Boyd, O. S., M. K. Savage, A. F. Sheehan, C. H. Jones, 2007, Illuminating the plate interface structure beneath Cook Strait, New Zealand with receiver functions
Wilson, Jones, Sheehan, Molnar, and Boyd, 2004, Distributed deformation in the lower crust and upper mantle beneath a continental strike-slip fault zone: Marlborough Fault System, South Island, New ZealandBoyd, Savage, Sheehan, and Jones, 2004, Deep Seismic Discontinuity Structure Beneath New Zealand
Wilson, C. K., C. H. Jones, P. Molnar, A. F. Sheehan, and O. S. Boyd, Distributed deformation in the lower crust and upper mantle beneath a continental strike-slip fault zone: Marlborough Fault System, South Island, New Zealand, Geology, 2004.
Converted phases from teleseisms recorded by a seismic array spanning the northern half of the Marlborough fault system, South Island, New Zealand show a continuous, unbroken Moho underlying a seismically anisotropic lower crust beneath the two northernmost faults of the fault system. These observations suggest that distributed deformation, not slip on a narrow vertical fault, accommodates displacement in the lower crust below the 120-480 km of right-lateral slip across the Wairau fault, one splay of the Marlborough fault system and the northward continuation of the Alpine fault. Beneath the Wairau Fault, the Moho dips 25-30° southeast from a depth of ~26 km northwest of the fault to a depth of ~34 km southeast of the fault. Farther to the southeast, Ps conversions from the Moho continue under the Awatere fault (34±10 km of slip) with a constant amplitude and depth of ~34 km. Across the two faults, converted energy from 16-20 km depth varies with back-azimuth in a manner suggesting the presence of anisotropy in the lower crust. These observations imply that one of the tenets of plate tectonics, that faults defining plate boundaries pass through both crust and upper mantle, does not apply to New Zealand, nor to continents in general.
Using teleseismic receiver functions derived from broadband seismic arrays on the north end of the South Island and south end of the North Island of New Zealand, we image seismic impedance discontinuities in the upper mantle beneath Cook Strait using common conversion point (CCP) and Kirchhoff migration methods. Our primary findings are observations of discontinuities associated with the top of the subducting Pacific Plate. Our results suggest that contrary to recent suggestions, the plate is continuous under the northern South Island through the region of seismicity deeper than 50 km. West of Cook Strait, the slab dips steeply to the northwest. We find evidence for a low-velocity layer at the top of the slab, near which much of the seismicity is concentrated. We see hints of two crustal discontinuities, consistent with observations from previous studies. We also see substantial and continuous energy on the transverse receiver function stacks above the mantle wedge extending to the northwestern edge of our stacks, which may indicate seismic anisotropy above 50 km depth.
Boyd, O S, M. K. Savage, A. F. Sheehan, C. H. Jones, Deep Seismic Discontinuity Structure Beneath New Zealand, EOS, 85 (47) (fall meeting supplement), abstract T21B-0523, 2004. (link to AGU abstract)
We have created receiver functions using broadband seismic stations on either side of Cook Strait, New Zealand. These waveforms have been stacked in common conversion point (CCP) bins and reveal lateral variations in seismic structure above 700 km depth. The tectonic relationships beneath New Zealand vary from westward subduction in the north through transform along the Southern Alps to eastward subduction in the very south. The precise geometry of plate interactions at depth is not well known and is only loosely constrained by seismicity, shear wave splitting, seismic tomography, gravity studies, and plate reconstructions. Several outstanding questions include whether the northern subducting plate continues to subduct south of recorded seismicity in the Nelson region and over what lateral extent does the subducting plate penetrate the mantle transition zone. We image the top of the subducting Pacific Plate as well as several mid mantle seismic discontinuities including the 410 km and 660 km upper mantle transition zone discontinuities. Synthetics are generated for our ray geometry and compared with the CCP stacks and suggest that the subducting plate dips steeply under the southern extent of our stations, the southwestern edge of the Marlborough Fault System. The upper mantle transition zone discontinuities are consistent with the penetration of a cold slab southwest beyond recorded seismicity, i.e. the transition zone is thickened in the region of the cold subducted slab. We interpret the significant northeast deepening of the 410 km seismic discontinuity along the subducted plate to be due to differences in the downward component of subduction rate and the kinetics of the olivine phase transition. We observe splitting of the P to S-wave conversion from the 250 km and deeper discontinuities which indicates seismic anisotropy above 250 km depth oriented approximately north-northeast. The orientation reflects transform relative plate motion and/or mechanisms producing a trench parallel fast axis of anisotropy.
Wilson, C. K., C. H. Jones, P. H. Molnar, A. F. Sheehan, O. Boyd, M. Savage, and T. Stern, Evidence for distributed lower crustal deformation within a continental strike-slip fault zone: Marlborough Fault System, South Island, New Zealand, EOS Trans. AGU, 84(46), Fall Mtg. Suppl., Abstract S22A-0422, 2003 (link to AGU abstract)
Teleseismic converted wave images from a passive seismic imaging experiment (2000-2002) across the Marlborough fault system, South Island, New Zealand show a continuous, unbroken Moho beneath the two northernmost faults of the fault system, suggesting that accommodation of lower crustal deformation occurs through distributed, ductile deformation and not by slip on a narrow vertical fault. Beneath the northernmost fault, the Wairau fault (~450 km offset), the Moho dips between 25 to 30 degrees from a depth of ~25 km northwest of the fault to a depth of 34 km southeast of the Wairau fault. Further to the southeast, the Moho arrival appears with a constant amplitude and depth of 34 km beneath the Awatere fault (~30 km offset). Mid crustal arrivals appear to stretch across the Awatere fault at 10, 17, and 27 km depth but their continuation across the Wairau is not clear, possibly indicating a change in the depth of transition to ductile deformation north of the Wairau. Images derived using common conversion point stacking schemes will lose coherence and resolution in the presence of either large, lateral variations in seismic velocity or interface topography with wavelength similar to the smallest bin size of the stacking algorithm. To test the possibility that our image is actually produced by an offset in the Moho, we construct synthetic converted-wave images from seismograms calculated for models with either a Moho step or dip using the same station-event geometry as the processed data set. The synthetic image produced from the dipping Moho model matches our results, but that with a step does not. Large velocity variations associated with the terrain boundary represented by the Wairau fault could affect the coherence of the Moho conversion across the Wairau. Although, restacking with several different velocity models does not affect the lateral continuity of the Moho. The observation of a continuous but dipping Moho under the Wairau Fault and its 450 km of displacement implies distributed strain over a broader region of weak, ductile lower crust.
Wilson, C. K., C. H. Jones, A. F. Sheehan, Lithospheric Structure of a Continental Strike-Slip Boundary, Marlborough Fault Zone, South Island, New Zealand, EOS, Fall 2001 AGU Meeting, 2001
Between December 2000 and July 2002, fifty short period and seven broadband sensors are deployed across the Marlborough Fault Zone of the South Island, New Zealand, in an effort to test competing models of continental boundary deformation using receiver function analysis. Such models can range from a narrow, near-vertical plate boundary through the entire lithosphere to a broad deforming zone. Observations of offsets of discontinuities across these faults and anisotropy in the lithosphere will distinguish among the possibilities. The Marlborough region, containing four main faults, was chosen because it is an active, well-developed, continental strike-slip boundary that accumulated ~460 km of slip over the last 12 million years. Most of the net slip is on the northernmost strand, the Wairau Fault, but Holocene slip rates are higher on the more southerly strands of the fault zone. In addition, the region experiences both abundant local intermediate depth seismicity (providing high-frequency body waves with steep angles of incidence) and well distributed teleseismic events. Sensors have been deployed in five L-shaped arrays and three freestanding broadband seismometers along a transect from north of the Wairau Fault to near the Awatere Fault; a second deployment will carry across the remaining faults of the system. Beams of short period seismograms are constructed to reduce the effect of scattering from topography in the region; these are then used to construct receiver functions. The amplitude and timing of radial and transverse receiver function arrivals can be used to determine crustal anisotropy. The demonstration of negligible lower crustal anisotropy, or of anisotropy but with an orientation of the faster S wave notably different from the strikes of the faults (which are parallel to relative plate motion), will suggest that faults in the upper crust continue as localized shear zones into the lower crust and probably into the mantle. A demonstration of an offset of the Moho at the faults will corroborate such an inference. Conversely, the demonstration of anisotropy implying shear on approximately horizontal planes (or planes dipping gently away from the faults) with a fast orientation parallel to the strikes of the faults would imply that deformation in the upper mantle is distributed and that it connects to upper crustal blocks by shear on sub-horizontal planes in the lower crust. Variations in depth of Moho, and patterns of crust and mantle anisotropy constrain whether the deeper levels of the plate boundary are discrete or not. Broadband stations provide additional frequency range to examine possible frequency dependence of conversions and anisotropy.
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C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder
Last modified on October 1, 2007