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Tectonics and Structural Geology

Minds over Methods

Minds over Methods: Experimental seismotectonics

Minds over Methods: Experimental seismotectonics

For our next Minds over Methods, we go back into the laboratory, this time for modelling seismotectonics! Michael Rudolf, PhD student at GFZ in Potsdam (Germany), tells us about the different types of analogue models they perform, and how these models contribute to a better understanding of earthquakes along plate boundaries.

 

Credit: Michael Rudolf

Experimental seismotectonics – Seismic cycles and tectonic evolution of plate boundary faults

Michael Rudolf, PhD student at Helmholtz Centre Potsdam – German Research Centre for Geosciences GFZ

The recurrence time of large earthquakes that happen along lithospheric-scale fault zones such as the San Andreas Fault or Chile subduction megathrusts, is very long (≫100 yrs.) compared to human timescales. The scarcity of such events over the instrumental record of around 60 years is unfortunate for a statistically sound analysis of the earthquake time series.

So far, only few megathrust events have been monitored in detail with near-field seismic and geodetic networks. To circumvent this lack of observational data, we at Helmholtz Tectonic Laboratory use analogue modelling to understand plate boundary faulting on multiple time-scales and the implications for seismic hazard. We use models of strike-slip zones and subduction zones, to investigate several aspects of the seismic cycle. Additionally, numerical simulations accompany and complement each experimental setup using experimental parameters.

 

Seismotectonic scale models
In my project, we develop experiments that can model multiple seismic cycles in strike-slip conditions. Our study employs two types of experimental setups both are using the same materials. The first is simpler (ring shear setup) and is able to show the on-fault rupture propagation. The second is geometrically more similar to the natural system, but only the surface deformation is observable.

To model rupture propagation, we introduce deformable sliders in a ring shear apparatus. Two cylindrical shells of ballistic gelatine (Ø20 cm), representing the side walls, rotate against each other, with a thin layer (5 mm) of glass beads (Ø355-400µm) in between representing an annular fault zone. A see-through lid connected to force sensors holds the upper shell in place, whereas the machine rotates the lower shell. Through the transparent lid and upper shell, we directly observe the fault slip. We can vary the normal stress on the fault (<20 kPa) and the loading velocity (0.0005 – 0.5 mm/s).

The next stage of analogue model, features depth-dependent normal stress and a rheological layering mimicking the strike-slip setting in the uppermost 25-30 km of the lithosphere (see also Mehmet Köküm’s blog post). A gelatine block (30x30cm) compressed in uniaxial setting represents the elastic upper crust under far-field forcing. Embedded in the block is a thin fault filled with quartz glass beads. The ductile lower crust is modelled using viscoelastic silicone oil. The model floats in a tank of dense sugar solution, to guarantee free-slip, stress-free boundaries.

 

Figure 2 – Setup and monitoring technique during an experiment. Several cameras record the displacement field and the ring shear tester records the experimental results. Credit: Michael Rudolf

 

Analogue earthquakes
Both setups generate regular stick-slip cycles including minor creep. Long phases of quiescence, where no slip or very slow creep occurs, alternate with fast slip events sometimes preceded by slow slip events. The moment magnitude of analogue earthquake events is Mw -7 to -5. The cyclic recurrence of slip events is an analogue for the natural seismic cycle of a single-fault system.

 

Figure 3 – Detailed setup and results from the ring shear tester experiments. The upper right image shows a snapshot of an analogue earthquake rupture along the fault zone. The plot shows the recorded shear forces and slip velocities over one hour of experiment. Credit: Michael Rudolf

 

Optical cameras record the slip on the fault and the deformation of the sidewalls. Using digital image correlation techniques, we are able to visualize accurately deformations on the micrometre scale at high spatial and temporal resolution. Accordingly, we can verify that analogue earthquakes behave kinematically very similar to natural earthquakes. They generally nucleate where shear stress is highest, and then propagate radially until the seismogenic width is saturated. In the end, the rupture continues laterally along the fault strike. Our experiments give insight into the role of viscoelastic relaxation, interseismic creep, and slip transients on the recurrence of earthquakes, as well as the control of loading conditions on seismic coupling and rupture dynamics.

 

Figure 4 – Setup and Results for the strike-slip geometry. The surface displacement field is very similar to natural earthquakes. The plot shows that due to technical limitations of this setup, fewer events are recorded but the slip velocities are higher. Credit: Michael Rudolf

 

Future developments
Together with our partners in the Collaborative Research Centre (CRC1114 – Scaling Cascades in Complex Systems) we employ a new mathematical and numerical description of the fault system, to simulate our experiments and get a physical understanding of the empirical friction laws. In the future, we want to use this multiscale spatial and temporal approach to model complex fault networks over many seismic cycles. The experiments serve as benchmarks and cross-validation for the numerical code, which in the future will be using natural parameters to get a better geological and mathematical understanding of earthquake slip phenomena and occurrence patterns in multiscale fault networks.

Minds over Methods: Reconstructing oceans lost to subduction

Minds over Methods: Reconstructing oceans lost to subduction

Our next Minds over Methods article is written by Derya Gürer, who just finished a PhD at Utrecht University, the Netherlands. During her PhD, she used a combination of many methods to reconstruct the evolution of the Anadolu plate, which got almost entirely lost during closure of the Neotethys in Anatolia. Here, she explains how the use of these multiple methods helped her to obtain a 3D understanding of the Anatolian double subduction system and the demise of the Anadolu plate. From May 2018, Derya will be joining the School of Earth and Environmental Sciences at The University of Queensland, Brisbane, Australia, to investigate the tectonic evolution of other subduction-dominated systems, such as the Eastern Australian margin and the Southwest Pacific region.

 

Credit: Derya Gürer

Reconstructing oceans lost to subduction

Derya Gürer, lecturer at the School of Earth and Environmental Sciences, University of Queensland, Brisbane, Australia. 

Subduction represents the single biggest recycling process on Earth and takes place at convergent plate boundaries. One plate subducts underneath another into the mantle, generating volcanism, earthquakes, tsunamis and associated hazards. Subduction zones come and go, and nearly half of the subduction zones active today formed in the Cenozoic (after ~65 Ma) (Gurnis et al., 2004). The negative buoyancy of subducted lithosphere (‘slab pull’) is thought to be the major driver of plate tectonics (Turcotte and Schubert, 2014). Changes in the configuration of subduction zones thus change the driving forces of plate tectonics, making the reconstruction of the kinematic evolution of subduction key to understanding past plate motions. Such reconstructions make use of data preserved in the modern oceans (marine magnetic anomalies and fracture zone patterns). But because subduction is a destructive process, the surface record of subduction-dominated systems is naturally incomplete, and more so backwards in time. Sometimes, relicts of subducted lithosphere are preserved in active margin mountain belts, holding valuable information to restore past plate motions and the dynamic evolution of subduction zones.

But how does one recognize a plate that has been almost entirely lost to subduction? And how do we reconstruct the evolution of subduction zones through space and time?

 

Archives of plates that were (almost) lost due to subduction

Subduction occurs in a variety of geometries and leaves behind a distinct geological record that holds key elements for the analysis of the past kinematics of now-subducted plates. Where subduction occurred below oceanic lithosphere, fragments of the leading edge of this overriding lithosphere may be left behind as remnants of oceanic crust (ophiolites). Subduction of oceanic plates may also be associated with accretion of its volcano-sedimentary cover to the overriding plate as an accretionary complex (Matsuda and Isozaki, 1991). Forearc basins associated with intra-oceanic subduction zones form on top of ophiolites and accretionary complexes and may record permanent deformation (syn-kinematic) of the overriding plate in response to tectonic interaction with the down-going plate (e.g., accretion, subduction erosion, slab roll-back) (Fig. 1).

Fig. 1: The location of archives of the evolution of “lost” oceanic plates (ophiolites, accretionary complexes, forearc basins) in a subduction zone setting.Credit: Derya Gürer.

The sedimentary infill of forearc basins implicitly records the nature and stress state of the overriding plate. Forearc basins may therefore hold the most complete record of the motion of the oceanic plate relative to the trench. However, many accretionary complexes and forearcs are deeply submerged and buried below sediments, making them highly inaccessible, and therefore expensive to study. As a consequence, our understanding of such systems is primarily based on well-studied examples in the East Pacific (e.g. Franciscan Complex, California (Wakabayashi, 2015)). Other such systems exist in the Mediterranean realm – for example in the geological record of Anatolia. The unique and direct archive of past plate motion in the geological record of Central and Eastern Anatolia is independent from constraints provided by marine magnetic anomalies, and provides a key region to unravel the evolution of destructive plate boundaries.

 

How many oceans were lost in Anatolia?

Fig. 2: The multidisciplinary approach used in my PhD research consisted of structural field analysis and stratigraphy of Anatolian sedimentary basins with focus on syn-kinematic deformation (top) with time constraints provided by absolute age dating of accessory minerals and biostratigraphy (middle). Paleomagnetic analysis (bottom left) provided information about vertical axis rotations. The combined information from these methods were integrated in a kinematic reconstruction and tested against mantle tomography (bottom right). Credit: Derya Gürer

To answer this question, I studied the deformation of sedimentary basins overlying Anatolian ophiolites (remnants of oceanic crust), and the deformation record of rocks which were buried and exhumed below these ophiolites. The Cenozoic deformation of the Anatolian orogen allowed for identifying the timing of arrest of the subduction history and revealed the simultaneous activity of two subduction zones in Late Cretaceous time. These two subduction zones bound a separate oceanic plate within the Neotethys Ocean – the Anadolu Plate (Fig. 3, Gürer et al., 2016). The aim of my PhD research was to reconstruct the birth, evolution and destruction of this oceanic plate.

Tectonic problems require a multidisciplinary approach, in order to study the evolution of orogens and associated sedimentary basins. My research involved the integration of (1) structural analysis, (2) stratigraphy, (3) geochronology, (4) paleomagnetism, (5) plate reconstruction, and (6) mantle tomography (Fig. 2). The main goal was to obtain new data on the evolution of the Central and Eastern Anatolian regions through the analysis of spatial and temporal relationships of deformation archived in the geological record.

First, I collected kinematic data from sedimentary basins (Fig. 2) overlying ophiolitic relicts of the oceanic Anadolu Plate, as well as from the underlying accretionary complex (Gürer et al., 2018a). Here, it was especially useful to focus on syn-kinematic deformation recorded by sediments. To constrain the timing of this deformation, I used geochronological data coming from absolute age dating and biostratigraphy. The integrated reconstruction of the kinematic history of basins was used to develop concepts quantitatively constraining the tectonic history of the Anadolu Plate and its surrounding trenches in 2D (Gürer et al., 2016).

 

Fig. 3: The Ulukışla Basin (Central Anatolia) represents a forearc basin in Late Cretaceous to Eocene time which recorded the evolution of the Anadolu Plate. The basin has subsequently been strongly deformed during Eocene and younger collisional processes and is juxtaposed against the Aladağ range along the Ecemiş Fault. Credit: Derya Gürer.

 

There are, however, large vertical axis rotations constrained through paleomagnetic analysis within Anatolia, not taken into account in the workflow described in the previous paragraph. Therefore, paleomagnetic data from the Late Cretaceous to Miocene sedimentary basins were collected. These data identified coherently rotating domains and major tectonic structures that accommodated differential rotations between tectonic blocks (Gürer et al., 2018b).

Fig. 4: Simplified interpretation of the Late Cretaceous double subduction geometry in Anatolia and the Anadolu Plate.Credit: Derya Gürer.

Subsequently, a kinematic reconstruction of Anatolia back to the Late Cretaceous was built (Fig. 4) incorporating the timing of deformation obtained through structural analysis, stratigraphy, geochronology, and vertical axis rotations. This reconstruction provided first-order implications for the timing and geometry of subduction zones and revealed that the demise of the Anadolu Plate and collision in Anatolia was variable along the strike of the orogen, younging from the west to the east. The exact timing of collision in Eastern Anatolia will require future studies applying structural field geology, systematic analysis of the age and nature of magmatism, and thermochronology to constrain timing of regional exhumation, as well as detrital geochronology, providing information on the relative proximity of tectonic blocks through the provenance of sediments.

 

Finally, the resulting 2D kinematic reconstruction was tested against a mantle tomographic model (UU-07, Amaru, 2007; van der Meer et al., 2017) to gain insights into its 3D geometry. Mantle tomography images the present-day structure and positive seismic anomalies (blue colours in Fig. 5), which may be interpreted as subducted slabs. Comparing the convergence estimate obtained from the kinematic reconstruction with the imaged subducted lithosphere allowed to infer that the mantle structure in the Eastern Mediterranean holds record of not only the two strands of the Neotethys Ocean that existed in Anatolia, but also of the Paleotethys Ocean.

 

Fig. 5: Map view tomographic structure below the Eastern Mediterranean region at variable depths (increasing in depth from left to right). Blue colours generally represent positive, whereas red colours represent negative wave speed anomalies. Credit: Derya Gürer & Wim Spakman.

The combination of methods to unravel the geological record of Anatolia quantitatively constrained the evolution of subduction zones and of the Anadolu Plate. The reconstruction of the Anatolian double subduction system that existed in Late Cretaceous time has implications for the dynamics of multiple simultaneously active subduction zones.

 

References

Amaru, M.L., 2007. Global travel time tomography with 3-D reference models. PhD thesis, Utrecht University, The Netherlands.

Gürer, D., van Hinsbergen, D.J.J.D.J.J., Matenco, L., Corfu, F., Cascella, A., 2016. Kinematics of a former oceanic plate of the Neotethys revealed by deformation in the Ulukışla basin (Turkey). Tectonics 35, 2385–2416. https://doi.org/10.1002/2016TC004206

Gürer, D., Plunder, A., Kirst, F., Corfu, F., Schmid, S.M., van Hinsbergen, D.J.J., 2018a. A long-lived Late Cretaceous–early Eocene extensional province in Anatolia? Structural evidence from the Ivriz Detachment, southern central Turkey. Earth Planet. Sci. Lett. 481. https://doi.org/10.1016/j.epsl.2017.10.008

Gürer, D., Hinsbergen, D.J.J. van, Özkaptan, M., Creton, I., Koymans, M.R., Cascella, A., Langereis, C.G., 2018b. Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia. Solid Earth 9, 1–27. https://doi.org/10.5194/se-9-1-2018

Gurnis, M., Hall, C., Lavier, L., 2004. Evolving force balance during incipient subduction. Geochemistry Geophys. Geosystems 5, Q07001. https://doi.org/10.1029/2003GC000681

Matsuda, T., Isozaki, Y., 1991. Well-documented travel history of Mesozoic pelagic chert in Japan: from remote ocean to subduction zone. Tectonics 10, 475–499.

van der Meer, D.G., van Hinsbergen, D.J.J., Spakman, W., 2018. Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics 723, 309–448.

Wakabayashi, J., 2015. Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales. Int. Geol. Rev. 37–41. https://doi.org/10.1080/00206814.2014.998728

 

Minds over Methods: Making ultramylonites

Minds over Methods: Making ultramylonites

“Summer break is over, which means we will continue with our Minds over Methods blogs! For this edition we invited Andrew Cross to write about his experiments with a new rock deformation device – the Large Volume Torsion (LVT) apparatus. Andrew is currently working as a Postdoctoral Research Associate in the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA. He did his PhD at the University of Otago, New Zealand, although he is originally from the UK. His main research interest lies in understanding how micro-scale deformation processes influence the evolution of Earth’s lithosphere and tectonic plate boundaries. Hopefully we will be seeing more of him in the very near future” – Subhajit Ghosh.

Credit: Andrew Cross

Investigating strain-localisation processes in high-strain laboratory deformation experiments

Andrew Cross, Postdoctoral Research Associate at the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA.

Below the upper few kilometres of the Earth’s surface – where rocks break and fracture under stress – elevated temperatures and pressures enable solid rocks to flow and bend, like a chocolate bar left outside on a warm day. This ductile flow of rocks and minerals plays a crucial role in many large-scale geodynamic processes, including mantle convection, the motion of tectonic plates, the flow of glaciers and ice sheets, and post-seismic and post-glacial rebound.

Fig. 1: Creep deformation occurs over very long timescales in the Earth. To replicate these processes on observable timescales, we must increase the rate of deformation in the laboratory. Credit: Andrew Cross

Unlike seismogenic slip that periodically accommodates large displacements over very short timescales, ductile flow occurs continuously, and at an almost imperceptibly slow rate: for example, rocks in the Earth’s interior creep at a rate roughly 10 billion times slower than that of the long-running pitch drop experiment1. Since few researchers are willing to wait millions of years to observe creep deformation in nature, we need ways of replicating these processes on much shorter timescales. Fortunately, by increasing temperature and the rate of deformation in the laboratory, we can generate creep behaviour in small samples of rock over timescales of a few hours, days, or weeks (Fig. 1).

In the Experimental Studies of Planetary Materials (ESPM) group at Washington University in St. Louis, we have spent the last couple of years developing a new rock deformation device – the Large Volume Torsion (LVT) apparatus (Fig. 2) – for performing torsion (twisting) experiments on geologic materials. By twisting small, disk-shaped rock samples, we are able to apply much more deformation (“strain”) than by squashing cylindrical samples end-on: this enables us to replicate deformation processes that operate in high-strain regions of the Earth (along the boundaries between tectonic plates, for instance).

Fig. 2: The Large Volume Torsion (LVT) apparatus. A 100-ton hydraulic ram applies a confining pressure, while electrical current passes through a graphite tube around the sample, generating heat through its electrical resistance. A screw actuator (typically used to raise and lower drawbridges) is used to rotate the lower platen and twist the sample, held between two tungsten-carbide anvils. Credit: Andrew Cross

Using the LVT apparatus, we are starting to investigate the microstructural and mechanical processes that lead to the formation of mylonites and ultramylonites: intensely deformed rocks that comprise the high-strain interiors of ductile shear zones and tectonic plate boundaries. It is widely thought that dramatic grain size reduction during (ultra)mylonite formation causes strain localisation, since strain-weakening deformation mechanisms (i.e., diffusion creep and grain boundary sliding) dominate at small grain sizes. However, grain size reduction (and therefore strain-weakening) is counteracted by the tendency of grains to grow over time, in the same way that bubbles in soapy water merge and grow over time.

An effective way of limiting grain growth is through “Zener pinning”, whereby the intermixing of grains of different mineral phases prevents grain boundary migration (and therefore growth). However, despite its suspected importance for ultramylonite formation and the occurrence of localised deformation on Earth (and possibly other planetary bodies), the processes leading to interphase mixing remain somewhat poorly understood and quantified.

Fig. 3: A comparison between our experimentally deformed calcite-anhydrite samples2 (backscattered electron (BSE) images), and natural metagranodiorite mylonites from Gran Paradiso, Western Alps3 (quartz grains, in black, mapped using electron backscatter diffraction (EBSD). Credit: Andrew Cross and Kilian et al., 2011.

To investigate phase mixing processes, we recently performed torsion experiments on mixtures of calcite and anhydrite. By deforming these mixtures to different amounts of strain, and then analysing the deformed samples in a scanning electron microscope, we were able to observe and quantify the evolution of deformation microstructures and mechanisms leading to ultramylonite formation. Backscattered electron (BSE) images show that clusters of the different minerals stretch out to form very thin, fine-grained layers, similar to foliation in natural shear zones (Fig. 3). At relatively large shear strains (17 < γ < 57) those layers disaggregated to form a fine-grained and homogeneously mixed aggregate. Electron backscatter diffraction (EBSD) analysis showed that calcite crystals became progressively more randomly oriented during phase mixing, indicative of a transition to the strain-weakening diffusion creep and grain boundary sliding regime.

The fact that a large amount of strain is required for phase mixing – and therefore strain-weakening – suggests that 1) only mature (highly-strained) shear zones are likely to maintain their weakness over long periods of geologic time, and 2) these features are therefore more likely to be reactivated after periods of quiescence. Inherited, long-lived mechanical weakness may well explain why tectonic plate boundaries are often reactivated over multiple cycles of continent accretion and rifting.

 

http://smp.uq.edu.au/content/pitch-drop-experiment

 Cross, A. J., & Skemer, P. (2017). Ultramylonite generation via phase mixing in high‐strain experimentsJournal of Geophysical Research: Solid Earth122(3), 1744-1759.

 3 Kilian, R., Heilbronner, R., & Stünitz, H. (2011). Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creepJournal of Structural Geology33(8), 1265-1284.

Minds over Methods: Block modeling of Anatolia

 

How can we use GPS velocities to learn more about present-day plate motions and regional deformation? In this edition of Minds over Methods, one of our own blogmasters Mehmet Köküm shares his former work with you! For his master thesis at Indiana University, he used block modeling to better understand the plate motion and slip rates of Anatolia and surrounding plates.

 

credit: Mehmet Köküm

Using block modeling to constrain present-day deformation of Anatolia and slip rates along the North Anatolian Fault

Mehmet Köküm, researcher at Firat University, Turkey

Until the late 1980’s, geological features such as offset of geomorphological markers were mainly used to determine historical slip rates along faults. Since the mid 1990’s, however, GPS has been widely used since it gives more accurate estimates of present-day slip rates by calculating strain accumulation at the crust. In this work, I use a GPS derived velocity field of Anatolia including data from 1988 to 2005 by Reilinger et al. (2006).

Turkey (Anatolian Plate) is located in the center of the Alpine fold and thrust belt. Due to the closure of different branches of the Neo-Tethys Ocean, main tectonic features of the Anatolian Plate are complicated by interactions between several tectonic plates.  The Arabian plate collides with the African plate in the south and the Eurasian plate in the north while the African plate subducts beneath the Anatolian plate along the Hellenic-Cyprus trench. As a result of these complex tectonic structures, the Anatolian plate displays various tectonic styles simultaneously.

Modeling and Data
Kinematic block modeling of interseismic surface motions has been used in different formats by several authors (e.g., McClusky et al. 2000; Westaway 2000; Barka and Reilingier 1997, 2006). The block modeling approach used here is described by Johnson and Fukuda (2010). In this study we used an elastic block model, which is a traditional block model that assumes no long-term deformation of the blocks. For simplicity, all faults are vertical, plates are considered as blocks and are assumed to be rigid. Block boundaries are defined from historic earthquakes, mapped faults and seismicity. Many of the major structures in Anatolia are well known except for a few submarine structures.

 

Map showing selected block model including of 14 blocks (or plates). Credit: Mehmet Köküm

 

Locking Depth
Locking depths indicate the depth for which a fault is completely locked above and creeping below. Estimates of these locking depths are output of the modeling studies and should correlate with the depth of major earthquakes along related faults. Meade and Hager (2005) suggest that there is a relation between locking depth and fault slip rates. Shallower locking depths correlate with slower slip rate estimates; therefore, GPS velocities near locked faults have slower velocities (Reilinger et al., 2006).

 

Elastic-half-space model showing fault creep at surface, locked (nonslipping) fault at depth, and freely sliding zone at great depth. (source: SFSU CREEP Project)

 

Results
On the basis of the GPS velocity field, the Anatolia and Aegean blocks show counterclockwise motion with respect to the Eurasian plate and the rate of the motion increases towards the west. The locking depth variations of the work are between 20-25 km, which correlates with the focal depths of significant earthquakes. The major fault slip rates are consistent with some of the geological slip rate estimates.

 

Results of the model. Figure shows Anatolian plate motion and slip rate estimates of major faults. Credit: Mehmet Köküm

Minds over Methods: Reconstruction of salt tectonic features

Minds over Methods: Reconstruction of salt tectonic features

What is the influence of salt tectonics on the evolution of sedimentary basins and how can we reconstruct such salt features? Michael Warsitzka, PhD student at the Friedrich Schiller University of Jena, explains which complementary methods he uses to better understand salt structures and their relation to sedimentary basins. Enjoy!

 

Credit: Michael Warsitzka

Reconstruction of salt tectonic features from analogue models and geological cross-sections

Michael Warsitzka, PhD student, Institute of Geosciences, Friedrich Schiller University Jena

Salt tectonics, as a sub-discipline of structural geology, describe deformation structures developing due to the special deformation behaviour of salt (as synonym for a sequence of evaporitic rocks). Salt behaves like a viscous fluid over geological time scales and, therefore, it may flow due to lateral differences in thickness and density of the supra-salt layers. This influences the structural evolution of sedimentary basins, because salt flow can modify the amount of regional subsidence of the basin. Local sinks (“minibasins”) develop in regions from where salt is squeezed out and salt structure uplifts, e.g. diapirs or pillows evolve in regions of salt influx. Unfortunately, temporal changes of salt flow patterns are often difficult to reconstruct owing to enigmatic ductile deformation structures in salt layers. Understanding the evolution of salt-related structures requires either forward modelling techniques (e.g. physically scaled sandbox experiments) or restoration of sedimentary and tectonic structures of the supra-salt strata.

In my PhD thesis, I tried to integrate both, analogue modelling and restoration, to investigate salt structures and related minibasins developed in the realm of extensional basins. The sandbox model is a lab-scale, simplified representative of natural salt-bearing grabens, e.g. the Glückstadt Graben located in the North German Basin (Fig. 1). A viscous silicone putty and dry, granular sand were used to simulate ductile salt and brittle overburden sediments. Cross sections were cut through the model at the end of each experiment to conduct reconstruction of the final experimental structures. The material movements were monitored with a particle tracking velocimetry (PIV) technique at the sidewalls of the experimental box.

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Fig 1: 2D restoration of the supra-salt (post-Permian) strata in the Glückstadt Graben (Northern Germany). Credit: Michael Warsitzka

Using experimental and geological cross sections, structures in the overburden of the ductile layer can be reconstructed, if present-day layer geometries and lithologies of the overburden strata can be identified. From natural clastic and carbonatic sediments we know that they compact with burial, reducing the layer thickness. Therefore, the reconstruction procedure sequentially removes the uppermost layer and layers beneath are decompacted and shifted upwards to a horizontal surface (Fig. 2). The sequence of decompaction and upward shifting is then repeated until the earliest, post-salt stage is reached (Fig. 1). It intends to restore the initial position, shape and thickness of each reconstructed layer.

In analogue experiments, no decompaction is necessary, because the compressibility of the granular material is insignificant for depths of a few centimetre. Restoration can be directly applied to coloured granular layers revealing detailed layer geometries for each experimental period (Fig. 2a). The PIV technique displays coeval material movement and strain patterns occurring during the subsidence of the experimental minibasins (Fig. 2b). Based on the observation that the experimental structures resemble those reconstructed from the natural example (Glückstadt Graben during the Early Triassic, Fig. 1), it can be inferred that strain patterns observed in the experiments took place in a similar manner during the early stage of extensional basins. This demonstrates the advantage of applying both methods. First, original geometries of basin structures can be determined from the restoration and then reproduced in the model. If the restored geometries are suitably validated by the models, the kinematics observed in the model can be translated back to nature and help to understand the effect of salt flow on the regional subsidence pattern.

Fig 2: Result of an analogue model showing (a) reconstructed sand layers restored from a central cross section, and (b) monitored displacement and strain patterns in the viscous layer above the left basal normal fault. Credit: Michael Warsitzka

Minds over Methods: Sensing Earth’s gravity from space

Minds over Methods: Sensing Earth’s gravity from space

How can we learn more about the Earth’s interior by going into space? This edition of Minds over Methods discusses using satellite data to study the Earth’s lithospere. Anita Thea Saraswati, PhD student at the University of Montpellier, explains how information on the gravity of the Earth is obtained by satellites and how she uses this information to get to know more about the lithosperic structure in subduction zones.

 

Sensing Earth’s gravity from space

Anita Thea Saraswati – PhD student, Géosciences Montpellier

From the basic physics we all know that the value of the gravity is a constant 9.81 meter per second squared. This assumption would be true if the Earth were a smooth nonrotating spherical symmetric body made of uniform element and material. However, because of the Earth’s rotation, internal lateral density variation, and the diversity of the topography (including mountains, valleys, oceans and glaciers), the gravity  varies all over the surface. These tiny changes in gravity due to the mass variations could be a crucial hint for understanding the structure of the Earth, both on the surface and at depth.

The determination of Earth’s gravity field has benefited from various gravity satellite missions that have been launched recently. Among them are the Challenging Minisatellite Payload (CHAMP) (2000-2010), the Gravity Recovery and Climate Experiment (GRACE) (2002-recent), and most recently the Gravity field and steady-state Ocean Circulation Explorer (GOCE) (2009-2013). From these missions, finally a global high quality coverage of Earth’s gravity field became available. (Yay!)

GRACE observation data are very useful for the temporal analysis of changes in gravity. For example to detect the gravity signal before and after a big earthquake, like the Sumatra Mw 9.1 (2004) and Tohoku Mw 9.1 (2011) ones. By analyzing the changes of gravity signal during a certain period of time, it could also be used to detect the drought over a large scale area, which is used in several areas in Africa and Australia.

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Design of GOCE satellite observation. A geoid’s shape is showed on the bottom left. On the top right, the GOCE gravity gradients in six components. (Source : ESA)

 

Meanwhile, GOCE is very suitable for the construction of a static model of Earth’s gravity field. Since this satellite has a very low orbit, ~250 km above mean sea level, it has a better spatial resolution. Its accuracy is also better than the previous missions, up to 1 mGal. GOCE is equipped with a gradiometer, which measures the gravity acceleration in three directions (x, y, and z). Afterwards this information is processed into a gravity-gradient dataset containing six components (XX, XY, XZ, YY, YZ, ZZ).

This gravity gradient is the first derivative of the gravity acceleration, which provides us better information about the geometry of the earth’s structure than the gravity acceleration itself. For my PhD, I use this gravity gradient dataset to analyze the lithospheric structure of subduction zones. Before treating the GOCE observation data, I am developing a computational code to calculate the gravity and gravity gradient due to the effect of topography, also called the topographic reduction. The observed gravity and gravity gradient values will be reduced by this topography effect in order to get the anomaly signal. This means that only the signal due to other geodynamic phenomena over the observed area (e.g. slab, isostasy, mantle plum, etc.) is left. By doing further processing, we can obtain the lateral variations of the lithospheric structure in the study areas and then investigate the correlation with the occurrence of mega-earthquakes in these subduction zones.

Since there is still some ambiguity about the information that is produced by gravity data only, it is better to combine the use of them with others geophysical or geological measurements, e.g. seismic tomography measurements and magnetic field observations.

 

Global coverage of GOCE gravity gradient (in milliEötvös) in radial direction (ZZ) (Panet, I. et al., 2014)

 

Reference:

Panet, I., Pajot-Métivier, G., Greff-Lefftz, M., Métivier, L., Diament, M. and Mandea, M., 2014. Mapping the mass distribution of Earth/’s mantle using satellite-derived gravity gradients. Nature Geoscience7(2), pp.131-135.

Minds over Methods: studying dike propagation in the lab

Minds over Methods: studying dike propagation in the lab

Have you ever thought of using gelatin in the lab to simulate the brittle-elastic properties of the Earth’s crust? Stefano Urbani, PhD student at the university Roma Tre (Italy), uses it for his analogue experiments, in which he studies the controlling factors on dike propagation in the Earth’s crust. Although we share this topic with our sister division ‘Geochemistry, Mineralogy, Petrology & Volcanology (GMPV)’, we invited Stefano to contribute this post to ‘Minds over Methods’, in order to show you one of the many possibilities of analogue modelling. Enjoy!

 

dscn0024Using analogue models and field observations to study the controlling factors for dike propagation

Stefano Urbani, PhD student at Roma Tre University

The most efficient mechanism of magma transport in the cold lithosphere is flow through fractures in the elastic-brittle host rock. These fractures, or dikes, are commonly addressed as “sheet-like” intrusions as their thickness-length aspect ratio is in the range of 10-2 and 10-4 (fig.3).

Understanding their propagation and emplacement mechanisms is crucial to define how magma is transferred and erupted. Recent rifting events in Dabbahu (Afar, 2005-2010) and Bardarbunga (Iceland, 2014, fig.1) involved lateral dike propagation for tens of kilometers. This is not uncommon: eruptive vents can form far away from the magma chamber and can affect densely populated areas. Lateral dike propagation has also been observed in central volcanoes, like during the Etna 2001 eruption. Despite the fact that eruptive activity was mostly fed by a vertical dike to the summit of the volcano, several dikes propagated laterally from the central conduit and fed secondary eruptive fissures on the southern flank of the volcanic edifice (fig.2). Lateral propagation can hence occur at both local (i.e. central volcanoes) and regional (i.e. rift systems) scale, suggesting a common mechanism behind it.

fig-3mario-cipollini

Fig. 2 Lava flow near a provincial road, a few meters from hotels and souvenir shops, during the 2001 lateral eruption at Etna. Credit: Mario Cipollini

Therefore, it is of primary importance to evaluate the conditions that control dike propagation and/or arrest to try to better evaluate, and eventually reduce, the dike-induced volcanic risk. Our knowledge of magmatic systems is usually limited to surface observations, thus models are useful tools to better understand geological processes that cannot be observed directly. In particular, analogue modelling allows simulating natural processes using scaled materials that reproduce the rheological behavior (i.e ductile or brittle) of crust and mantle. In structural geology and tectonics analogue modelling is often used to understand the nature and mechanism of geological processes in a reasonable spatial and temporal scale.

d_grad_dike57_080Field evidence and theoretical models indicate that the direction of dike propagation is controlled by many factors including magma buoyancy and topographic loads. The relative weight of these factors in affecting vertical and lateral propagation of dikes is still unclear and poorly understood. My PhD project focuses on investigating the controlling factors on dike propagation by establishing a hierarchy among them and discriminating the conditions favoring vertical or lateral propagation of magma through dikes. I am applying my results to selected natural cases, like Bardarbunga (Iceland) and Etna (Italy). To achieve this goal, I performed analogue experiments on dike intrusion by injecting dyed water in a plexiglass box filled with pig-skin gelatin. The dyed water and the gelatin act as analogues for the magma and the crust, respectively. Pig-skin gelatin has been commonly used in the past to simulate the brittle crust, since at the high strain rates due to dike emplacement it shows brittle-elastic properties representative of the Earth’s crust. We record all the experiments with several cameras positioned at different angles, taking pictures every 10 seconds. This allows us to make a 3D reconstruction of the dike propagation during the experiment.

In order to have a complete understanding of the dike intrusion process it is essential to compare the laboratory results with natural examples. Hence, we went to the field and studied dikes outcropping in extinct and eroded volcanic areas, with the aim of reconstructing the magma flow direction (Fig. 3). This allows validating and interpreting correctly the observations made during the laboratory simulations of the natural process that we are investigating.

fig-1

Fig. 3 Outcrop of dikes intruding lava flows. Berufjordur eastern Iceland.

 

Minds over Methods: Experimental earthquakes

Minds over Methods: Experimental earthquakes

After our first edition of Minds over Methods, which was about Numerical Modelling, we now move to Rock Experiments! How can rock experiments be used to study processes within the Earth? We invited Giacomo Pozzi, PhD student at Durham University, to explain us how he uses rock experiments to study fault behaviour during earthquakes.

 

13072693_10207863372934990_7705005482414752149_oExperimental earthquakes to understand the weak behaviour of faults.

Giacomo Pozzi, PhD student at Durham University

As seismic slip along faults accommodates large deformations in the upper crust, the intriguing absence of significant heat flow anomalies (which are expected to be produced by intense energy dissipation during slip) along major geological bodies like the S. Andreas fault pushed the researchers to start conceiving a new, dynamic theory of friction, which eventually led to the concept of low frictional strength of faults during propagation of earthquakes.

rotary_apparatus

Fig 1. the Rotary apparatus

In the past two decades, the development of machines capable of shearing natural materials made it possible to achieve direct, experimental evidences of how friction in rocks (and gouges, when pulverised) drops from Byerlee’s values (μ=0.6-0.8) towards zero when approaching seismic velocities (>10 cm/s) and this independently of the rock composition.

However, even though a common bulk behaviour is witnessed, the weakening mechanisms that operate at the microscale are strongly dependent on the mineralogy and, despite a large amount of literature focused on this research, they are still poorly understood as their physic is an evergreen matter of debate.

My Ph.D. focuses on a weakening mechanism that has been recently proposed to occur in carbonate faults: viscous flow by grain boundary sliding, a diffusion creep dominated process particularly efficient in fine grained aggregates. In order to verify and characterise this hypothesis we try to reproduce coseismic shear conditions in pure calcite (CaCO3) gouges with a Low to High Velocity Rotary (LHVR) apparatus (Figure 1). This machine allows to simulate arbitrary amounts of slip in a thin volume of gouge, our experimental fault core, which is squeezed between two hollow cylinders. A piston located in the lower part of the apparatus lifts the lower cylinder producing an axial load (up to 25MPa) perpendicular to the plane of slip while the top cylinder spins at angular velocities up to 1500rpm (1.4 m/s tangential velocity at the reference radius).

rotary_lrDuring the experiments we record different mechanical parameters that can be processed to obtain: displacement, velocity, axial stress, shear stress, axial displacement and, with an opportune equation, the estimated temperature in the shear zone. The ratio between shear stress and axial stress gives the friction coefficient that produces a classic weakening profile when plotted against the displacement as in the graph of figure 2, where are evident two main stages: pre-weakening (μ>0.6) and weakening stage (μ<0.3).

At the end of each experiment we carefully remove the sheared sample in order to make microstructural analysis. We describe the architecture of the shear zone mainly by acquiring electron backscattered (EBS) images (figure 3) on polished sections of the samples using a scanning electron microscope. We are also planning to use cathodoluminescence and EBS diffraction to study in detail the distribution of strain, temperature and hidden geometries.

By coupling the mechanical data and the microstructural analysis of experiments stopped at different amounts of slip we are able to reconstruct the evolution of the shear zone, including the transition between a pre-weakening brittle behaviour to the steady state weakening stage where ductile-plastic processes are dominant. Understanding how the internal architecture of the shear zone changes with time and measuring its geometrical features is of paramount importance to achieve a quantitative description of the processes, which can lead to new physical laws.

With our experiments we are trying to link a qualitative description of complex natural processes and quantitative simulations based on the current physical knowledge. As a matter of fact, the obtained microstructures can be compared to natural equivalents while mechanical data and inferred laws can be implemented in numerical models.

weakening_profile

Fig 2. Weakening profile

sem_image

Fig 3. SEM BSE image of a cross section of the slip zone

Minds over Methods: Numerical modelling

Minds over Methods: Numerical modelling

Minds over Methods is the second category of our T&S blog and is created to give you some more insights in the various research methods used in tectonics and structural geology. As a numerical modeller you might wonder sometimes how analogue modellers scale their models to nature, or maybe you would like to know more about how people use the Earth’s magnetic field to study tectonic processes. For each blog we invite an early career scientist to share the advantages and challenges of their method with us. In this way we are able to learn about methods we are not familiar with, which topics you can study using these various methods and maybe even get inspired to use a multi-disciplinary approach! This first edition of Minds over Methods deals with Numerical Modelling and is written by Anouk Beniest, PhD-student at IFP Energies Nouvelles (Paris).

 

Approaching the non-measurable

Anouk Beniest, PhD-student at IFP Energies Nouvelles, Paris

‘So, what is it that you’re investigating?’ It’s a question every scientist receives from time to time. In geosciences, the art of answering this question is to explain the rather abstract projects in normal words to the interested layman. Try this for example: “A long time ago, the South American and African Plate were stuck together, forming a massive continent, called Pangea, for many millions of years. Due to all sorts of forces, the two plates started to break apart and became separated. During this separation hot material from deep down in the earth rose to the surface increasing the temperature of the margins of the two continents. How exactly did this temperature change over time, since the separation until present-day? How did this change affect the basins along continental margins?”

These are legitimate questions and not easy to answer, since we cannot measure temperature at great depth or back in time. In this first post on numerical methods, we will be balancing between geology and geophysics, highlighting the possibilities and limits of numerical modelling.

The migration of ‘temperature’ through the lithosphere is a process that takes time and depends heavily on the scale you look at. Surface processes that affect the surface temperature can be measured and monitored, yielding interesting results on the present-day state and variations of the temperature. The influence of mantle convection cycles and radiogenic heat production are already more difficult to identify, take much more time to evolve and might not even affect the surface processes that much. Going back in time to identify a past thermal state of the earth seems almost impossible. This is where numerical models can be of use, to improve, for example, our understanding on the long-term behaviour of ‘temperature’.

Temperature is a parameter that affects and is affected by a variety of processes. When enough physical principles are combined in a numerical model, we can simulate how the temperature has evolved over time. All kinds of different parameters need to be identified and, most importantly, they need to make sense and apply to the observation or process you try to reproduce. Some of these parameters can be identified in the lab, like the density or conductivity of different rock types. Others need to be extracted from physical or geological observations or even estimated.

Once the parameters have been set, the model will calculate the thermal evolution. It is not an easy task to decide if a simulation approaches the ‘real’ history and if we can answer the questions posed above. We should always realise that thermal model results at best approach the real world. We can learn about the different ways temperature changes over time, but we should always be on the hunt to find measurements and observations that confirm what we have learned from the simulations.

temperature_quick