Speaker: Prof. Dr. Ingo Grevemeyer, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
Title: Oceanic transform faults revisited – evidence for extensional tectonics and phases of magmatic accretion at so called conservative plate boundaries
Venue: Seminar rooms or join online via Zoom link
Abstract: Transform faults represent one of three classes of plate boundaries and show strike-slip tectonism where one plate moves past another and hence are called conservative plate margins where crust is neither formed nor destructed. The other two types of plate boundaries are divergent and hence extensional plate boundaries where new crust is formed (mid-ocean ridges, MOR) and convergent or destructive plate margins (subduction zones) where crust is recycled back into Earth’s interior. Researcher focused their attention on MOR and subduction zones, but transform faults garnered rather little attention over the last decades, especially in ocean basins.
Oceanic transform faults (OTF) are gigantic features – up to 900 km long – and without oceans masking the seafloor, they would be among the most prominent features on Earth, offsetting mid-ocean ridges, forming tens of kilometres wide and up to 7 km deep valleys on the ocean floor. Yet, they are defined as simple strike-slip faults, but how can a transcurrent plate boundary, generating magnitude 7+ strike-slip earthquakes, promote extension forming the deep and wide valleys? Furthermore, for over half a century, researchers failed to appreciate that OTF are always deeper than adjacent oceanic features of an older age, challenging a major concept of plate tectonics. Thus, instead of showing the predicted age-dependent subsidence, the seafloor shallows at ridge-transform intersections (RTI). It therefore might be reasonable to question if they are indeed conservative plate boundaries.
I will provide observational evidence suggesting that OTFs are highly dynamic features, showing two phases of accretion at both RTIs. In addition, they may suffer from extensional tectonics below their strike-slip surface fault zone. In my presentation I’m going to show that we may need to revise our concept of transform faults and will show constraints from a global compilation of bathymetric data, show predictions from numerical simulations and will dive into a pilot study of micro-seismicity at the the Oceanographer transform at the Mid-Atlantic Ridge near 35°N. The micro-earthquake activity captured by a network of ocean-bottom-seismometers reveals a diffuse activity over a broad area, cutting across the inside corner domain between the spreading centre and the transform fault before focusing along the trace of the fault. In the vicinity of the ridge-transform intersection, focal mechanisms reveal transform-normal extensional tectonics instead of showing transcurrent motion, while strike-slip tectonics occurs only >15 km away from adjacent spreading segments. These observations support a scenario based on numerical simulations showing that at RTIs the right-angular plate boundary at the seafloor develops into an oblique shear zone at depth, causing crustal thinning and consequently forming transform valleys. Away from RTIs, seismicity is focused at a narrow and segmented strike-slip fault system as predicted by plate tectonics. However, tectonic processes shaping transforms are divers, arguing for a revision of the concept of conservative plate boundaries to account for their morphology and strong lateral differences in seismic behaviour.
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