How Do Normal Faults Grow?

Normal faults accommodate extensional stress in the Earth’s crust. The Googlesphere indicates the precise definition of a normal fault varies massively; however, it is best described as a fracture across which two sides of rock move, with the hangingwall moving downward relative to the footwall (see images below). Or at least this is what I vaguely remember from 1st-year structural geology lectures. Blame @rocksquasher if I’m wrong. In these and associated 1st-year lectures (e.g. sedimentology and stratigraphy, basin analysis, economic geology) I also remember being taught about the importance of normal faults; for example, they can cause devastating earthquakes, they can control the geomorphology of the Earth, and they can location of economic quantities of minerals and hydrocarbons. One thing I distinctly remember NOT being taught about is the detailed geometry (i.e. branchlines, relay zones, related folding) or growth (i.e. kinematics) of normal faults. I essence, I had no real appreciation of how large they could be, or how they developed in space and time. I then did a PhD and my world changed.

Normal faults. Image on the left from: Image on the right: Geologists: you might need to point out the fault to your geophysical friends in the image on the right.

My PhD, which I undertook at the University of Manchester under Rob Gawthorpe (now at UiB), focused on the tectono-stratigraphic development of rift basins, using field and seismic reflection data from the Suez Rift, Egypt. ‘Tectono-stratigraphic analysis’ is a fancy term for studying how structures control the stratigraphic development of tectonically active basins, such as rifts. I mapped, I logged and I correlated, trying to understand how normal faults grew and how this impact the sedimentology and sequence stratigraphy of non-marine and shallow-marine syn-rift deposits. In short, I loved it, despite the regular bouts of food poisoning and the sometimes blistering temperatures. My PhD research seemingly showed that normal faults got longer as they accumulated displacement, a style of fault growth that was, at that time, the flavour-of-the-month, and which is now referred to as ‘isolated fault model’ (see below left; see also discussions by Walsh et al., 2002; Jackson & Rotevatn, 2013; Tvedt et al., 2016). This model has essentially dominated the structural geology literature for c. 30 years, being supported by and, in my view, largely underpinned by global displacement vs. length scaling laws. I was at least partly responsible for supporting this as the ‘normal fault growth model of choice’ via a couple of PhD-related publications

The two main ‘competing’ models for normal fault growth. Left: the isolated model. Right: the constant-length model. Note the two strongly contrasting growth patterns exhibited by faults that, at the end of deformation at time 4 (T4), have the same length and displacement (modified from Nicol et al. 2016).

Since the early-2000’s, there’s been a ‘new kid on the block‘; the ‘constant length model’. This model suggests that normal faults grow by a rapid establishment of their near-final length prior to significant displacement accumulation (see above right). This model was developed by the Fault Analysis Group (FAG) (previous at the University of Liverpool, now at University College Dublin), and was based on and has since been most strongly supported by, the analysis of 3D seismic datasets. The isolated and constant-length models make very different predictions regarding the geomorphic and tectonostratigraphic evolution of rifts, and the size and location of potentially hazardous earthquakes. So, to put it mildly, deciding which model is ‘correct’ is very important for a range of Earth Science disciplines. I have in recent years become increasingly obsessed by testing these models, at least partly because I think some of my PhD research, which is published and has been modestly cited, is wrong. I also think a lot of other people are applying the incorrect model to their data, or are simply unaware of the newer, less well-publicised, constant-length model.

Time-structure map of a Late Jurassic-Early Cretaceous normal fault network, Horda Platform, Norwegian North Sea (see Duffy et al., 2015).

Motivated by the above, one of my GSA Distinguished Lecturer Tour talks is entitled: ‘How Do Normal Faults Grow?“. In this talk I outline several techniques that can constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. I then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model. Analysis of growth strata represents the best way to test competing fault growth models; most studies utilising this approach support the constant-length fault model, suggesting it may be more applicable than currently assumed. Approximately 15 years on, I am minded to write a ‘Discussion’ paper in response to my own PhD papers…

Featured image at top of post is taken from:

Author: Christopher Aiden-Lee Jackson

I am Professor of Basin Analysis @imperialcollege. I ❤️ 🏃🏿, 🚴🏿 and @basinsIC (⛏). I obsess about the tectono-stratigraphic development of sedimentary basins. Why? Because I'm hopeless at everything else.

6 thoughts on “How Do Normal Faults Grow?”

  1. Kudos for quoting your own earlier research as wrong! That’s what a decent scientist should do, try – fail – learn – adapt. Very nicely written, now I’ll finally remember more on normal faults 😉 Looking forward to a post on strike slipp faults…

  2. I really enjoyed your post. I especially like the humility you showed towards adopting a new concept that contradicts your past publications. Sadly, this isn’t a trait seen in all scientists. I am hoping to start a PhD in the fall of 2017 analyzing extensional systems with an emphasis in tectonstratigraphy and sedimentary basin evolution. Looking forward to reading more posts.

    1. Thanks Aaron. We all need to feel comfortable, almost willing, to be proven wrong. It’s simply part of the scientific cycle. Your PhD will hopefully show that to you. Being wrong is sometimes painful, but it should inspire us to work harder, listen more, reflect, etc. Thanks for the comment!

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