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Where the Earth’s tectonic plates meet, they interact in complex geological, geochemical and

geophysical ways that are part of the plate tectonic cycle. In regions of convergence, one plate can sink under another in a subduction zone, or two colliding plates can form a mountain belt. At divergent boundaries, plates move away from each other and form mid-ocean ridges or major rift zones, and at strike-slip boundaries plates slide past each other. But none of these plate boundaries are static; instead, they change constantly and evolve over time. Now in Nature, Wannamaker and colleagues use electrical resistivity data to investigate a plate boundary in New Zealand where a subduction zone beneath the North Island is lengthening at the expense of a strike-slip plate boundary on the South Island1.

Instead of colliding head-on, the Pacific and Australian plates approach obliquely in the vicinity of New Zealand, leading to complex plate interactions (Fig. 1). On the South Island, both plates are made up of buoyant continental crust; therefore each plate resists sinking, and instead compression forms the Southern Alps mountain range2. Because the plates converge at an angle, the oblique component of motion between them is accommodated by horizontal sliding along a prominent strike-slip fault (the Alpine fault) that slices the South Island in two3. In the North Island region, in contrast, the Pacific plate consists of oceanic lithosphere that sinks beneath the buoyant continental lithosphere of the Australian plate to form the Hikurangi subduction zone4.

Wannamaker and colleagues1 focus on the Marlborough region in the north of the South Island, where a series of sub-parallel strike-slip faults mark the transition from the Alpine fault to the Hikurangi subduction zone. They used the magnetotelluric method5 to measure the electrical resistivity of the region’s lithosphere with naturally occurring radio waves. Fluid-rich zones of the Earth’s lithosphere show low-resistivity values, so these magnetotelluric data

are particularly sensitive to the presence of fluids. Analysis of the data reveals five zones of low resistivity in the underlying crust and mantle, indicative of the presence of fluids. These fluid-rich zones lie above the northwest-dipping Pacific slab, and extend to progressively greater depths from the southeast to the northwest, indicating upward fluid migration from sources along the subducting Pacific plate.

The fluid-rich zones in the crust seem to coincide with some of the strike-slip faults of the region. Wannamaker and colleagues suggest that the upward fluid flux in the region could have weakened the crust over time and contributed to the formation of the faults. This is consistent with a number of studies of the earthquake rupture process on major faults that suggest fluids can weaken faults by lowering the frictional strength6. Further evidence that the low-resistivity zones reflect weak regions comes from the general lack of large earthquakes in these areas, with deformation occurring by creep or many small earthquakes. The larger earthquakes can only occur in strong material, and are concentrated in the areas that fringe the low-resistivity regions.

The most northwesterly of the fluid-rich zones is also the one that penetrates to the greatest depth, extending from the shallowest crust to the top of the Pacific slab. Moderate earthquakes have occurred along faults in the shallow crust above this zone. But these faults dip at steep angles of over 45 degrees — a geometry that is not considered favourable for fracture in the absence of fluids.Wannamaker and colleagues suggest that the fluids migrating upward from depth permeate through the faults and lubricate them, facilitating slip along the faults.

The high influx of fluids in the early stage of subduction seems to be a key factor in the transformation of the plate boundary into a subduction zone. As shown by Wannamaker and colleagues, the fluids facilitate extensive faulting, leading to the development of a region

geOPhySicS

a plate boundary in fluxThe Pacific and Australian plates collide and interact in complex ways around New Zealand. Electrical resistivity data reveal that subduction-zone fluids exert an important influence on deformation in the region.

martyn Unsworth

NorthIsland

200 km

South Island

Hikurangitrench

Australianplate

Pacificplate

Transitional region between subduction and strike-slip boundary

Approximate location of the magnetotellurictransect

Alpine fault

Figure 1 | A complex plate boundary region. Topographic and bathymetric map of the New Zealand region showing the important tectonic features (background image courtesy of US Geological Survey). Brown colours show regions of high topography; blue and green colours show regions below sea level. The Hikurangi subduction zone is gradually expanding southwestward at the expense of the strike-slip and collisional boundary along the Southern Alps. Fluids released at the upper surface of the subducting Pacific slab migrate upward through the lithosphere of the overlying Australian plate. Wannamaker and colleagues document how these fluids influence deformation in the Marlborough region1. The black arrows indicate motion along the Alpine fault and the red arrow indicates the direction of motion of the Pacific plate; black triangles mark locations of important volcanoes associated with this plate boundary region.

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of distributed deformation. This process permits a significant reorganization of the plate motion. The fluids also trigger mantle melting that further weakens the crust, and ultimately leads to the formation of the volcanoes observed in many subduction zones around the Pacific ‘ring of fire’, including the North Island of New Zealand4.

The transition from crustal compression and strike-slip faulting to subduction, as studied by Wannamaker and colleagues in the Marlborough region1, is not a one-way journey:

subduction zones may themselves be transformed to other types of plate boundary. For example, when India collided with Asia, an active subduction zone was transformed into a collision that formed the Himalaya and Tibetan plateau7. Such a fate does not seem to be in store for the subduction zone in northern New Zealand, at least not in the near-term geological future. But, in light of the ever-evolving and dynamic interplay between Earth’s tectonic plates, it is clear that this subduction boundary will not last forever. ❐

Martyn Unsworth is in the Department of Physics and the Department of Earth and Atmospheric Sciences, University of Alberta, 11322-89 Avenue, Edmonton, Alberta T6G 2J1, Canada. e-mail: unsworth@phys.ualberta.ca

references1. Wannamaker, P. E. et al. Nature 460, 733–736 (2009).2. Molnar, P. et al. Science 286, 516–519 (1999).3. Walcott. R. I. Rev. Geophys. 36, 1–26 (1998).4. Smith, E. G. C., Stern, T. & Reyners, M. Pure Appl. Geophys.

129, 203–231 (1989).5. Simpson, F. & Bahr, K. Practical Magnetotellurics (Cambridge

Univ. Press, 2005).6. Byerlee, J. Geology 21, 303–306 (1993).7. Tapponnier, P. et al. Science 294, 1671–1677 (2001).

earth’s earliest evidence of life, considered to be putative microfossils from 3,465-million-year-old rocks,

strongly resembles the morphology of modern oxygen-producing bacteria. Recognizing the fossils of ancient photosynthetic microbes is important, as identifying the time of their formation would reveal the timing of the initial production of oxygen on our planet. Tiny filamentous structures found within the Apex chert, a rock unit in the Pilbara Craton, Western Australia, are considered to be the oldest known microfossils, based on their complex cellular structure and carbonaceous content1, but their palpability as early-life signatures has been heatedly debated. It has been suggested that similar microstructures could develop from hydrothermal alteration2, and comparable examples have even been reproduced in laboratory experiments3. On page 640 of this issue, Pinti and colleagues use microscale scanning electron microscope (SEM) imagery to describe the environment in which the Apex chert formed, and to identify the processes that led to the formation of some of the prominent microstructures4.

Chert — microcrystalline silica that has been compacted and cemented into rock — is preserved extensively within rock layers of Archaean age (that is, more than 2,500 million years old) on several continents. The abundance of

chert in Archaean rocks, relative to its proportion in younger rocks, can be explained by the lack of silica-utilizing organisms in Archaean sea water, and

by voluminous silica precipitation from hydrothermal fluids.

The putative microfossils in the Apex chert (Fig. 1) were found in fragments within a brecciated chert vein, which has been interpreted to have acted as a feeder from a hydrothermal vent to the overlying bedded cherts2. One vein sampled from the Apex chert did indeed produce a silicon isotope signature consistent with precipitation from hydrothermal fluids5. Although a deep hydrothermal environment was initially thought to be an unfavourable niche to most microbes, it is possible that microorganisms were carried downwards during intense hydrothermal circulation or that heat-loving bacteria colonized the veins6.

To clarify the formation history of the Apex chert, Pinti and colleagues collected two samples from the boundary between the vein where the purported microfossils were found and the overlying bedded chert4. Chemical and mineralogical analyses show that the rock is composed of a collection of minerals, native metals and cavity-bearing iron oxides, all of which would suggest that it formed at high temperatures2. However, using microscale SEM imagery, the authors demonstrate that the primary mineral assemblage of microcrystalline quartz, barite, plagioclase and native metals has been partially altered and replaced at low to medium temperatures (<150 ºC). Results

early BiOSPhere

magnifying ancient microstructuresPurported 3,465-million-year-old microfossils from Australia have been the subject of considerable debate. A method to distinguish between pristine fossils, mineral artefacts and subsequent microbial contamination will aid the search for ancient biogenic material.

Patricia corcoran

Figure 1 | The oldest life on Earth? This chert vein of the Apex chert, Western Australia contains the putative microfossils. Pinti and colleagues use microscale SEM imagery to evaluate possible biological microstructures in samples collected from the boundary between the chert vein and the overlying bedded chert4.

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