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Minds over Methods: The faults of a rift

Minds over Methods: The faults of a rift

Do ancient structures control present earthquakes in the East African Rift? 

Åke Fagereng, Reader in Structural Geology, School of Earth and Ocean Sciences, Cardiff University

For this edition of Minds over Methods, we have invited Åke Fagereng, reader in Structural Geology at the School of Earth and Ocean Sciences, Cardiff University. Åke writes about faults in the Malawi rift, and the seismic hazard they may represent. With his co-workers from the Universities of Cardiff, Bristol, and Cape Town, the Malawi Geological Survey Department, Chancellor’s College, and the Malawi University of Science and Technology, he has studied these faults in the field and in satellite imagery. Their methods span scales of observation from outcrop to rift valley, allowing insights on how rifts and rift-related faults grow.

 

Åke Fagereng looking at rocks by Lake Malawi. Credit: Johann Diener.

Introduction

East Africa hosts some of the longest faults on the continents, yet it is not a well-known location for significant seismic hazard. There is, however, historical record of large earthquakes (> M7; Ambraseys, 1991), and topographic evidence for major scarps (Jackson and Blenkinsop, 1997), so we ask where, and of what magnitude, could future earthquakes occur?

The instrumental record of East African earthquakes is short, and sparse instrumentation only detects moderate to large magnitude events. Recent, seismic deployments have, however, made great progress in understanding active seismicity in the southern East African rift (e.g. Gaherty et al., 2019; Lavayssière et al., 2019). Such studies have demonstrated that the seismogenic thickness – the depth range where earthquakes nucleate – spans the entire crust (up to 40 km depth). This large seismogenic thickness is important; it theoretically means that ~ 100 km long border faults could rupture along their entire length and throughout the thickness of the crust, creating > M8 earthquakes. Such events would have severe impact, particularly given rapid population growth, urbanization, and high vulnerability of the East African building stock (Goda et al., 2016).

There is a scale dependence to fault observations in rifts. On the rift valley scale, active extension broadly follows ancient plate boundaries marked by deformed, metamorphic rocks separating ancient continental building blocks of cratons and shields. However, does the same observation apply to individual faults, and at what depths? Is there evidence for large earthquake events? A hypothesis to test is whether ancient structures control the location and geometry of major rift border faults, and allow them to be long and continuous.

 

The Bilila-Mtakataka fault in Malawi (black), broadly following but locally cross-cutting foliations (red). Map reproduced after Hodge et al. (2018).

Methods and Results

In the PREPARE project, we focus on Malawi as a case study in the southern, non-volcanic, portion of the rift. Although we have established a network of campaign GPS stations, slow extension rates (< 3.5 mm/yr; Saria et al., 2014) imply that reliable geodetic data will take several more years to collect. Similarly, long earthquake recurrence times lead to a question of how representative the instrumental record can be. Therefore, we have explored other methods for understanding fault scarps and the likely behavior of their faults.

In his PhD work, Michael Hodge used a combination of data sets, including satellite imagery and digital elevation models, photogrammetry, and field observations to analyse the major Bilila-Mtakataka fault from outcrop to rift scale. This fault sits within an ancient ‘mobile belt’ and follows these structures in some places, but cross cuts them in others (Hodge et al., 2018). These observations show that the fault only exploited near-surface ancient structures where these were very well oriented for failure, while elsewhere the fault would take another orientation.

 

The Bilila-Mtakataka fault, illustrated by its ~ 10 m high scarp. Credit: Åke Fagereng.

 

To better understand the controls on fault reactivation and how our surface observations relate to faulting at depth, we place our field and satellite observations in a framework of fault mechanics. It is not uncommon that normal fault segments take two strike orientations: one parallel to pre-existing weaknesses, the other perpendicular to the extension direction (e.g. McClay and Khalil, 1998). However, although there is uncertainty around local extension directions, where fault segments in Malawi cross-cut ancient structures they do not seem to have a consistent, extension-perpendicular, strike (Hodge et al., 2018).

Hodge et al. (2018) tested whether a mid- to lower crustal ancient structure could control the average orientation of the Bilila-Mtakataka surface scarp. This test was based on whether model predictions fitted detailed measurements of scarp height and orientation using a digital elevation model. Such measurements are time consuming, but using a semi-automated method, can also be applied efficiently at the scale of multiple faults (Hodge et al., 2019). Here, the model supports a conclusion where long fault scarps form above localized, deep crustal structures, that are locally deflected near the surface. This model allows the long scarps to have formed in multiple smaller earthquakes rather than a single very big one.

 

Field observations of the Bilila-Mtakataka fault, which locally parallels (A) and locally cross cuts foliation (B). Map from Hodge et al. (2018), Michael Hodge and Hassan Mdala study the fault in A (credit Åke Fagereng) and Åke Fagereng measures a fracture in B (credit Johann Diener).

 

Model, simplified from Hodge et al. (2018), showing how upward propagation of a deep structure is consistent with a segmented surface trace, if the fault is deflected by well-oriented foliation locally and near –surface.

Future Work

Newly discovered fault scarps within the Zomba graben, further south in Malawi, furthermore show that fault orientations vary on the scale of multiple faults within a graben. Here, strike variations also lack a clear fit to expected orientations inferred from the rift-wide extension direction. Ongoing studies are now employing similar methods to analyze the relation between fault geometry and pre-existing structures in the field, on the scale of a complete graben, further bridging the outcrop and rift scales.

Geophysical and remote sensing approaches have been and continue to be invaluable to understand the regional and subsurface structure of the East African and other continental rifts. We emphasise, however, that fault scale structural observations aid interpretation of these data and allow deduction of fault growth mechanisms. Our methods in future work will therefore continue to span scales, and root interpretation in detailed fault-scale field studies linked to rift-scale satellite data.

 

Edited by Elenora van Rijsingen

 

References

  • Ambraseys, N.N., 1991. The Rukwa earthquake of 13 December 1910 in East Africa. Terra Nova 3, 202-211.
  • Gaherty, J.B., Zheng, W., Shillington, D.J., Pritchard, M.E., Henderson, S.T., Chindandali, P.R.N., Mdala, H., Shuler, A., Lindsey, N., Oliva, S.J. and Nooner, S., 2019. Faulting processes during early-stage rifting: seismic and geodetic analysis of the 2009–2010 Northern Malawi earthquake sequence. Geophysical Journal International 217, 1767-1782.
  • Goda, K., Gibson, E. D., Smith, H. R., Biggs, J., & Hodge, M. (2016). Seismic risk assessment of urban and rural settlements around Lake Malawi. Frontiers in Built Environment, 2, 30.
  • Hodge, M., Fagereng, Å., Biggs, J. and Mdala, H., 2018. Controls on early‐rift geometry: new perspectives from the Bilila‐Mtakataka fault, Malawi. Geophysical Research Letters 45, 3896-3905.
  • Hodge, M., Biggs, J., Fagereng, Å., Elliott, A., Mdala, H. and Mphepo, F., 2019. A semi-automated algorithm to quantify scarp morphology (SPARTA): application to normal faults in southern Malawi. Solid Earth 10, 27-57.
  • Jackson, J. and Blenkinsop, T., 1997. The Bilila‐Mtakataka fault in Malaŵi: An active, 100‐km long, normal fault segment in thick seismogenic crust. Tectonics 16, 137-150.
  • Lavayssière, A., Drooff, C., Ebinger, C., Gallacher, R., Illsley‐Kemp, F., Oliva, S.J. and Keir, D., 2019. Depth extent and kinematics of faulting in the southern Tanganyika Rift, Africa. Tectonics 38, 842– 862.
  • McClay, K. and Khalil, S..1998. Extensional hard linkages, eastern Gulf of Suez, Egypt. Geology 26, 563–566
  • Saria, E., Calais, E., Stamps, D.S., Delvaux, D. and Hartnady, C.J.H., 2014. Present‐day kinematics of the East African Rift. Journal of Geophysical Research 119, 3584-3600.

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures
In this Minds over Methods we don’t have one, but two scientists talking about their research! Michael Kettermann and Christopher Weismüller, both from Aachen University, explain us about the multidisciplinary approach they use to understand more about massively dilatant faults. How do they form and what do they look like at depth?

Massively dilatant faults in Iceland – from surface to subsurface structures

Michael Kettermann & Christopher Weismüller, RWTH Aachen University

Michael (left) and Christopher (right) in the field. Credit: Michael Kettermann and Marianne Sophie Hollinetz.

Iceland is a volcanic island in a unique setting on the Mid-Atlantic Ridge, separating the Eurasian and North American plates. A deep mantle plume lies beneath Iceland, and the combination of rift and plume leads to very active basaltic volcanism. Ubiquitous features along the rift zone are normal faults, often exquisitely exposed at the surface. Normal faults in basalts are also common in many volcanic provinces, like Hawaii, the East African Rift, and along mid ocean ridges. These faults often form as massively dilatant faults (MDF), which show apertures up to tens of meters at the surface and supposedly have large volumes of open voids in the subsurface.

Figure 1. View along-strike a massively dilatant fault in layered basalt. The geometry of the vertical fault faces is prescribed by the cooling joints. Several basalt columns have been loosened and dropped into the fault, now being stuck in the fault (top) or filling the cavity (bottom). Opening width < 3 m. Credit: Michael Kettermann.

These openings form pathways for fluids like magma or hydrothermal waters and consequently are of importance for volcanic plumbing systems, mineralization and geothermal energy supply.

Iceland provides a perfect natural laboratory to study MDF. Due to its position on the Mid-Atlantic Ridge, Iceland is cut by extensional fault systems roughly from southwest to north. A wide range of oblique extensional to pure extensional faults can be observed mostly in flood basalts, but also in sub-glacially formed hyaloclastites (weaker volcanic sediments), pillow lavas and occasionally sediment layers formed in warmer times. Outcropping rocks in Iceland are younger than 20 Ma distal from the rift (eastern and western Iceland), while tectonic and corresponding volcanic activity at the ridge (central Iceland) constantly causes the formation of new rocks. The rough climate hinders soil formation and vegetation to overgrow faults, providing unique outcrop conditions.

While it is relatively easy to access and study the faults at surface level, investigations into the subsurface are much more challenging. Direct observations are only possible down to depths of some tens of meters by climbing into the fractures. Cavities are often filled with rubble, sediments, water or snow (Fig. 1). Steep, open fractures with meter-scale aperture are hard to detect with geophysical methods (seismic reflection/refraction, ground penetrating radar, electrical resistivity tomography) at depths greater than some meters.

We therefore started the massively dilatant fault project, a multidisciplinary, integrated project bringing together remote sensing, fieldwork, analogue modelling and numerical simulations. In essence, we utilize a modelling approach to recreate the structure and evolution of MDF at depth, using real 3D surface data as input and comparison data set.

 

Drone mapping and photogrammetry

In a first step, we capture and analyse the surface expressions of MDF at a number of representative fault areas in Iceland. To this end, we flew 27 drone surveys during our five weeks long field season in summer 2017, covering a total length of more than 42 km of faults. Luckily, in the Icelandic summer the days are very long, so the National Park Service allowed us to fly the drones early in the morning and late in the evening outside of tourist hours. For each area, we took several hundred to thousands of overlapping photographs (e.g. Fig. 2). We processed these sets with photogrammetry software, applying the Structure from Motion (SfM) technique. SfM is an increasingly popular, fast and cheap technique to reconstruct high-resolution 3D information from 2D images. This allows us to recreate digital elevation models and ortho-rectified photo-mosaics of the faults in resolutions better than 15 cm per pixel (Fig. 3). The largest area at the famous Thingvellir fissure swarm covers a length of almost 7 km with an average resolution of 11 cm per pixel.

We use these digital elevation models and ortho-photos to retrieve a wide range of structural data. Mapping the fault traces in a GIS software allows for the measurement of fault opening width, throw, orientation and length. From throw and aperture, we can then estimate the fault dip at depth. Digital elevation models further provide surface dip data that we then compare with observations from analogue models.

Figure 2. A drone photograph facing South of the Almannagjá fault in Thingvellir, where the Thingvallavegur road crosses the fault. The Almannagjá fault resembles the western shoulder of the Thingvellir graben system with locally > 50 m opening width and 40 m vertical offset. Credit: Christopher Weismüller. .

Figure 3. Digital elevation model created from drone photographs using photogrammetry software. It contains the faults at Sandvik on the Reykjanes Peninsula (SW Iceland). The detail panes (red square) show the DEM (right) at a higher zoom level and the corresponding ortho-rectified photograph (left). The bridge crossing the fault depicted in the detail panes is a famous touristic spot, known as „The bridge between the continents“, since the fault symbolically divides the North American and Eurasion plates. Credit: Michael Kettermann.

 

 

 

 

 

 

 

Figure 4. Sideview of an analogue model showing three timesteps of the development of a massivley dilatant fault and associated fractures in hemihydrate (Bücken, 2017). Note the tilted block developing at the surface of the model and the dilatant jogs and voids in the subsurface. The opening at the surface is not directly linked to the fault at depth, but caused by the rotation of the tilted block. Credit: Daniel Bücken..

Modelling approach

For the analogue modelling approach on the hundreds to thousand meter scale, we use cohesive powders as modelling material (Bücken, 2017). Especially hemihydrate powder has been proven suited to model dilatant fractures (Holland et al., 2006; Kettermann et al., 2016; van Gent et al., 2010) as it has a well characterized true cohesion and tensile strength. Faults in Iceland transform from opening mode fractures to shear mode faults at depth when overburden stress is high enough. As we are interested in the upper dilatant parts of the faults, i.e. above the shear mode faulting, we chose a basement-fault controlled approach, where a rigid basement represents the shear mode fault. It moves down-dip along a predefined surface, deforming the powder sieved on top. The basement fault dip follows the data we derived from the field and is set to 60° – 65°. The scale of the models calculates from strength and weight of the natural prototype and the modelling material. 1 cm of powder equals about 50 m of basalt.

 

Comparison of models and nature

Results show a close similarity between field and experiment at the surface structures. Open fractures form with large apertures at the surface and often we observe the formation of tilted blocks (Fig. 4). The existence and scaled dimensional similarity of fractures and tilted blocks in the field and using a scaled material suggest a validity of other observations in the models. Glass sidewalls in the analogue models provide the opportunity to examine how the faults evolve at depth. We observe that large caves form underneath these blocks and we predict that these must exist in the field as well, albeit potentially filled with rubble. Our models corroborate earlier predictions that extensional faults are open down to 800 – 1000 m (Gudmundsson and Bäckström, 1991). We also learned that below that, a hybrid failure zone exists where dilational jogs, open extensional fractures between shear mode faults, provide lateral pathways for magma or water, even at depths where the overburden stress prevents the formation of purely extensional faults.

 

Outlook

The previously shown experiments investigated MDF at a larger scale in purely dip-slip kinematics. However, faults at rifts often have strike-slip components, forming normal faults with oblique kinematics. In further experiments, we therefore explored the effect of varying basement fault obliquities, i.e. the range between dip-slip normal faults and strike-slip faults (Bitsch, 2017). As expected, early phases of faulting are dominated by Riedel shears. Surprisingly, the surface structure of mature faults, however, does not change distinctly up to obliquities of 60°, but the subsurface connectivity decreases with increasing obliquity.

Figure 5. Analogue model resembling successive layers of lava flows with cooling joints created by carefully stacking several layers of dried corn starch slurry (Winhausen, 2018). The dip of the basement fault is prescribed by the apparatus. The fault geometry generated in the model is very similar to the ones observed in Iceland. Large cavities develop and are partially refilled by loosened columns, as shown in figure 1. Tensile fractures develop on the surface of the footwall, similar to the hemihydrate model and the field. Credit: Lisa Winhausen.

Zooming in on the faults, an inherent mechanical anisotropy (orthotropy) of basalts gains more influence on the macroscale structure of faults. Due to the shrinking during cooling of flood basalts, polygonal to blocky columns form and present regular weak zones in the rockmass. Introducing mechanical anisotropy into a stronger modelling material (dried corn-starch slurry) beautifully illustrates how the small-scale structure of the faults is affected by the layering of flood basalts and cooling fractures therein (Fig. 5; Winhausen, 2018). Close to the surface the strong basalt does not fracture, but propagating faults rather localize at the pre-existing cooling joints. This causes a jagged structure of the fault, formation of caves, and eroded basalt columns filling the opening fractures.

We are currently working on implementing all these learning points into discrete element simulations, where we can adjust material properties in a way that allows for modelling deeper parts of the faults with better mechanical control.

 

 

References

Bitsch, N.D., 2017. Massively dilatant faults in oblique rift settings – an analogue modeling study (MSc Thesis). RWTH Aachen University, Germany, Aachen.

Bücken, D.H., 2017. Effect of mechanical stratigraphy on normal fault evolution – Insights from analogue models and natural examples in Iceland (MSc Thesis). RWTH Aachen University.

Gudmundsson, A., Bäckström, K., 1991. Structure and development of the Sveinagja graben, Northeast Iceland. Tectonophysics 200, 111–125. https://doi.org/10.1016/0040-1951(91)90009-H

Holland, M., Urai, J.L., Martel, S., 2006. The internal structure of fault zones in basaltic sequences. Earth Planet. Sci. Lett. 248, 301–315. https://doi.org/10.1016/j.epsl.2006.05.035

Kettermann, M., von Hagke, C., van Gent, H.W., Grützner, C., Urai, J.L., 2016. Dilatant normal faulting in jointed cohesive rocks: a physical model study. Solid Earth 7, 843–856. https://doi.org/10.5194/se-7-843-2016

van Gent, H.W., Holland, M., Urai, J.L., Loosveld, R., 2010. Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder. J. Struct. Geol. 32, 1375–1391. https://doi.org/10.1016/j.jsg.2009.05.006

Winhausen, L., 2018. Influence of columnar joints on normal fault geometry and evolution An analog modeling study Master Thesis (MSc thesis). RWTH Aachen University.

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

For this first Minds over Methods of 2019, we invited Christopher Spencer, Senior Research Fellow at Curtin University in Australia, to tell us something about tectonochemistry. By applying geochemistry to tectonic processes, it is possible to get more insight into the different stages of the rock cycle. By combining fieldwork and geochemical analyses of the Oman/UAE ophiolite, Chris and his co-workers believe they found the first direct and in-situ evidence of sediment melting in the mantle.

 

Credit: Christopher Spencer

Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Christopher Spencer, Senior Research Fellow, Curtin University, Australia

The rock cycle is the first thing we learn in Geology 101. Magma and lava cool to form igneous rocks. Igneous rocks then erode to form sediment, which forms sedimentary rocks as it is compacted. Increasing pressure and heat then create metamorphic rocks, which eventually will melt. In each of the transitions described in the rock cycle, tectonics is usually involved. Granite batholiths form in subduction zones and are uplifted and eroded in collision zones. The sediments derived therefrom are deposited along continental margins that are often then returned to subduction zones where they contribute to new magmatic systems. There is a wide array of tools that we can use to evaluate the role of tectonics in the rock cycle, of which geochemistry is able to provide insight into each stage of the process.

Applying geochemistry to tectonics is (unsurprisingly) referred to as tectonochemistry. Similar to tectonophysics, where geophysics is applied to address large-scale tectonic questions, tectonochemistry provides a unique view into geochemical proxies of tectonic processes. The melting of sediment along convergent margins is a classic tectonochemical problem, as the unique chemical signature of sediment found in a granite provides unequivocal evidence for the melting of a sedimentary rock. In collisional systems, like the Himalaya, tectonochemistry has been used to constrain the melting of meta-sedimentary rocks as crustal thickening and decompression drives dehydration of micas which leads to melting. Collisional systems provide clear and in situ evidence for sediment melting.

Figure 1: Clockwise from top left: tourmaline-bearing leucogranite from the Himalaya in NW India, leucogranite dykes intruding meta-sedimentary rocks exposed at 5000m altitude, in situ melting of meta-pelite and formation of leucogranite, incongruent melting of muscovite + plagioclase + quartz to form leucogranite but leaving the biotite behind. Credit: Christopher Spencer.

 

Sediments are also thought to melt in subduction systems, but given the difficulty of accessing the asthenosphere directly, it is more challenging to constrain the processes occurring deep in a subduction zone. The incorporation of sediment in subduction zones is often constrained using the geochemistry of the resulting magmatic rocks. The chemical signature of sediment provides a clear indication of its incorporation in the magma, but it is often unclear whether the contamination is occurring in the asthenospheric wedge or in the upper crust. For example, many granite batholiths contain zircon grains that are foreign to the host magma and whose age spectra match the detrital zircon age spectra of the adjacent sedimentary units. This relationship is a clear indication that sedimentary contamination occurred in the upper crust. Unfortunately, the geochemical proxies used to establish the sedimentary contamination only provide indirect evidence for the subduction of sedimentary material into the asthenospheric wedge. Such indirect evidence includes seismic stratigraphy showing sedimentary units being subducted beneath the forearc and whiffs of sedimentary geochemical signals in arc volcanics. Although these evidences point towards sediment being subducted deep into the asthenospheric wedge where it melts and contaminates the magmas coming off the subducting slab, they do not preserve direct evidence of sediment melting in the mantle.

To acquire direct evidence of processes happening deep in the mantle, I set my sights on the Oman/UAE ophiolite, where a thick succession of mantle peridotite is preserved beneath a complete stratigraphic section of oceanic crust. Previous work has shown that this ophiolite not only preserves an intact record of oceanic crustal stratigraphy, but also geochemical features of a subduction zone in the oceanic crust. This implies the ophiolite formed in a supra-subduction setting, where during the earliest phase of subduction, extension in the upper plate caused rifting and formation of oceanic crust above a subduction zone.

 

Figure 2: Oceanic crustal stratigraphy of the Oman/UAE ophiolite comprised of (clockwise from top left): pillow basalts, sheeted dykes, layered gabbros, and mantle peridotite. Credit: Christopher Spencer.

 

During fieldwork in the ophiolite, while traversing the 8-15 km thickness of the mantle peridotite, I encountered a number of granitoid dykes that cross cut the peridotite, but do not cross the petrologic Moho. Many of these dykes contained tourmaline, muscovite, biotite, and even andalusite, minerals that would be expected from the melting of sedimentary material. Finding these minerals in the mantle indicates these grantoid dykes formed from the melting of sedimentary material and here they were within the mantle! Subsequent analysis of zircon grains from these granitoid dykes revealed the age of these dykes was equivalent to the age of the overlying ophiolite providing bullet-proof evidence that they intruded while the ophiolite was forming above a subduction zone. To provide the nail in the coffin for a sedimentary origin, I performed oxygen isotope analysis of the zircon and quartz. Sedimentary material has a distinct oxygen isotopic composition and igneous rocks that are thought to have experienced sediment contamination have δ18O values that lie along mixing lines between a sediment end member and the mantle. The oxygen isotopic analyses of the sub-Moho granitoids of the Oman/UAE ophiolite revealed the highest δ18O values ever measured in igneous rocks, providing unequivocal evidence that these granitoids represent pure sediment melts. In a paper published in Geology (Spencer et al., 2017), my coauthors and I argue these igneous rocks represent the first direct and in situ evidence of sediment melting in the mantle. Lucky for us, we have just scratched the surface of the exciting things left to learn about these fascinating granitoids and I look forward to the opportunity to return to the Oman/UAE ophiolite.

Figure 3: Sub-Moho granitoids of the Oman/UAE ophiolite: A) Cathodoluminescence image of a zircon shown with location and result of δ18O analyses. B) Photograph of sub-Moho granitoids. C) Hand sample of granite with tourmaline and lepidolite (lithium-bearing mica). Credit: Christopher Spencer.

 

Minds over Methods: Linking microfossils to tectonics

Minds over Methods: Linking microfossils to tectonics

This edition of Minds over Methods article is written by Sarah Kachovich and discusses how tiny fossils can be used to address large scale tectonic questions. During her PhD at the University of Brisbane, Australia, she used radiolarian biostratigraphy to provide temporal constraints on the tectonic evolution of the Himalayan region – onshore and offshore on board IODP Expedition 362. Sarah explains why microfossils are so useful and how their assemblages can be used to understand the history of the Himalayas. And how are new technologies improving our understanding of microfossils, thus advancing them as a dating method?

 

                                                                          Linking microfossils to tectonics

Credit: Sarah Kachovich

Sarah Kachovich, Postdoctoral Researcher at the School of Earth and Environmental Sciences, The University of Queensland, Australia.

Radiolarians are single-celled marine organisms that have the ability to fix intricate, siliceous skeletons. This group of organism have captured the attention of artist and geologist alike due to their skeletal diversity and complexity that can be observed in rocks from the Cambrian to the present. As a virtue of their silica skeletons, small size and abundance, radiolarian skeletons can potentially exist in most fine-grained marine deposits as long as their preservation is good. This includes mudstones, hard shales, limestones and cherts. To recover radiolarians from a rock, acid digestion is commonly required. For cherts, 12-24 hours in 5 % hydrofluoric acid is needed to liberate radiolarians. Specimens are collected on a 63 µm sieve and prepared for transmitted light or scanning electron microscope analysis.

Animation of radiolarian diversity. Credit: Sarah Kachovich

Scale and diversity of modern radiolarians. Credit: Sarah Kachovich (radiolarians from IODP Expedition 362) and Adrianna Rajkumar (hair).

 

 

 

 

 

 

 

 

 

 

Improving the biostratigraphical potential of radiolarians

The radiolarian form has changed drastically through time and by figuratively “standing on the shoulders of giants”, we correlate forms from well-studied sections to determine an age of an unknown sample. A large effort of my PhD was aimed to progress, previously stagnant, research in radiolarian evolution and systematics in an effort to improve the biostratigraphical potential of spherical radiolarians, especially from the Early Palaeozoic. The end goal of this work is to improve the biostratigraphy method and its utility, thus increasing our understanding of the mountain building processes.

The main problem with older deposits is the typical states of preservation, where radiolarians partly or totally lose their transparency, which makes traditional illustration with simple transmitted light optics difficult. Micro-computed tomography (µ-CT) has been adopted in fields as diverse as the mineralogical, biological, biophysical and anatomical sciences. Although the implementation in palaeontology has been steady, µ-CT has not displaced more traditional imaging methods, despite its often superior performance.

Animation of an Ordovician radiolarian skeleton in 3D imaged through µ-CT. Credit: Sarah Kachovich

To study small complex radiolarian skeletons, you need to mount a single specimen and scan it at the highest resolution of the µ-CT. The µ-CT method is much like a CAT scan in a hospital, where X-rays are imaged at different orientations, then digitally stitched together to reconstruct a 3D model. The vital function of the internal structures provides new insights to early radiolarian morphologies and is a step towards creating a more robust biostratigraphy for radiolarians in the Early Paleozoic.

Linking radiolarian fossils to tectonics

Radiolarian chert is important to Himalayan geologists as it provides a robust tool to better document and interpret the age and consumption of oceanic lithosphere that once intervened India and Asia before their collision.The chert that directly overlies pillow basalt in the ophiolite sequence (remnant oceanic lithosphere) represents the minimum age constraint of its formation. In the Himalayas, over 2000 km of ocean has been consumed as India rifted from Western Australia and migrated north to collide with Asia. Only small slivers of ophiolite and overlying radiolarian cherts are preserved in the suture zone and it is our job to determine how these few ophiolite puzzle pieces fit together.

Another way I have been able to link microfossils to Himalayan tectonics is by studying the history and source of erosion from the Himalayas on board IODP Expedition 362. Sedimentation rates obtained from deep sea drilling can provide ages of various tectonic events related to the India-Asia collision. For example, we were able to date various events such as the collision of the Ninety East Ridge with the Sumatra subduction zone, which chocked off the sediment supply to the Nicobar basin around 2 Ma as the ridge collided with the subduction zone.

Left: Results from the McNeill et al. (2017) of the sedimentation history of Bengal Fan (green dots) and Nicobar Fan (red dots). Middle/right: Reconstruction of India and Asia for the past 9 million years showing the sediment source from the Himalayas to both basins on either side of the Ninety East Ridge.

 

 

 

 

 

 

 

 

 

 

 

 

Lastly, on board Expedition 362 we were able to use microfossils to understand how and why big earthquakes happen. We targeted the incoming sediments to the Sumatra subduction zone that were partly responsible for the globally 3rd largest recorded earthquake (Mw≈9.2). This earthquake occurred in 2004 and produced a tsunami that killed more than 250,000 people.

From the seismic profiles (see example below), we found that the seismic horizon where the pre-decollement formed coincided with a thick layer of biogenetically rich sediment (e.g. radiolarians, sponge spicules, etc.) found whilst drilling. Under the weight of the overlying Nicobar Fan sediments, this critical layer of biogenic silica is undergoing diagenesis and fresh water is being chemically released into the sediments. The fresh water within these sediments is moving into the subduction zone where it has implications to the physical properties of the sediment and the morphology of the forearc region.

The Sumatra subduction zone. The dark orange zone represents the rupture area of the 2004 earthquake. Also shown are the drill sites of IODP Expedition 362 and the location of seismic lines across the plate boundary.

Seismic profile: The fault that develops between the two tectonic plates (the plate boundary fault) forms at the red dotted line. Note the location of the drill site.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Hüpers, A., Torres, M. E., Owari, S., McNeill, L. C., Dugan, & Expedition 362 Scientists, 2017. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip. Science, 356: 841–844. doi:10.1126/science.aal3429

McNeill, L. C., Dugan, B., Backman, J., Pickering, K. T., & Expedition 362 Scientists 2017. Understanding Himalayan Erosion and the Significance of the Nicobar Fan. Earth and Planetary Science Letters, 475: 134–142. doi:10.1016/j.epsl.2017.07.019

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