TS
Tectonics and Structural Geology
Derya Gürer

Derya Gürer

Derya Gürer is a Lecturer of Earth Sciences at the School of Earth and Environmental Sciences, University of Queensland, Brisbane, Australia. Her research evolves around tectonics and the evolution of Earth's lithosphere at various spatio-temporal scales. She combines field-based data (structural geology, stratigraphy) with laboratory analyses (geochronology, paleomagnetism) to build kinematic reconstructions and compare those to the structure of the underlying mantle.

Minds over Methods: Mineral reactions in the lab

Minds over Methods: Mineral reactions in the lab

 

Mineral reactions in the lab

André Niemeijer, Assistant Professor, Department of Earth Sciences at Utrecht University, the Netherlands

In this blogpost we will go on a tour of the High Pressure and Temperature (HPT) Laboratory at Utrecht University and learn about some of the interesting science done there.

André Niemeijer next to a striated fault surface. Credit: André Niemeijer.

André’s main interest is fault friction and all the various processes that are involved in the seismic cycle. This includes the evolution of fault strength over long and short timescales, the evolution of fault permeability and the effects of fluids. His current research is aimed at understanding earthquake nucleation and propagation by obtaining a better understanding of the microphysical processes that control friction of fault rocks under in-situ conditions of pressure, temperature and fluid pressure.

Most of the deformation in the Earth’s brittle crust occurs on and along faults. Fault movement produces fine-grained wear material or gouge, which is very prone to fluid-rock interactions and mineral reactions (Wintsch, 1995). It has long been recognized that the presence of a fluid allows for deformation to occur at much lower differential stresses than without.

Pressure solution

One of the mechanisms by which this deformation occurs is pressure solution (alternatively termed “solution-transfer creep” or “dissolution-precipitation creep”). This mechanism operates through the dissolution of materials at sites of elevated stress, diffusion along grain boundaries and re-precipitation at low stress sites (e.g. pores). Pressure solution is an important diagenetic process in sandstones and carbonates as evidenced by the presence of stylolites in many carbonate rocks, which are often used as counter tops and floors (particularly in banks, I noticed). In addition, it has been suggested that pressure solution plays an important role in the accommodation of (slow) shear deformation of faults (Rutter & Mainprice, 1979) and possibly in controlling the recurrence interval of earthquakes (Angevine, 1982).

Fluid-rock interactions in the lab

Experimentally, it is challenging to activate pressure solution or mineral reactions in the laboratory, because they are typically slow processes. Moreover, it is difficult to find evidence of their operation. We have used a unique hydrothermal rotary shear apparatus, which is capable of temperatures up to 700 °C to activate pressure solution in fine-grained quartz gouges. We were able to prove that new material was precipitated by using a combination of state-of-art electron microscopy techniques that involve cathodoluminescence (CL).

The hydrothermal rotary shear apparatus at the HPT laboratory at Utrecht University, the Netherlands. Credit: André Niemeijer.

Signature of pressure solution

The CL signal of a mineral depends on the type and level of impurities and defects that are present. We used quartz derived from a single crystal which showed relatively uniform CL. Because our apparatus has various metal alloy parts, small amounts of aluminium are present in the fluid. Aluminium can be incorporated in newly precipitated quartz, which gives a different CL signal. This allows us to map the locations where quartz has newly formed and link this to the experimental data. Taken together, we can use these to derive and constrain microphysical models for fault slip that can be used to extrapolate to natural conditions (e.g. Chen & Spiers 2016, van den Ende et al., 2018).

RGB overlay of secondary electron and cathodoluminescence signals in a deformed quartz sample. Newly precipitated quartz shows up in a blue colour. Credit: Maartje Hamers.

Mineral reactions

Outcrops of natural faults often show evidence for enhanced mineral reactions with increasing shear strain. For instance, the Zuccale fault (Isle of Elba, Italy) has a high content of talc in the highest strained portion of the fault (Collettini & Holdsworth, 2004). Talc is a frictionally weak mineral and its presence in the Zuccale fault provides an explanation for the possibility of slip along this low-angle normal fault. We were able to produce talc experimentally from mixtures of dolomite and quartz in only 3-5 days of shearing at low velocity. This shearing was accompanied by major weakening, with friction dropping from 0.8 to as low as 0.3. The reaction to talc is sensitive to temperature and fluid composition. At slightly higher temperature, we produced diopside and forsterite which are frictionally unstable and generated audible laboratory earthquakes.

Identifying reaction products

We tried a whole range of different analytical techniques to identify the reaction products. Despite the obvious frictional weakening that we observed, talc was only observed in two samples with x-ray diffraction (XRD). Fourier-transform Infrared analysis, on the other hand, proved to be very sensitive to talc and has the big advantage that only a small amount of material is needed (~70 mg). Electron microscopy with EDS-analysis (Energy Dispersive X-ray Spectroscopy) proved helpful to some extent, because it shows the phase distribution. However, the small size of reaction products gives a mixed chemistry, which complicates the identification of reaction products. Finally, to positively identify the various phases in the different samples, we employed Raman mapping.

RGB overlays of EDS analyses of samples deformed at 300 °C (left) and 500 °C (right). Dolomite appears in yellow, quartz in blue, calcite in red, talc in cyan in the left image, while dolomite is orange, calcite is red, diopside is purple and forsterite is cyan in the right image. Credit: André Niemeijer.

Outlook

Our studies have shown that reactions can be quite rapid in fine-grained fault gouges. These reactions can have a profound effect on both fault strength and stability but are typically ignored in large-scale models of the seismic cycle. Incorporating reactions requires models that can account for the effect of stress and grain size reduction on the development of faults, which is not an easy task, but is a necessary ingredient to understand the long-term behavior of faults.

References

  • Angevine, C. L., Turcotte DL, Furnish MD. (1982) Pressure solution lithification as a mechanism for the stick-slip behavior of faults. Tectonics 1 (2), 151-160 doi:10.1029/TC001i002p00151.
  • Chen, J. and Spiers CJ. (2016) Rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model. Journal of Geophysical Research: Solid Earth 121 (12), 8642-8665 doi:10.1002/2016JB013470.
  • Collettini, C. and Holdsworth RE. (2004) Fault zone weakening and character of slip along low-angle normal faults: Insights from the Zuccale fault, Elba, Italy. Journal of the Geological Society 161 (6), 1039-1051 doi:10.1144/0016-764903-179.
  • E H Rutter, D H Mainprice (1979)On the possibility of slow fault slip controlled by a diffusive mass transfer process. Gerlands Beitr. Geophysik, Leipzig 88 (1979) 2, S. 154-162.
  • van den Ende, M. P. A., Chen J, Ampuero J., Niemeijer AR. (2018) A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip. Tectonophysics 733, 273-295 doi:10.1016/j.tecto.2017.11.040.
  • Wintsch, R. P., Christoffersen R, Kronenberg AK. (1995) Fluid-rock reaction weakening of fault zones. Journal of Geophysical Research: Solid Earth 100 (B7), 13021-13032 doi:10.1029/94JB02622.

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. 

Credit: Derya Gürer

Reconstructing oceans lost to subduction

Derya Gürer, Postdoctoral Researcher, Utrecht University, the Netherlands. 

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