Molnar Peter H.

No Thumbnail Available
Last Name
Molnar
First Name
Peter H.
ORCID

Filters

2011 - 2014 3
2 1
3
Reset filters

Settings

Search Results

Now showing 1 - 3 of 3
  • Article
    Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere
    (John Wiley & Sons, 2014-10-23) Collins, John A. ; Molnar, Peter H.
    Pn travel times from regional earthquakes recorded both by stations on New Zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region. The largest anisotropy of ~8% (±2%, 1σ) lies beneath the deforming part of the South Island to just off its West Coast, a zone roughly 100–200 km wide. The fastest propagation is aligned N60°E (±3°), essentially parallel to the largely strike-slip relative plate motion since 20 Ma, also ~ N60°E. The magnitude of anisotropy decreases abruptly northwest and southeast of this zone, and on the southeast side of the island, the orientation of fastest propagation is between N32°W and N-S. The ~ N60°E orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the Pacific plate past the Australia plate is spread over a region with a width of 100–200 km. The agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500–800°C, where Peierls creep seems to be the likely deformation mechanism. Such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form inline image, where inline image is the average strain rate and τ is the operative deviatoric stress, with an average value of n ≈ 3–10. Presumably, the NW-SE fast propagation in the region southeast of the island results from strain that precedes the Cenozoic deformation that has shaped the island.
  • Preprint
    Multibeam bathymetric surveys of submarine volcanoes and mega-pockmarks on the Chatham Rise, New Zealand
    ( 2011-04) Collins, John A. ; Molnar, Peter H. ; Sheehan, Anne F.
    Multibeam bathymetric surveys east of the South Island of New Zealand present images of submarine volcanoes and pockmarks west of Urry Knolls on the Chatham Rise, and evidence of submarine erosion on the southern margin of the Chatham Rise. Among numerous volcanic cones, diameters of the largest reach ~2000 m, and some stand as high as 400 m above the surrounding seafloor. The tops of most of the volcanic cones are flat, with hints of craters, and some with asymmetric shapes may show flank collapses. There are hints of both northeast-southwest and northwest-southeast alignments of volcanoes, but no associated faulting is apparent. Near and to the west of these volcanoes, huge pockmarks, some more than ~1 km in diameter, disrupt bottom topography. Pockmarks in this region seem to be confined to sea floor shallower than ~1200 m, but we see evidence of deeper pockmarks at water depths of up to 2100 m on profiles crossing the Bounty Trough. The pockmark field on the Chatham Rise seems to be bounded on the south by a trough near 1200 m depth; like others, we presume that contour currents have eroded the margin and created the trough.
  • Article
    Upper mantle seismic anisotropy at a strike-slip boundary : South Island, New Zealand
    (John Wiley & Sons, 2014-02-05) Zietlow, Daniel W. ; Sheehan, Anne F. ; Molnar, Peter H. ; Savage, Martha K. ; Hirth, Greg ; Collins, John A. ; Hager, Bradford H.
    New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi-shear wave nearly parallel to relative plate motion, with increased obliquity to this orientation observed farther from shore. On the eastern side of the island, fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast. Uniform delay times between the fast and slow quasi-shear waves of nearly 2.0 s onshore continue to stations approximately 100 km off the west coast, after which they decrease to ~1 s at 200 km. Stations more than ~300 km from the west coast show little to no splitting. East coast stations have delay times around 1 s. Simple strain fields calculated from a thin viscous sheet model (representing distributed lithospheric deformation) with strain rates decreasing exponentially to both the northwest and southeast with e-folding dimensions of 25–35 km (approximately 75% of the deformation within a zone 100–140 km wide) match orientations and amounts of observed splitting. A model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if, at depths of 100–130 km below the lithosphere, typical grain sizes are ~ 6–7 mm.