TS
Tectonics and Structural Geology

Tectonics and Structural Geology

Features from the field: Folding

 

The folds of the rocks. The folds of the rocks at position Apoplystra, between the village of St. Paul and the sandhills (Greece) are remarkably colorful.Photo credit: © by G. Shuttleworth via geologyin.

 

Folding is one of the most common geologic phenomena in the world. I should start with defining the term ‘deformation’ in order to understand the folding process better.

In geology, deformation is an alteration of the size or shape of rocks. Deformation is caused by stress, the scientific term for force applied to a certain area. Stresses on rocks can stem from various sources, such as changes in temperature or moisture, shifts in the Earth’s plates, sediment buildup or even gravity.

Z folds in the Alba Syncline. Did they really make it? They are geologist so they can 🙂 Photo credit: by Erin Kennedy distrubted via  geology.blogs.brynmawr.

There are three types of rock deformation. Elastic deformation is temporary and is reversed when the source of stress is removed. Ductile deformation is irreversible, resulting in a permanent change to the shape or size of the rock that persists even when the stress stops. A fracture is considered as brittle deformation, whereas folding is considered as ductile deformation. The third one type is viscous deformation is the behavior of the fluids such as magma.

Certain factors determine which type of deformation rocks will exhibit when exposed to stress. These factors are rock type, strain rate, pressure and temperature. For instance, higher temperatures and pressures encourage ductile deformation. This is common deep within the Earth, where, due to higher temperatures and pressure than nearer the surface, rocks tend to be more ductile.

But, nowadays we find rocks from deep regions exposed at the surface. How? The answer is ‘uplift’, the balance between the rate of magma intrusion into the crust, erosion, and the relative densities of the continental crust and the mantle.

Anticline Trap. Anticline is a structural trap for petroleum.  Image reproduced from original source.

Folding is a manner for sedimentary and metamorphic rocks. Different layers in those rocks help geologist to understand structures.

Last but not least, Anticlines (type of folding) are important types of “structural traps” in petroleum geology.

To sum it up, Folds are significant structures for either in structural or economic geology. They are, moreover, remarkable phenomenon for people due to their great looking like many other geologic structure.

Teaching in the 21st century – a PICO session

Teaching in the 21st century – a PICO session

With the progress in the digital world there are more and more e-tools available for research and teaching. What are smart ways to make use of new techniques in teaching? For inspiration and learning, Hans de Bresser, Janos Urai and Neil Mancktelow convened a PICO session at the EGU 2017 General Assembly to showcase present-day e-learning opportunities to improve the efficiency and quality of teaching structural geology and tectonics. Despite an 8h30 morning slot and a limited number of abstracts, all spots during the 2 min madness were taken and all authors were talking the full 90 minutes – and some even well into the coffee break – about what they are using and how. Inspirational and fun!

Hans, you’re somewhat of an expert on teaching in Utrecht as former head of teaching in geosciences, how would you say that teaching has changed from back when you were a student?

”In my time as a student, I spent hours learning to identify rocks and recognizing minerals in thin sections, browsing back and forward in text books packed with determination tables and graphs of which more than half was not relevant for me. I also invested a lot of time in making maps in the field, carefully adding measurements and colors, hoping that I did it right the first time (never) and that I wouldn’t spill coffee over my precious products (it happened). And I sat in classes in which professors talked for hours, repeating the content of books that I had in front of me, while my level of activity in class was so low that preventing to fall asleep was a serious challenge.  I really learned a lot, definitely had a lot of fun, but looking back I feel it could have been done more efficiently. New styles of teaching, such as blended learning and flipping the class room, and state-of-the-art e-tools for data collection, modelling and visualization now help us to be very efficient and improve the quality of teaching. And it can be fun”.

 

Broadly speaking the presented aids can be divided in the following 3 categories, for each category we give examples below.

1. How to bring the real and experimental world into the classroom?

2. How to make life easier for a teacher?

3. Can we add extra information using the virtual world?

 

Category 1:  How to bring the real and experimental world into the classroom?

Benjamin Craven explaining his PICO: Fieldwork Skills in Virtual Worlds

Virtual landscape, presented by Benjamin Craven: a 3D model of a field area, to bring the real world in the classroom, and increasing the efficiency of real world field teaching. Different packages of open source software make it easy to design your own landscapes, though you can also make use of the already made world. One could also teach students photogrammetry, to enable them to make their own 3D models using photos made with a smart phone, of rocks or other items in the classroom. One idea is to ask students to create a geological map in the virtual “field”, but then to give each student only a limited amount of time to do so. This allows them to learn how to plan their time in mapping projects.

Drones and 3D models , presented by Thomas Blenkinsop. Using drones one can make 3D surface models, which can be combined with cross sections to create a 3D MOVE (TM Midland Valley) project. It starts with the regular manual work, but adding the digital models improves 3D thinking as well as it allows students to check their own cross sections.

Deforming ice with students, presented by Dave Prior. A low-cost ice deformation rig, designed to be used by student teams. It brings Dave’s own research into the class room. Through a questionnaire teams were designated by Dave to get the right mix of skills to eventually present a poster. Some results are of high enough quality to publish.

 

Thomans Blenkinsop explaning about using drones, Lidar measurements and 3D models for undergraduate teaching

Category 2: How to make life easier for a teacher?

Jupyter notebooks, presented by Florian Wellmann: software for those who can’t program. It makes it easier to create exercises, and to allow students to play around with parameters. Automatic and manual exercise grading are both easy.

STEREOVIDEO, presented by Jose A. Alvarez-Gomez. This is a (currently Spanish only) channel with various Youtube video instructions on how to use stereonets. The channel will bring more subjects later. It is very popular in Latin America.

 

Category 3:  Can we add extra information using the virtual world?

Zappar, presented by Friedrich Hawemann. Using an icon on a poster and a free download app one can add extra layers of information to images. It also recognizes objects such as a polished rock, allowing the teacher to add arrows, circles etc. to highlight features

 

By Anne Pluymakers (just a visitor) and Hans de Bresser (session convener)

Folding and Fracturing of Rocks – 50 years of research since the seminal textbook of JG. Ramsay

Folding and Fracturing of Rocks – 50 years of research since the seminal textbook of JG. Ramsay

John G. Ramsay1 wrote his seminal textbook on the folding and fracturing of rocks in 1957, almost 20 years before I was born (and I don’t count myself as young!). So why did I co-convene a session at EGU in 2017 to celebrate the book? Because the book, in many ways, expresses the legacy that John has given to structural geology. He followed it with a series of books of the same ilk – Ramsay and Huber ‘The Techniques of Modern Structural Geology’, published in two volumes (1984 and 1987), was a main stay for me as an undergraduate and postgraduate and well actually now; although, it is often not to be found on my bookshelf, but on the desk of one of my graduate students. The Techniques of Modern Structural Geology, like the Folding and Fracturing of Rocks, is beautifully illustrated and describes geometrically and mathematically many of the now commonly used techniques in structural geology. So to me Ramsay, although I did not know him personally, was the name behind this interesting subject structural geology.

John Ramsay opening the session on his book “Folding and Fracturing of Rocks”

It was as a postgraduate student that I met John at a Tectonic Studies Group (TSG) meeting and had the pleasure of dancing with him after the conference dinner! Many years on I am now the Chair of TSG, one of several specialist groups affiliated to the Geological Society of London. John was instrumental in founding TSG in 1970 @TSG_since1970 , when he was working at Imperial College London. As the Chair of TSG I was approached by several people enquiring about plans to celebrate 50 years since the publication of the Folding and Fracturing of Rocks. Given John’s European career and global impact EGU seemed the place to celebrate and I contacted some of John’s PhD students and collaborators to join me in convening a session to celebrate the book, John and structural geology.

Early on the morning of April 25th John gave the opening paper for the session to a large audience. He spoke not to a series of power point slides but to the audience about the book, his interests and his life as a structural geologist. I think one of the key messages for me is the one that he ended his paper with; that fieldwork is fundamental to structural geology and models need to be derived from and tested against field observations. John was followed by a series of speakers from Rod Graham, of Ramsay and Graham (1970)2 on shear zones from the field to seismic interpretation, to Richard Lisle (co-author with John on the third volume in the Techniques of Modern Structural Geology series)3. Richard gave an overview of the citations for the Folding and Fracturing of Rocks; it is the most cited structural geology textbook. Further papers were given in the afternoon poster session, celebrating and giving modern, and old, twists on aspects of the book.

Afterwards at the reception

John joined many well-wishers at a reception and dinner after the session. John has given a legacy to structural geology, through his focus on the geometry and mathematics of structures backed up with detailed field observations, work that very much forms part of modern structural geology today.

The Folding and Fracturing of Rocks is currently published by Blackburn press.

 

Click here to go to the webpage of EGU Session TS1.3.

 

 

Blog by Clare Bond,

University of Aberdeen

Thanks to TecTask and TSG for sponsorship

 

1.John was born in 1931 in London, he was educated at Imperial College, London gaining a first class degree in 1952 and a PhD, working on superposed folds at Loch Monar Scotland. After military service he returned to the staff at Imperial, before taking up Professorial positions at the University of Leeds and the ETH in Zurich. He has been recognized for his work in advancing structural geology with many awards: Bigsby (1973) and Wollaston (1986) medals of the Geological Society of London, the Société Géologique de France Prestwich Medal in 1989, Sir Arthur Holmes Medal of the European Union of Geosciences (EGU) in 1984, C. T. Clough medal (1962) of the Geological Society of Scotland, the University of Liège medal in 1988. In 1992 he was named a Commander of the Most Excellent Order of the British Empire in the Queen’s Honours list. He is a Fellow of the Royal Society(elected 1973), and holds Honorary Fellowships of the Geological Society of America, the Société Géologique de France, the Indian National Science Academy, the American Geophysical Union, the US National Academy of Sciences and the Geological Society of London.

2.Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences7(3), pp.786-813.

3.Ramsay, J.G. and Lisle, R.J., 2000. The techniques of modern structural geology. Volume 3: Applications of continuum mechanics in structural geology. Academic, San Diego, Calif.

 

You’re an early career scientist and you want to go somewhere… but where?

You’re an early career scientist and you want to go somewhere… but where?

Only a few more days and the General Assembly of the EGU 2017 will start! Five exciting days with science and the opportunity to meet your colleagues and collaborators, both the old and the new. Earlier this week the outgoing TS President Susanne Buiter and the incoming TS President Claudio Rosenberg posted a blog with TS highlights, but what are the must-see for the Early Career Scientists? This is where we, outgoing TS ECS reps, come in! So we provide here tips for the hottest TS ECS events the coming week.

 

Meetings/socials:

Don’t forget to come for dinner with the other TS ECS at Brandauers Bierbogen (Heiligenstädter Str. 31) on Monday, at 20h.

We welcome everyone to the ECS corner at the icebreaker on Sunday. Pick up your badge, meet old and new friends at Foyer E, from 18h30 to 21h.

http://meetingorganizer.copernicus.org/EGU2017/session/25204

The TS Division Meeting is on Wednesday, 12h15-13h15. Lunch will be provided. What happened within the TS Division last year? What are the plans for next year?

http://meetingorganizer.copernicus.org/EGU2017/session/25276

Come give feedback! Your new TS ECS representative Anouk Beniest organizes an informal get-together over lunch on Thursday. We meet at 12h15 at the EGU flags just outside the building, we’ll get a sandwich and find a nice spot to sit and chat on how to improve things for TS ECS. All are welcome!

Meet your reps! The EGU Division President Claudio Rosenberg and the outgoing ECS Representatives Anne Pluymakers and João Duarte plus the incoming ECS Representative Anouk Beniest! They will all be at the EGU booth on Thursday from 14h15 to 15h.

 

Short courses:

For those of you who have not been to Vienna before, start the week with a crash course on how to navigate EGU as a first-timer, on Monday from 8h30 to 10h in Room -2.31. http://meetingorganizer.copernicus.org/EGU2017/session/25606#

If you like fully open access and transparent publishing, you should consider the course on how to publish in EGU journals: Solid Earth and Earth Surface Dynamics. Meet the Editors on Monday, 13h30 – 15h in room -2.91.

http://meetingorganizer.copernicus.org/EGU2017/session/25617

Of course we would love to see more TS-related research, also by the youngsters! So come to ‘How to write a successful ERC Grant proposal’ on Tuesday, 13h30 – 15h, in room -2.91.

http://meetingorganizer.copernicus.org/EGU2017/session/25127

Publish more pretty pictures to show of those beautiful structures with ‘Virtual Polarizing Microscopy in Petrology and Microtectonics’ on Wednesday 10h30 – 12h00, in room -2.16. http://meetingorganizer.copernicus.org/EGU2017/session/25156

 

Debates:

After open access publishing, we seem to transition now towards open science: all data available for everyone. Is this really the way to go? Come to the panel debate on Thursday, 15h30-17h in Room E1.

http://meetingorganizer.copernicus.org/EGU2017/session/25038

With the decreased amounts of funding and less and less permanent jobs the pressure of ‘publish or perish’ is mounting. What is the best way of judging early career scientists? Come to the group debates on Wednesday 19h-20h30 in room G1.

http://meetingorganizer.copernicus.org/EGU2017/session/25040

If you have a disability or chronic condition and want to find out more about the Chronically Academic Network and the type of support it provides, this is the meeting to attend. This meeting will also gather some information that will be provided to EGU to make the GA and EGU in general more accessible – so your input is much appreciated. Come come! Thursday 12h15 – 13h15 to room Room 2.61

http://meetingorganizer.copernicus.org/EGU2017/session/25644

 

Sessions:

Every TS session is an ECS session! So browse the program, and look for something that captures your interest, either directly inside your field or just outside. Due to its size EGU is the perfect conference to look across the boundary into other fields. Create your program online and download it into the EGU2017 app for Apple and Android.

http://meetingorganizer.copernicus.org/EGU2017/sessionprogramme

http://meetingorganizer.copernicus.org/egu2017/app

If you have no other plans, come to the Arne Richter Award lecture by outgoing ECS rep João Duarte “The Future of Earth’s Oceans: consequences of subduction invasion in the Atlantic” on Wednesday 9h30–10h in Room D3.

http://meetingorganizer.copernicus.org/EGU2017/session/25355

Equal opportunities for all! On Friday there are talks from women in geosciences from 10h30 to 12h15 in Room L4/5  and on equal opportunities in general from 13h30–17h in Room L4/5.

http://meetingorganizer.copernicus.org/EGU2017/session/23685

http://meetingorganizer.copernicus.org/EGU2017/session/23686

 

These and other sessions for ECS across all program groups can be found at http://meetingorganizer.copernicus.org/EGU2017/sessions-of-special-interest/ECS

 

written by: Anne Pluymakers and João Duarte, outgoing TS ECS reps

photo credits: TS Twitter and Facebook pages and EGU blog

Highlights at EGU 2017 from the division for Tectonics and Structural Geology

Susanne Buiter (outgoing TS president, Geological Survey of Norway) and Claudio Rosenberg (incoming TS president, UPMC France)

It is with great pleasure that we write this blog welcoming everyone to EGU’s upcoming General Assembly in Vienna, and especially to the many events organised by our division for Tectonics and Structural Geology! We are highlighting some of the week’s many events below, though it is in fact really difficult to choose highlights as of course the contribution of every individual to the success of the meeting is a highlight!

The TS programme can be found directly at these two links:

http://meetingorganizer.copernicus.org/EGU2017/meetingprogramme/TS

http://egu2017.eu/EGU2017_schedule_TS.pdf

We would encourage you to download the meeting app (http://app.egu2017.eu) and/or make a personal programme (http://meetingorganizer.copernicus.org/egu2017/personal_programme) in addition to taking the above pdf schedule along to Vienna.

In the list below we emphasized award lectures, PICO sessions, poster-only sessions, short courses that TS co-organises, and of course the opportunities where you can give us feedback, such as the division meeting and Meet EGU. But please don’t forget that we are running several scientific sessions in parallel throughout the day!

At the General Assembly and throughout the year, you can follow the division via our webpage (http://www.egu.eu/ts/home/), Facebook (http://www.facebook.com/TSdivision), twitter (https://twitter.com/EGU_TS) and the division mailing list (http://lists.egu.eu/cgi-bin/mailman/listinfo/ts).

Don’t forget to tweet about the General Assembly using #EGU17TS and #EGU17!

We wish you a great conference!

Susanne and Claudio

 

Monday 24 April

  • Start of the TS scientific programme at 08:30 with three sessions in parallel: TS1.4 “New geochronological approaches for quantification of geological processes”, 3 “Structures and patterns in fractured and porous media: witnesses for paleostress and fluid flow”, and TS7.4 “Probing the subduction plate interface”
  • Short course SC48 “Publishing in EGU journals: Solid Earth and Earth Surface Dynamics – Meet the Editors”, 13:30-15:00, room -2.91
  • PICO TS8.1 “Digital mapping and 3D visualization approaches in the Earth Sciences”, 15;30-17:00, PICO spot 5a
  • TS division Early Career Scientists event at Brandauers Bierbögen (Heiligenstädter Str 31), starting at 20:00

 

 

Tuesday 25 April

  • TS1.3 “Folding and Fracturing of Rocks – 50 years of research since the seminal text book of JG. Ramsay”, 08:30 – 10:00/D3 and 17:30-19:00/Hall X2
  • PICO TS8.3 “Analogue and numerical modelling of tectonic processes”, 10:30-12:00, PICO spot 5a
  • The Stephan Mueller medal lecture by Cees Passchier, “Panta Rhei – the changing face of rocks “, 16:00-17:00, room D3
  • Poster-only session TS1.1 “Open Session on Tectonics and Structural Geology”, 17:30 – 19:00, Hall X2

 

 

Wednesday 26 April

  • PICO TS1.2 “Teaching Structural Geology and Tectonics in the 21st century”, 08:30-10:00, PICO spot 1
  • The Arne Richter Award lecture by João Duarte “The Future of Earth’s Oceans: consequences of subduction invasion in the Atlantic”, 09:30-10:00, room D3
  • Short course SC9/TS10.1 “Virtual Polarizing Microscopy in Petrology and Microtectonics”, 10:30-12:00 room -2.16
  • Division meeting of Tectonics and Structural Geology: Your chance to provide us with feedback on the division and our programme. A light lunch will be provided. 12:15 – 13:15, room G1
  • PICO TS3.3 “Microstructure and texture analysis: New methods and interpretations”, 13:30-15:00, PICO spot 5b
  • Poster-only sessions:
    • TS8.4 “Learning from failed models and negative results”
    • TS9.2 “Oceanic and continental transform plate boundaries: nucleation, evolution and tectonic significance”

 

 

Thursday 27 April

  • PICO TS8.2 “Unravelling the Earth subsurface structure from seismic imaging and interpretation, geological observations, and numerical Experiments” 10:30-12:00, PICO spot A
  • Arthur Holmes medal lecture by Jean-Pierre Brun, “The extending lithosphere”, 12:15-13:15, room E1
  • Meet the EGU Division Presidents of Tectonics and Structural Geology (us!), 13:30-14:15, EGU Booth
  • Meet the incoming EGU Division President (Claudio) and the ECS Representatives of Tectonics and Structural Geology (Anne Pluymakers and João Duarte (both outgoing), and Anouk Beniest (incoming)), 14:15-15:00, EGU Booth

 

Friday 28 April

  • 17:30 – 19:00 Friday evening poster sessions! All are in Hall X2:
    • TS5.1 “Bridging Earthquakes and Tectonics: give-and-take”
    • TS5.4 “Advances in understanding earthquake processes and hazards in regions of slow lithospheric deformation”
    • TS6.1 “The evolution and architecture of rifts, rifted passive margins, and mid oceanic ridges: from mantle dynamics to surface processes”
    • TS7.5 “The Caledonian orogen of the North Atlantic region: understanding tectonic processes in collisional belts”
    • TS7.6 “Lithospheric and crustal dynamics of the Wilson Cycle: The Iberia case study”

 

(photos by Susanne Buiter)

Strike Slip Faults Classification

Strike Slip Faults Classification

A strike slip faults is a fault on which most of the movement is parallel to the fault strike (Bates and Jackson, 1987). The term ‘wrench fault’ is also popularized in some researchers. Sylvester (1988) suggest not using wrench fault term for defining strike slip fault as general term because wrench fault was defined by Anderson (1905) as deep seated, regional and vertical faults. Many major strike slip faults; however, are not vertical and do not cut the lithosphere on the continental crust.

Strike slip faults are clas sify by two major groups by Sylvester (1988) with regard to where they occur: Transform faults are general term that cut the whole lithosphere and Transcurrent faults are general term do not cut the lithosphere.

Sylvester (1988) classification of the strike slip faults is the most used and convenient way to determine the type of the strike slip faults.

Table 1. Classification Strike Slip Faults by Sylvester (1988).

Figure 1. Plate tectonic setting of major classes of strike slip faults by Sylvester (1988).

Figure 2. Plate tectonic setting of major classes of strike slip faults by Sylvester (1988).

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!

 

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)

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

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.

Features from the Field: Growth Faults

Features from the Field: Growth Faults

Growth faults are syndepositional or syn-sedimentary extensional faults. Growth faults develop when sediments are being deposited, are key elements in understanding deformation processes. Indeed, successively deposited sedimentary layers are involved in the different stages of the growth of the structure and produce a record of the deformation history. Their fault plane dips mostly toward the basin and has long-term continuous displacement.

As the fault grows upward, it cuts through the newly formed sedimentary layers at the top. Therefore, the overall displacement along the fault plane is not the same. Further, the lowermost layer has higher displacement than the uppermost layer while the intermediate layer displacement lies in between (See Figure). Because the fault plane flattens into décollement, the downthrown block moves basinward and the displaced sedimentary layer of the downthrown block bends close to the fault plane forming rollover anticline, synthetic and antithetic faults.

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.

 

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