Owen Weller

Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

The Trans-Hudson orogen of North America is a circa 1,800 million year old, middle Palaeoproterozoic continental collisional belt. The orogen may represent an ancient analogue to the Himalayan orogen, which began forming 50 million years ago and remains active today. Both mountain belts exhibit similar length scales of deformation and timescales of magmatism and metamorphism. A notable divergence in this correlation has been the absence of high-pressure, low-temperature metamorphic rocks in the Trans-Hudson compared with the Himalaya. It has been debated whether this absence reflects a secular tectonic change, with the requisite cool thermal gradients precluded by warmer ambient mantle temperatures during the Palaeoproterozoic, or a lack of preservation. Here we identify eclogite rocks within the Trans-Hudson orogen. These rocks, which typically form at high pressures and cool temperatures during subduction, fill the gap in the comparative geologic record between the Trans-Hudson and Himalayan orogens. Through the application of phase equilibria modelling and in situ U–Pb monazite dating we show that the pressure–temperature conditions and relative timing of eclogite-facies metamorphism are comparable in both orogenies. The results imply that modern-day plate tectonic processes featuring deep continental subduction occurred at least 1,830 million years ago. This study highlights that the global metamorphic rock record (particularly in older terrains) is skewed by overprinting and erosion. The timing of onset of modern-style plate tectonics on Earth is unclear. Identification of eclogite rocks—typically formed during subduction—in the Trans-Hudson orogen implies modern-style tectonics may have been active 1,830 million years ago.

The structure of the Sumatran Fault revealed by local seismicity

The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ∼15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches.