Fault Nucleation Research Articles

Nucleation and growth of geological faults

Abstract. We present a new model of fault nucleation and growth based on the Weibull theory, already widely used in fracture research engineering. We propose that, according to a birth-and-growth process, germs (nuclei) are born at random instants at random spatial locations and then grow with time. This leads to a satisfactory formulation of fault length distribution, different from classical statistical laws. Especially, this formulation reconciles previous analyses of fault datasets displaying power-law and/or exponential behaviors. The Weibull parameters can be statistically estimated in a simple way. We show that the model can be successfully fitted to natural data in Kenya and Ethiopia. In contrast to existing descriptive models developed for geological fault systems, such as fractal approaches, the Weibull theory allows to characterize the strength of the material, i.e. its resistance to deformation. Since this model is very general, we expect that it can be applied in many situations, and for simulations of geological fracture processes. The model is independent of deformation intensity and type and therefore allows a better constraint of the seismic risk in threatened regions.

Anomalous deformation twinning in fcc metals at high temperatures

Nanotwinned structures have shown strong promise as optimal motifs for strength, ductility, and grain stability in fcc metals—in sharp contrast to their nano-grained counterparts where gains in strength are disappointingly offset by loss of ductility. However, their high temperature stability has remained relatively unaddressed. Here we investigate the high temperature response of twin boundaries that constitute these nanostructured metals, by way of molecular dynamics simulations. At low and intermediate temperatures, the twin boundaries exhibit normal motion coupled to shear deformation as expected. However, our simulations at higher temperatures (above 0.5–0.7 Tm), reveal considerable deformation twinning, an occurrence that has not been observed before in fcc metals. Although the origins of this intriguing behavior are not yet clear to us, we discuss a possible conjecture by addressing the following questions: (i) Why is the high temperature response of some fcc metals different? (ii) Why do we observe a transition from twin migration to stacking fault nucleation and subsequent twin formation at high temperatures?

Determining the uniqueness of best-fit trishear models

We show the application of a simulated annealing algorithm to trishear inverse modeling. The algorithm traverses the parameter space in search for best-fit models without being trapped in local minima, and thus sampling for more possible solutions globally. Simulated annealing is a robust and efficient technique to determine the uniqueness of best-fit trishear models; the spread of possible trishear models that can fit a structure. We first apply the algorithm to a decameter-size, contractional fault-propagation fold in west-central Taiwan, for which there is an exceptional exposure of pre-growth and growth strata. Simulated annealing shows that even for this complete fold dataset with low uncertainties, there is a range of models and fault slip/uplift histories that can fit the data, with the consequent implications for the assessment of seismic hazard. We then apply the algorithm to a kilometer-size, extensional fault-propagation fold, the Hadahid monocline, Gulf of Suez Rift, Egypt. In this monocline there is only surface coverage in the footwall anticline areas and the algorithm was used to delimit the range of possible models that can fit the data and their uncertainties, thus avoiding biases in the interpretation. Simulated annealing suggests that the along-strike structural variability of the monocline can result from along-strike variability in fault slip, fault propagation to fault slip ratio and depth of fault nucleation. Both examples illustrate the benefits of searching for a possible range of models rather than a precise best-fit model when modeling fault-propagation folds. In an attempt to understand which parameters control fault development, and also how the spread of possible solutions varies with fold growth, we apply the algorithm to four sequential stages of a published, analog clay model of an extensional forced fold. The inversions of the natural examples and the analog model suggest that the spread of the possible models is a manifestation of the data uncertainties, the suitability of the trishear model, fold evolution, and rock mechanical properties.

Collateral damage: Evolution with displacement of fracture distribution and secondary fault strands in fault damage zones

resulting in an apparently more gradual decay with distance, and (3) a change in apparent decay and fault zone thickness becomes evident in faults that have displaced more than ∼150 m. This last observation is consistent with a stochastic model where strand formation is related to the number of fractures within the damage zone, which in turn is a function of displacement. These three observations together suggest that the apparent break in scaling between small and large faults is due to the nucleation of secondary faults and not a change in process.

Flexural extension and top/fault seal failure in the Bonaparte Basin: input form multi-scale deformation models

Overcoming uncertainties associated with top/fault seal effectiveness still represents a critical challenge for conventional hydrocarbon exploration in the Bonaparte Basin. Cainozoic faults’ development/reactivation and dip-linkage processes through top seal have been well established as primary causes of the development of conduits, leading to fluid flow from reservoirs. Constraining predictions on structural traps integrity and preservation of hydrocarbon column, however, requires further understanding and quantification of the background mechanism behind the Cainozoic structural activity. In this paper, we use 2D elastic models to evaluate the evolution of the flexural deformation affecting the Bonaparte Basin petroleum system during the Cainozoic development of the Timor foreland basin. Associated stress distribution is quantified using multi-scale deformation modelling and used to predict the locus of deformation and fault distribution within the Cretaceous top seal and Cainozoic limestone. Modelling results associated with paleogeographic reconstructions and field-scale structural analysis support that the distribution of a flexural extension front triggers the Cainozoic structural activity. At the reservoir-scale, the horizontal extension results in new fault nucleation within the Tertiary limestone and reactivation of older faults bounding the reservoirs. Shear failure and linkage occurs between the initially detached two sets of faults due to the propagation of fault tip damage zones. The spatial relationship between the two sets, the Cainozoic stress tensor and the mechanical anisotropy of the stratigraphic column might significantly impact on such propagation and fault connectivity. The modelling of the perturbed stress field may provide prediction of potential fracture zones in the top seal and risking of traps structural trap integrity.

Stability Analysis of fcc Crystal under Local Share Deformation by Molecular Dynamics

We propose a framework that can be used to study the local thermodynamic stability of materials at finite temperatures, by reconstructing free energy surface based on metadynamics, constraint molecular dynamics and local atomic deformation tensor analysis methods. We apply the proposed framework to fcc embedded atom copper models, and estimate the activation energies, volumes, and critical local deformation tensor for a stacking fault nucleation event in copper single crystal.

Rupture modes in laboratory earthquakes: Effect of fault prestress and nucleation conditions

Seismic inversions show that earthquake risetimes may be much shorter than the overall rupture duration, indicating that earthquakes may propagate as self‐healing, pulse‐like ruptures. Several mechanisms for producing pulse‐like ruptures have been proposed, including velocity‐weakening friction, interaction of dynamic rupture with fault geometry and local heterogeneity, and effect of bimaterial contrast. We present experimental results on rupture mode selection in laboratory earthquakes occurring on frictional interfaces, which were prestressed both in compression and in shear. Our experiments demonstrate that pulse‐like ruptures can exist in the absence of a bimaterial effect or of local heterogeneities. We find a systematic variation from crack‐like to pulse‐like rupture modes with both (1) decreasing nondimensional shear prestress and (2) decreasing absolute levels of shear and normal prestress for the same value of nondimensional shear prestress. Both pulse‐like and crack‐like ruptures can propagate with either sub‐Rayleigh or supershear rupture speeds. Our experimental results are consistent with theories of ruptures on velocity‐weakening interfaces, implying that velocity‐weakening friction plays an important role in governing the dynamic behavior of earthquake ruptures. We show that there is no measurable fault‐normal stress decrease on the fault plane due to the nucleation procedure employed in experiments, and hence, this is not a factor in the rupture mode selection. We find that pulse‐like ruptures correspond to the levels of nondimensional shear prestress significantly lower than the static friction coefficient, suggesting that faults hosting pulse‐like ruptures may operate at low levels of prestress compared to their static strength.

Influence of pre-existing fabrics on fault kinematics and rift geometry of interacting segments: Analogue models based on the Albertine Rift (Uganda), Western Branch-East African Rift System

This study aims at showing how far pre-existing crustal weaknesses left behind by Proterozoic mobile belts, that pass around cratonic Archean shields (Tanzania Craton to the southeast and Congo Craton to the northwest), control the geometry of the Albertine Rift. Focus is laid on the development of the Lake Albert and Lake Edward/George sub-segments and between them the greatly uplifted Rwenzori Mountains, a horst block located within the rift and whose highest peak rises to >5000 m above mean sea level. In particular we study how the southward propagating Lake Albert sub-segment to the north interacts with the northward propagating Lake Edward/George sub-segment south of it, and how this interaction produces the structures and geometry observed in this section of the western branch of the East African Rift, especially within and around the Rwenzori horst. We simulate behaviour of the upper crust by conducting sandbox analogue experiments in which pre-cut rubber strips of varying overstep/overlap connected to a basal sheet and oriented oblique and/or orthogonal to the extension vector, are placed below the sand-pack. The points of connection present velocity discontinuities to localise deformation, while the rubber strips represent ductile domain affected by older mobile belts. From fault geometry of developing rift segments in plan view and section cuts, we study kinematics resulting from a given set of boundary conditions, and results are compared with the natural scenario. Three different basal model-configurations are used to simulate two parallel rifts that propagate towards each other and interact. Wider overstep (model SbR3) produces an oblique transfer zone with deep grabens (max. 7.0 km) in the adjoining segments. Smaller overlap (model SbR4) ends in offset rift segments without oblique transfer faults to join the two, and produces moderately deep grabens (max. 4.6 km). When overlap doubles the overstep (model SbR5), rifts propagate sub-orthogonal to the extension direction and form shallow valleys (max. 2.9 km). Relative ratios of overlap/overstep between rift segments dictate the kind of transition zone that develops and whether or not a block (like the Rwenzoris) is captured and rotates; hence determining the end-member geometry. Rotation direction is controlled by pre-existing fabrics. Fault orientation, fault kinematics, and block rotation (once in play) reinforce each other; and depending on the local kinematics, different parts of a captured block may rotate with variable velocities but in the same general direction. Mechanical strength anisotropy of pre-structured crust only initially centres fault nucleation and propagation parallel to the grain of weakness of the basement, but at later stages of a protracted period of crustal extension, such boundaries are locally defied.

The role of bedding in the formation of fault–fold structures, Portalegre‐Esperança transpressional shear zone, SW Iberia

AbstractFold‐fault structures within a major transpressional shear zone of the SW Iberian Massif were investigated by combining geological mapping, cross‐section analysis and microtectonic studies. A significant example of contractional deformation is displayed in the Portalegre‐Esperança Shear Zone (PESZ) where a heterogeneous Ordovician stratigraphic sequence, showing a strong competence contrast between quartzites, slates and quartzo‐feldspathic rocks favoured strain localization and fault nucleation that controlled fold formation. The presence of pelitic layers within the thick‐bedded quartzites had probably provided weakness zones that were more favourable for the strain localization than the previous foliation present in the quartzites. The quartzites and the quartzo‐feldspathic rocks (granites and volcaniclastic rocks) accommodated heterogeneous high strain developing different degrees of mylonitization. The quartzites with protomylonitic textures are dominant and represent coarse‐grained siliciclastic sediments that suffered metamorphism and partial dynamic recrystallization. Ultramylonites occur within discrete high‐strain shear zones. It is probable that the strain localization in the PESZ involved both the effect of having layers of different competence and layers or stratigraphic contacts with rocks that experienced grain size reduction dominated by cataclasis and dislocation creep.Folds within quartzites with sub‐horizontal to gently plunging hinges vary from closed to open in thick‐bedded quartzites and from tight to closed in thin‐bedded quartzites. Observed changes in structural style of deformed quartzites, slates and quartzo‐feldspathic rocks are interpreted to result from the constraints imposed by the mechanical properties of the different lithologies.As the folds tightened during shortening, the alternating zones of contrasting competence favoured the disruption of the bedding. The deformation history in the PESZ was dominated by thrusting and strike‐slip faulting along incompetent layers that locally developed staircase geometry and transected the lower Ordovician stratigraphic sequence. The kinematic model proposed for the PESZ is consistent with the models of transpressional shear zones stretched along strike with the development of coeval strike‐slip and low angle oblique‐slip faults associated to active buckle folds with axes parallel to the principal extension direction. Copyright © 2010 John Wiley & Sons, Ltd.

Oblique normal faulting along the northern edge of the Majella Anticline, central Italy: Inferences on hydrocarbon migration and accumulation

Along the northern edge of the Majella anticline, in a large quarry originally excavated by ancient Romans, we studied the deformation mechanisms and internal architectures of faults characterized by both normal and lateral components of slip. These oblique normal faults, which crosscut Miocene carbonate grainstones, are associated with hydrocarbons in the form of tar. Within the faults, tar is present in the breccia of the fault cores, as well as in the surrounding fractured and faulted damage zones. Outside of them, tar is found within the porous carbonate beds flanking the oblique normal faults. We propose a conceptual model of fault nucleation and development. In this model, incipient faulting was characterized by shearing of the pre-existing pressure solution seams and formation of two main sets of tail pressure solution seams. With ongoing faulting, exhumation, and growth of the Majella anticline, the main deformation mechanism switched to predominant opening-mode failure in the form of tail joints and veins within the evolving oblique normal faults. This processes allowed the linkage of isolated slip surfaces across contiguous carbonate beds, and the development of isolated pods of fragmented rocks along the evolving faults. Brecciation and cataclasis localized only along the main slip surfaces of the oblique normal faults, forming a zone of intense deformation, the fault core, surrounded by less-deformed fractured, faulted and fragmented carbonates of the damage zone. Tar distribution was used as a proxy for fault and fracture permeability. Well-developed oblique normal faults, as a whole, form combined barrier-conduit structures to fluid flow. The cataclastic rocks, if continuous along the fault cores, form seals for cross-fault fluid flow, whereas the fault breccia and the surrounding carbonate damage zones act as conduits to fluid flow. Less-evolved oblique normal faults may form either distributed or localized conduits for fluid flow, depending on the presence (distributed) or absence (isolated) of fragmented carbonates around the conductive slip surfaces. Due to their orientation, these faults enhance the overall fault parallel fluid flow, which is thus prominent in the damage zones of the larger faults. The fundamental structural elements with greater tar content are sub-parallel to the current σ hmax of central Italy, reflecting the possible control exerted by the stress state on the overall fault and fracture permeability. At a larger scale, as suggested by the structural location of the study quarry, hydrocarbons are channeled primarily within the releasing jogs of interacting oblique normal faults.

Multiscale simulation of the dislocation emissions of single Ni crystal in nanoindentation

The simulation of nanoindentation into single nickel crystal is performed by using quasi continuum method. The strain energy-displacement and load-displacement curves are presented to study the mechanical behavior of the dislocation nucleation. The characteristics of the stacking fault and dislocation nucleation are determined by calculating the centro-symmetry parameters and analyzing the displacement field of the atoms beneath the indenter. The structure of the stacking fault and the characteristics of dislocation obtained in the simulation by quasicontinuum method are reproduced in the simulation by molecular dynamics.

A dynamic model for fault nucleation and propagation in a mechanically layered section

When a mechanically layered section of rock is subject to a horizontal strain, faults often nucleate preferentially in one or more layers before propagating through the rest of the section. The result is a high density of small, low-throw faults within these layers, and a much smaller number of large, through-cutting faults which nevertheless accommodate most of the strain due to their much larger displacement. A dynamic model of fault nucleation and propagation has been created by combining analytical and finite element techniques to calculate the energy balance of these propagating faults. This model shows that: 1) faults may nucleate in either mechanically weak layers, or in stiff layers with a high differential stress; 2) fault propagation may be halted either by strong layers (in which the sliding friction coefficient is high), or by layers which deform by flow and thus have low differential stress. This model can predict quantitatively the horizontal strain required for faults to nucleate, and to propagate across mechanical layer boundaries. The model is able to explain the complex pattern of fault nucleation and propagation observed in a mechanically layered outcrop in Sinai, Egypt.

Oblique normal faulting along the northern edge of the Majella Anticline, central Italy: Inferences on hydrocarbon migration and accumulation

Along the northern edge of the Majella anticline, in a large quarry originally excavated by ancient Romans, we studied the deformation mechanisms and internal architectures of faults characterized by both normal and lateral components of slip. These oblique normal faults, which crosscut Miocene carbonate grainstones, are associated with hydrocarbons in the form of tar. Within the faults, tar is present in the breccia of the fault cores, as well as in the surrounding fractured and faulted damage zones. Outside of them, tar is found within the porous carbonate beds flanking the oblique normal faults. We propose a conceptual model of fault nucleation and development. In this model, incipient faulting was characterized by shearing of the pre-existing pressure solution seams and formation of two main sets of tail pressure solution seams. With ongoing faulting, exhumation, and growth of the Majella anticline, the main deformation mechanism switched to predominant opening-mode failure in the form of tail joints and veins within the evolving oblique normal faults. This processes allowed the linkage of isolated slip surfaces across contiguous carbonate beds, and the development of isolated pods of fragmented rocks along the evolving faults. Brecciation and cataclasis localized only along the main slip surfaces of the oblique normal faults, forming a zone of intense deformation, the fault core, surrounded by less-deformed fractured, faulted and fragmented carbonates of the damage zone. Tar distribution was used as a proxy for fault and fracture permeability. Well-developed oblique normal faults, as a whole, form combined barrier-conduit structures to fluid flow. The cataclastic rocks, if continuous along the fault cores, form seals for cross-fault fluid flow, whereas the fault breccia and the surrounding carbonate damage zones act as conduits to fluid flow. Less-evolved oblique normal faults may form either distributed or localized conduits for fluid flow, depending on the presence (distributed) or absence (isolated) of fragmented carbonates around the conductive slip surfaces. Due to their orientation, these faults enhance the overall fault parallel fluid flow, which is thus prominent in the damage zones of the larger faults. The fundamental structural elements with greater tar content are sub-parallel to the current σ hmax of central Italy, reflecting the possible control exerted by the stress state on the overall fault and fracture permeability. At a larger scale, as suggested by the structural location of the study quarry, hydrocarbons are channeled primarily within the releasing jogs of interacting oblique normal faults.

アコースティック・エミッション

Since the similarity in size distribution of earthquakes and that of acoustic emissions (AEs) was pointed out in 1960s, a number of rock fracture experiments on the temporal evolution of AE activity prior to the final rupture have been conducted. We briefly review these studies mainly focusing on those on the space-time distribution of AEs. Recent development of the experimental technique and the measurement equipment have made it possible to locate sufficient number of AE events for investigating the formation process of final shear fault under triaxial compression. Detailed structure of the fault nucleation zone was revealed by using these AE data, which consists of two portions; the process zone in front of the fault tip, and the damage zone behind it. Evolution of statistical parameters of the spatial distribution of AEs was also studied, and discussions relating to intermediate- and short-term earthquake prediction were made based on the experimental results.

Geomechanics of polygonal fault systems: a review

ABSTRACT Layer-bound systems of polygonal faults are found in sequences of very fine-grained sediments that have typically undergone passive subsidence and burial. In the absence of tectonic extension, the heave of the faults must be complemented by horizontal compaction of the sediments. Density inversion, syneresis and low coefficients of friction on fault planes have all been proposed as causal mechanisms for the development of polygonal fault systems, but most sequences that contain polygonal faults are not underlain by sediments of lower density and there is a lack of evidence to support the idea that syneresis is responsible. Laboratory measurements of clay properties and a recent field test based on well data strongly suggest that low coefficients of residual friction in fine-grained sediments are key to the growth of faults that eventually develop into polygonal systems. However, coefficients of residual friction apply to faults only after initial slip has taken place, so some other mechanism must be responsible for the initial nucleation of the faults. Various speculative suggestions have been made, but there is no evidence that nucleation of those faults that evolve into polygonal systems differs fundamentally from the processes involved in the nucleation of other faults in soft sediments.

Temperature Dependence of Shockley Stacking Fault Expansion and Contraction in 4H-SiC p-i-n Diodes

The nucleation and expansion of Shockley stacking faults (SSFs) in 4H-SiC is known to induce an increase in the forward voltage drop (Vf) of bipolar devices such as pin diodes. However, recent annealing experiments have shown that SSFs can not only expand, but that low temperature annealing (210-7000C) induces a contraction of the SSFs that is coupled with a full and repeatable recovery of the Vf drift. Here we report that following extended periods of forward bias operation that the Vf drift of 10kV 4H-SiC pin diodes saturates, with the saturation Vf drift dropping with increasing stressing temperature. Upon reaching saturation, increases in temperature during forward bias operation at the same injection conditions also lead to a partial recovery of the Vf drift. Furthermore, the magnitude of this current-induced recovery is dependent upon the injection current, as reductions in the current cause a slower, but larger overall Vf drift recovery. All of these results clearly indicate that the current driving force models for SSF expansion are either incomplete or incorrect and that further efforts are required for a more complete understanding of SSF dynamics to be obtained.

The Benagerie Shear Zone: 1100 Myr of reactivation history and control over continental lithospheric deformation

A newly identified northwest–southeast oriented, deeply-rooted, steep to vertical, large-scale structural system within the Proterozoic Curnamona Province, Australia, which we term the “Benagerie Shear Zone”, is imaged in regional magnetic and gravity datasets. In this study, we use a combination of field analysis and quantitative geophysical methods, to establish a 1100 Myr history of activity along the Benagerie Shear Zone during which the location of younger geological structures are influenced by the pre-existing shear zone. This deformational system is interpreted to have 1) aided ascent and emplacement of the ca. 1600 Ma Ninnerie (magmatic) Supersuite; 2) controlled the loci of nucleation of normal faults during rifting and continental breakup at ca. 800 Ma; and 3) influenced the development of fold structures as well as acting as a plane co-linear to the rotation axis of pre-existing normal faults such that they were steepened and reactivated as strike slip structures during the ca. 500 Ma Delamerian Orogeny. We interpret that the Benagerie Shear Zone has not undergone uni-directional propagation during its evolution but rather through reactivation was a primary influence on controlling the nucleation of Neoproterozoic rift faults, thereby playing a major role in accommodating strain over a significant period of the evolution of the Curnamona Province. This study demonstrates that crustal-scale shear zones can evolve over hundreds of millions of years, have strike-lengths of hundreds to thousands of kilometers, and have vastly different surface expressions along strike.

The Grenchenberg conundrum in the Swiss Jura: a case for the centenary of the thin-skin décollement nappe model (Buxtorf 1907)

When Buxtorf in 1907 proposed his decollement hypothesis which visualized the Jura fold belt as a “folded decollement nappe” pushed by the Alps, he met with both fervent support of a few and skepticism by the many. As an illustration of recalcitrant problems within the Jura decollement nappe, that remain after 100 years, a model of the Grenchenberg complex is presented within the frame of 3D decollement kinematics, based on a set of rules gleaned from recurrent features in the Jura: (1) that generally progression of decollement was in sequence from south to north, (2) that in any given structure thrusting (with attendant ramp folding) preceded more generalized folding, (3) that progression of decollement was held up at “anchor points” (asperities) where the emerging thrusts and folds developed inflections with dextral transpression in the western and sinistral transpresssion in the eastern flank, (4) that at such asperities more southerly structures, riding piggyback on the moving decollement sheet, often collided with more northerly ones and even merged with them. The asperities occur on fault/flexure lines of Paleogene origin, the Pierre Pertuis anchor point on the Vicques and the Grenchenberg extended anchor domain on the “Schwarzwald” Line (the continuation of the eastern border of the Rhinegraben). These lines produced deformations of the decollement surface in the middle Triassic evaporites which acted as boundary conditions at the bottom boundary of the decollement nappe, which led to stress concentrations and the nucleation of faults. Although it is now widely recognized that these lines were reactivated during late Miocene Jura decollement, it ought to be stressed that this reactivation affected the decollement nappe only and there individual asperities or groups of asperities rather than the lines as a whole. Because in the course of nappe displacement the originally autochthonous asperities as expressed in the sedimentary cover moved into an allochthonous position, their original autochthonous location in the basement has to be found by retrodeformation enabled by a map-aspect kinematic model. The Grenchenberg structure developed by collision of the Montoz and Chasseral ranges and was affected by at least three different anchors. At one of these anchors a belt of brachyanticlines developed along which material was dextrally transferred from the Montoz- to the Chasseral fold, resulting in the disappearance of the former and the strengthening of the latter, which in turn merged with the rising Weissenstein fold.

Nucleation, development and petrophysical properties of faults in carbonate grainstones: Evidence from the San Vito Lo Capo peninsula (Sicily, Italy)

Detailed field mapping and microstructural and textural analyses carried out in Lower Pleistocene grainstones in the San Vito Lo Capo peninsula (in north-western Sicily) revealed document failure modes and fault development in porous carbonate grainstones. Individual compactive shear bands represent the simplest fundamental shear structures, while pressure solution processes commonly localize within previously developed compactive shear bands. In the Lower-Pleistocene carbonate grainstones of San Vito Lo Capo peninsula, composed of eroded carbonate and marl fragments, pressure solution processes localize mostly grain-to-grain, leading to the formation of zones of weakness which facilitate slip and significant displacement. The transition from one deformation process to another is likely controlled by changing material properties and anisotropy within the bands. Finally, laboratory analyses of representative fault rock samples show that the structures described above have sealing capacity with respect to the host rock, and may compartmentalize any geofluid reservoir.

Evolution of a poly-deformed relay zone between fault segments in the eastern Southern Alps, Italy

Abstract In the eastern Southern Alps (NE Italy), Liassic north–south extensional structures are prominent. The southern Trento Platform also experienced extension during the Palaeogene, when reactivation of some pre-existing faults occurred, coupled with nucleation of new faults. During Neogene shortening, these structures were reactivated once again, but with strike-slip kinematics. In this framework, the Gamonda–Tormeno restraining stepover represents the final result of an overlap zone which evolved through time. In the first stage (Lias to Early Cretaceous) a prominent splay developed at the tip of the Gamonda Fault by lateral propagation and breaching of independent segments. At the same time, there was kinematic interaction between the antithetic Gamonda and Tormeno faults, followed by diachronous motion on crossing faults and the development of a narrow graben. During the second stage of extensional tectonics (Palaeocene to Early Oligocene), the reactivation and propagation of the overlapping faults along with the generation of new faults led to deepening of the graben. In the third stage (Miocene to present), the final structure of a strike-slip restraining stepover was accomplished. Due to the mechanical stratigraphy and complex inherited architecture of the relay zone where stratigraphic sequences with different rheological properties are juxtaposed, the style of shortening is different in the western and eastern sides of the stepover. The Gamonda–Tormeno structure represents a unique example of how a relay zone may change through time.