ◽

Normal Fault ◽

Geothermal Gradient ◽

European Alps ◽

Normal Faulting ◽

Heat Advection ◽

Tauern Window ◽

The North ◽

Cooling Pattern ◽

Mountain Belts

Abstract Rapid rock exhumation in mountain belts is commonly associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks toward Earth’s surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages. Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The north-south–trending Brenner fault defines the western margin of the Tauern window (Austria) and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps. We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drill core. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ca. 8 to ca. 10 Ma and from ca. 7 to ca. 9 Ma, respectively, but differ by only ∼1 m.y. in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active at 19 ± 2 Ma and caused 35 ± 10 km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2 ± 0.9 km/m.y. and became inactive at 8.8 ± 0.4 Ma. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.

Download Full-text

New constraints on the exhumation history of the western Tauern Window (European Alps) from thermochronology, thermokinematic modeling, and topographic analysis

International Journal of Earth Sciences ◽

10.1007/s00531-021-02094-w ◽

2021 ◽

Author(s):

Reinhard Wolff ◽

Ralf Hetzel ◽

István Dunkl ◽

Aneta A. Anczkiewicz

Keyword(s):

Hanging Wall ◽

Thermal Relaxation ◽

Slip Rate ◽

Normal Fault ◽

European Alps ◽

Valley Bottom ◽

Topographic Analysis ◽

Tauern Window ◽

History Of ◽

Exhumation History

AbstractThe Brenner normal fault bounds the Tauern Window to the west and accommodated a significant portion of the orogen-parallel extension in the Eastern Alps. Here, we use zircon (U–Th)/He, apatite fission track, and apatite (U–Th)/He dating, thermokinematic modeling, and a topographic analysis to constrain the exhumation history of the western Tauern Window in the footwall of the Brenner fault. ZHe ages from an E–W profile (parallel to the slip direction of the fault) decrease westwards from ~ 11 to ~ 8 Ma and suggest a fault-slip rate of 3.9 ± 0.9 km/Myr, whereas AFT and AHe ages show no spatial trends. ZHe and AFT ages from an elevation profile indicate apparent exhumation rates of 1.1 ± 0.7 and 1.0 ± 1.3 km/Myr, respectively, whereas the AHe ages are again spatially invariant. Most of the thermochronological ages are well predicted by a thermokinematic model with a normal fault that slips at a rate of 4.2 km/Myr between ~ 19 and ~ 9 Ma and produces 35 ± 10 km of extension. The modeling reveals that the spatially invariant AHe ages are caused by heat advection due to faulting and posttectonic thermal relaxation. The enigmatic increase of K–Ar phengite and biotite ages towards the Brenner fault is caused by heat conduction from the hot footwall to the cooler hanging wall. Topographic profiles across an N–S valley in the fault footwall indicate 1000 ± 300 m of erosion after faulting ceased, which agrees with the results of our thermokinematic model. Valley incision explains why the Brenner fault is located on the western valley shoulder and not at the valley bottom. We conclude that the ability of thermokinematic models to quantify heat transfer by rock advection and conduction is crucial for interpreting cooling ages from extensional fault systems.

Download Full-text

The Schneeberg Normal Fault Zone: Normal faulting associated with Cretaceous SE-directed extrusion in the Eastern Alps (Italy/Austria)

Tectonophysics ◽

10.1016/j.tecto.2005.02.005 ◽

2005 ◽

Vol 401 (3-4) ◽

pp. 143-166 ◽

Cited By ~ 42

Author(s):

Helmuth Sölva ◽

Bernhard Grasemann ◽

Martin Thöni ◽

Rasmus Thiede ◽

Gerlinde Habler

Keyword(s):

Fault Zone ◽

Normal Fault ◽

Eastern Alps ◽

Normal Faulting

Download Full-text

Timing of fluorite mineralization and exhumation events in the east Central Alborz Mountains, northern Iran: constraints from fluorite (U–Th)/He thermochronometry

Geological Magazine ◽

10.1017/s0016756821000169 ◽

2021 ◽

pp. 1-17

Author(s):

Behnam Shafiei Bafti ◽

István Dunkl ◽

Saeed Madanipour

Keyword(s):

Thermal Relaxation ◽

Geothermal Gradient ◽

Source Rocks ◽

Lower Amount ◽

Mining District ◽

Northern Iran ◽

East Central ◽

Central Alborz ◽

Ore Mineralization ◽

History Of

Abstract The recently developed fluorite (U–Th)/He thermochronology (FHe) technique was applied to date fluorite mineralization and elucidate the exhumation history of the Mazandaran Fluorspar Mining District (MFMD) located in the east Central Alborz Mountains, Iran. A total of 32 fluorite single-crystal samples from four Middle Triassic carbonate-hosted fluorite deposits were dated. The presented FHe ages range between c. 85 Ma (age of fluorite mineralization) and c. 20 Ma (erosional cooling during the exhumation of the Alborz Mountains). The Late Cretaceous FHe ages (i.e. 84.5 ± 3.6, 78.8 ± 4.4 and 72.3 ± 3.5 Ma) are interpreted as the age of mineralization and confirm an epigenetic origin for ore mineralization in the MFMD, likely a result of prolonged hydrothermal circulation of basinal brines through potential source rocks. Most FHe ages scatter around the Eocene Epoch (55.4 ± 3.9 to 33.1 ± 1.7 Ma), recording an important cooling event after heating by regional magmatism in an extensional tectonic regime. Cooling of the heated fluorites, as a result of thermal relaxation in response to geothermal gradient re-equilibration after the end of magmatism, or exhumation cooling during extensional tectonics characterized by lower amount of erosion are most probably the causes of the recorded Eocene FHe cooling ages. Oligocene–Miocene FHe ages (i.e. 27.6 ± 1.4 to 19.5 ± 1.1 Ma) are related to the accelerated uplift of the whole Alborz Mountains, possibly as a result of the initial collision between the Afro-Arabian and Eurasian plates further to the south.

Download Full-text

The 4 December 2015 Mw 7.1 Normal-Faulting Antarctic Plate Earthquake and Its Seismotectonic Implications

Bulletin of the Seismological Society of America ◽

10.1785/0120190249 ◽

2020 ◽

Vol 110 (3) ◽

pp. 1090-1100

Author(s):

Ronia Andrews ◽

Kusala Rajendran ◽

N. Purnachandra Rao

Keyword(s):

Body Wave ◽

Focal Depth ◽

Normal Fault ◽

Normal Faults ◽

Oceanic Plate ◽

Normal Faulting ◽

Transform Faults ◽

Wave Inversion ◽

Spreading Ridges ◽

Southeast Indian Ridge

ABSTRACT Oceanic plate seismicity is generally dominated by normal and strike-slip faulting associated with active spreading ridges and transform faults. Fossil structural fabrics inherited from spreading ridges also host earthquakes. The Indian Oceanic plate, considered quite active seismically, has hosted earthquakes both on its active and fossil fault systems. The 4 December 2015 Mw 7.1 normal-faulting earthquake, located ∼700 km south of the southeast Indian ridge in the southern Indian Ocean, is a rarity due to its location away from the ridge, lack of association with any mapped faults and its focal depth close to the 800°C isotherm. We present results of teleseismic body-wave inversion that suggest that the earthquake occurred on a north-northwest–south-southeast-striking normal fault at a depth of 34 km. The rupture propagated at 2.7 km/s with compact slip over an area of 48×48 km2 around the hypocenter. Our analysis of the background tectonics suggests that our chosen fault plane is in the same direction as the mapped normal faults on the eastern flanks of the Kerguelen plateau. We propose that these buried normal faults, possibly the relics of the ancient rifting might have been reactivated, leading to the 2015 midplate earthquake.

Download Full-text

Seismic-Scale Evidence of Thrust-Perpendicular Normal Faulting in the Western Outer Carpathians, Poland

Minerals ◽

10.3390/min11111252 ◽

2021 ◽

Vol 11 (11) ◽

pp. 1252

Author(s):

Jan Barmuta ◽

Krzysztof Starzec ◽

Wojciech Schnabel

Keyword(s):

Large Scale ◽

Fault Plane ◽

Normal Fault ◽

Normal Faulting ◽

Seismic Profiles ◽

Horizontal Movement ◽

Outer Carpathians ◽

Differential Uplift ◽

Seismic Scale ◽

Detachment Surface

Based on the interpretation of 2D seismic profiles integrated with surface geological investigations, a mechanism responsible for the formation of a large scale normal fault zone has been proposed. The fault, here referred to as the Rycerka Fault, has a predominantly normal dip-slip component with the detachment surface located at the base of Carpathian units. The fault developed due to the formation of an anticlinal stack within the Dukla Unit overlain by the Magura Units. Stacking of a relatively narrow duplex led to the growth of a dome-like culmination in the lower unit, i.e., the Dukla Unit, and, as a consequence of differential uplift of the unit above and outside the duplex, the upper unit (the Magura Unit) was subjected to stretching. This process invoked normal faulting along the lateral culmination wall and was facilitated by the regional, syn-thrusting arc–parallel extension. Horizontal movement along the fault plane is a result of tear faulting accommodating a varied rate of advancement of Carpathian units. The time of the fault formation is not well constrained; however, based on superposition criterion, the syn -thrusting origin is anticipated.

Download Full-text

Validation of Coupled FDM-DEM Approach on the Soil-Raft Foundation Interaction Subjected to Normal Faulting

10.5194/egusphere-egu21-9783 ◽

2021 ◽

Author(s):

Fang Ru-Ya ◽

Lin Cheng-Han ◽

Lin Ming-Lang

Keyword(s):

Active Fault ◽

Ground Deformation ◽

Numerical Models ◽

Hanging Wall ◽

Normal Fault ◽

Small Scale ◽

Foot Wall ◽

Normal Faulting ◽

Raft Foundation ◽

Overburden Soil

Recent earthquake events have shown that besides the strong ground motions, the coseismic faulting often caused substantial ground deformation and destructions of near-fault structures. In Taiwan, many high-rise buildings with raft foundation are close to the active fault due to the dense population. The Shanchiao Fault, which is a famous active fault, is the potentially dangerous normal fault to the capital of Taiwan (Taipei). This study aims to use coupled FDM-DEM approach for parametrically analyzing the soil-raft foundation interaction subjected to normal faulting. The coupled FDM-DEM approach includes two numerical frameworks: the DEM-based model to capture the deformation behavior of overburden soil, and the FDM-based model to investigate the responses of raft foundation. The analytical approach was first verified by three&#160; benchmark cases and theoretical solutions. After the verification, a series of small-scale sandbox model was used to validate the performance of the coupled FDM-DEM model in simulating deformation behaviors of overburden soil and structure elements. The full-scale numerical models were then built to understand the effects of relative location between the fault tip and foundation in the normal fault-soil-raft foundation behavior. Preliminary results show that the raft foundation located above the fault tip suffered to greater displacement, rotation, and inclination due to the intense deformation of the triangular shear zone in the overburden soil. The raft foundation also exhibited distortion during faulting. Based on the results, we suggest different adaptive strategies for the raft foundation located on foot wall and hanging wall if the buildings are necessary to be constructed within the active fault zone. It is the first time that the coupled FDM-DEM approach has been carefully validated and applied to study the normal fault-soil-raft foundation problems. The novel numerical framework is expected to contribute to design aids in future practical engineering.Keywords: Coupled FDM-DEM approach; normal faulting; ground deformation; soil-foundation interaction; raft foundation.

Download Full-text

The Luco dei Marsi deep-seated gravitational deformation: first evidence of a basal shear zone in the central Apennine mountain belt (Italy)

10.5194/egusphere-egu21-5083 ◽

2021 ◽

Author(s):

Emiliano Di Luzio ◽

Marco Emanuele Discenza ◽

Maria Luisa Putignano ◽

Mariacarmela Minnillo ◽

Diego Di Martire ◽

...

Keyword(s):

Rock Mass ◽

Shear Zone ◽

Engineering Geology ◽

Slope Deformation ◽

European Alps ◽

Structural Constraints ◽

Normal Faulting ◽

Central Apennines ◽

Gravity Driven ◽

Basal Shear

The nature of the boundary between deforming rock masses and stable bedrock is a significant issue in the scientific debate on Deep-Seated Gravitational Slope Deformations (DSGSDs). In many DSGSDs the deforming masses move on a continuous sliding surface or thick basal shear zone (BSZ) [1-3].&#160;This last feature is due to viscous and plastic deformations and was observed (or inferred) in many worldwide sites [4]. However, no clear evidence has been documented in the geological context of the Apennine belt, despite the several cases of DSGSDs documented in this region [5-6].This work describes a peculiar case of a BSZ found in the central part of the Apennine belt and observed at the bottom of a DSGSD which affects the Meso-Cenozoic carbonate ridge overhanging the Luco dei Marsi village (Abruzzi region). The NNW-SSE oriented mountain range is a thrust-related Miocene anticline, edged on the east by an intramountain tectonic depression originated by Plio-Quaternary normal faulting. The BSZ appears on the field as a several meters-thick cataclastic breccia with fine matrix developed into Upper Cretaceous, biodetritic limestone and featuring diffuse rock damage.The gravity-driven process was investigated through field survey, aerial photo interpretation and remote sensing (SAR interferometry) and framed into a geological model which was reconstructed also basing on geophysical evidence from the CROP 11 deep seismic profile. The effects on slope deformation determined by progressive displacements along normal faults and consequent unconfinement at the toe of the slope was analysed by a multiple-step numerical modelling constrained to physical and mechanical properties of rock mass.The model results outline the tectonic control on DSGSD development at the anticline axial zone and confirm the gravitational origin of the rock mass damage within the BSZ. Gravity-driven deformations were coexistent with Quaternary tectonic processes and the westward (backward) migration of normal faulting from the basin margin to the inner zone of the deforming slope.References[1] Agliardi F., Crosta G.B., Zanchi A., (2001). Structural constraints on deep-seated slope deformation kinematics. Engineering Geology 59(1-2), 83-102. https://doi.org/10.1016/S0013-7952(00)00066-1.[2] Madritsch H., Millen B.M.J., (2007). Hydrogeologic evidence for a continuous basal shear zone within a deep-seated gravitational slope deformation (Eastern Alps, Tyrol, Austria). Landslides 4(2), 149-162. https://doi.org/10.1007/s10346-006-0072-x.[3] Zangerl C., Eberhardt E., Perzlmaier S., (2010). Kinematic behavior and velocity characteristics of a complex deep-seated crystalline rockslide system in relation to its interaction with a dam reservoir. Engineering Geology 112(1-4), 53-67. https://doi.org/10.1016/j.enggeo.2010.01.001.[4] Crosta G.B., Frattini P., Agliardi F., (2013). Deep seated gravitational slope deformations in the European Alps. Tectonophysics 605, 13-33. https://doi.org/10.1016/j.tecto.2013.04.028.[5] Discenza M.E., Esposito C., Martino S., Petitta M., Prestininzi A., Scarascia-Mugnozza G., (2011). The gravitational slope deformation of Mt. Rocchetta ridge (central Apennines, Italy): Geological-evolutionary model and numerical analysis. Bulletin of Engineering Geology and the Environment,70(4), 559-575. https://doi.org/10.1007/s10064-010-0342-7.[6] Esposito C., Di Luzio E., Scarascia-Mugnozza G., Bianchi Fasani G., (2014). Mutual interactions between slope-scale gravitational processes and morpho-structural evolution of central Apennines (Italy): review of some selected case histories. Rendiconti Lincei. Scienze Fisiche e Naturali 25, 161-155. https://doi.org/10.1007/s12210-014-0348-3.

Download Full-text