Lithosphere tearing at STEP faults: response to edges of subduction zones
Introduction
Near the majority of horizontal terminations of subduction trenches, continual tearing of the lithosphere is a geometric consequence [1]. With time, the intersection between the subduction fault and the transform segment propagates through the lithosphere, thereby effectively increasing the area of the transform-like fault (Fig. 1). We refer to the tearing transform fragment as a Subduction-Transform Edge Propagator (STEP) fault.
Following the identification of current plate boundaries, plate-tearing configurations were hypothesized (āscissors type faultingā [2], āhinge faultingā [3], [4]). Tearing has been occurring in the following regions in the last couple of million years (Fig. 2); the northern termination of the Tonga trench, the southern end of the New Hebrides trench, the northern and southern ends of the Lesser Antilles trench, the northern end of the South Sandwich trench, the southern end of the Hikurangi trench (North Island, New Zealand), the southern end of the Vrancea trench (Carpathians), on both ends of the Calabria trench (Sicily), both ends of the Hellenic trench, and possibly the Gibraltar arc. This list is not complete in that it does not include collision examples.
From their inventory of seismicity in STEP regions, Bilich et al. [5] find a āconspicuous scarcityā of strike-slip earthquakes. This finding is consistent with the nature of STEP faults, as explained next (Fig. 1aāb). A STEP fault allows subduction to continue. The amount of transform motion is not necessarily uniform along the STEP fault. To illustrate this point, consider a tectonic setting like the Central Mediterranean, where relative motion between continental Europe and Africa has nearly come to a halt and subduction of the Ionian slab is accompanied by slab roll-back. In such āland-locked basinsā [6], the overriding plate shows back-arc extension in response to the movement of the trench. Relative horizontal motion across the STEP fault only occurs up to the back-arc domain. STEP propagation acts like a wave traveling through the landscape; at any location along the STEP path, strains and rotations build up until it has passed by. Accurate timing of deformation and paleomagnetic rotation is therefore crucial to detect STEP propagation geologically. In the alternative case that rigid motion of the overriding plate is possible (Fig. 1c), strike-slip motion will occur across the STEP fault that in this case behaves more like a classical transform plate boundary. Whether the overriding plate can rigidly trail the trench during rollback, or even actively overrides the subducting plate without deforming internally, depends on boundary conditions (left side of Fig. 1aāc) and internal strength.
Schellart et al. [7] use analogue models to study the geological consequences of trench retreat for the North Fiji back-arc basin. Implicit in their experimental approach is the assumption that the STEP fault at the southern end of the New Hebrides trench propagates easily. Prescribed saloon door kinematics on the edge of the basin are shown to reproduce the observed structural development within the overriding plate. Return flow around slab edges is addressed in some recent model studies. Kincaid and Griffiths [8] show that return flow depends on velocities of subduction, slab-steepening, and rollback. Vice versa, the kinematics of subduction are significantly affected by the presence of a slab edge, resulting in a geometry of edge-bounded subduction zones that differs from the typical concave shape towards the overriding plate [9], [10], [11].
We first review some of the main characteristics of existing STEP regions. The inferred range of plate tectonic settings in which STEPs occur, inspires end-member dynamic models of STEP regions. These models are intentionally simple, and without a focus on any specific region. We will show that STEP faults are stable in that, once a STEP geometry exists, it will grow upon itself except in relatively extreme cases. Another outcome of the models is that very particular patterns of surface deformation result from tear propagation, which may be geodetically and geologically detectable. Finally, we predict some first order geological imprints from the models.
Section snippets
North Fiji Basin (Fig. 2a)
Following the classical work by Isacks et al. [2], [3], Millen and Hamburger [12] show that seismicity and focal mechanisms are indicative of progressive down-warping and tearing of the Pacific plate as it enters the northernmost segment of the Tonga subduction zone. Dip-slip faulting along shallow (18ā57 km) near-vertical planes that are oriented parallel to the slab edge is inferred from large (5.6ā7.5 mb) earthquakes. Sinistral strike-slip activity on the STEP fault tapers towards the
STEP model setup
We use a finite element (FE) method to solve the mechanical equilibrium equation for three-dimensional displacements. The code was developed from TECTON version 1.3 (1989) [52], [53]. Constitutive equations are based on (compressible) elastic and non-linear, incompressible viscous flow;
Discussion
Once established, STEPs are stable features in that they continue to propagate as long as the lithospheric strength is less than or equal to the slab strength. As subduction results from instability of the cold and strong boundary layer, STEPs will mostly be stable as seems to be evident from the length of STEP faults in Fig. 2. However, subduction of progressively younger lithosphere may result in a combination of substantial slab pull and a weak shallow slab causing slab break-off rather than
Conclusions
We identify a dozen or so STEP-type edges of subduction zones, where continued subduction requires tearing of the lithosphere. Observational support for STEP propagation varies strongly per region, partly because of the remoteness or inaccessibility of the area, but also because lithosphere tearing events seem to occur infrequently. STEPs are basically different from transform plate boundaries, although they may sometimes mimic the kinematic behavior of transforms. Once established, STEPs are
Acknowledgements
This research was performed as part of the ISES program. The work was partly supported by the EUROMARGINS Program of the European Science Foundation, 01-LEC-EMA22F WESTMED project. Reviews by Michael Hamburger, Ray Russo and Wouter Schellart are greatly appreciated.
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