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VERSION:2.0
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CALSCALE:GREGORIAN
X-WR-CALNAME:Geological Oceanography Seminar: Dr. Hanchao Jian (WHOI)
X-WR-TIMEZONE:Eastern Time (US & Canada)
BEGIN:VEVENT
DTSTAMP:20260819T142111Z
UID:tag:localist.com\,2008:EventInstance_31709857958477
DTSTART:20191101T180000Z
DTEND:20191101T190000Z
DESCRIPTION:Join us for a Geological Oceanography Seminar featuring Dr. Han
 chao Jian of Woods Hole Oceanographic Institution titled\, “Magmatic cru
 stal accretion at the ultraslow spreading Southwest Indian Ridge.”\n\n\n
 Abstract: The oceanic crust is formed by a combination of magmatic and tec
 tonic processes at mid-ocean ridges. Under ultraslow spreading environment
 \, however\, observations of thin crust and mantle rocks on the seafloor s
 uggest that a large portion of crust is formed mainly by tectonic processe
 s\, with little magmatism. Although the thermodynamic models predict a lar
 gely reduced melt supply at ultraslow ridges\, the strong along-axis varia
 tions of gravity and bathymetry indicate that melt focusing should exist t
 o form large volcanic centers. In order to understand the role of magmatis
 m in crustal accretion at ultraslow ridges\, I study a magmatically robust
  segment of the Southwest Indian Ridge at 50o28’E (segment 27) with ocea
 n bottom seismometer data.\n\nA three-dimensional (3-D) travel time tomogr
 aphy at the shallow segment 27 reveals a round-shaped (diameter of ~7 km) 
 low velocity anomaly (LVA) in the lower crust beneath the segment center\,
  accompanied by an unusually thick crust (~9.5 km)\, indicating an extreme
 ly magmatic accretion. The resolution of the LVA is further improved by a 
 2-D full waveform inversion of an across-axis profile running over the seg
 ment center\, which shows a strong velocity contrast at the top boundary o
 f the LVA\, suggesting the correspondence to an axial magma chamber (AMC) 
 and the accumulation of melt at the roof. I also perform a 2-D travel time
  tomography of an along-axis profile that extends from segment 27 to a non
 -transform discontinuity (NTD) and segment 28 on the west side. The result
  shows a significant along-axis variation in the crustal thickness from ~9
 .5 km at the center of segment 27 to less than 4 km beneath the neighborin
 g NTD\, within a 30-50 km distance\, requiring a highly-focused melt deliv
 ery from the mantle. While the NTD crust exhibits a typical structure comp
 osed of a thin\, intensely fractured volcanic crust overlying a partially 
 serpentinized mantle\, it may have been fed by the AMC at segment 27 throu
 gh shallow processes\, such as lateral dyke propagation. The evidence of s
 uch processes is found in the 3-D tomography results as a small low-veloci
 ty anomaly at the mid-crustal depth on the west side of the central AMC.\n
 \nI conclude that the extremely magmatic accretion at segment 27 is due to
  a focused melt flow towards the large volcanic center\, which has been en
 hanced by the significant large-scale variation in the lithospheric thickn
 ess along the ultraslow spreading ridge. The focused melt from the mantle 
 forms the localized AMC and thick lower crust at the large volcanic center
 \, whereas the lateral redistribution of melt from the crustal AMC through
  shallow processes forms the relatively uniform upper crust along a long s
 tretch of the ridge axis. However\, due to the wide spacing of large volca
 nic centers and the oblique spreading at NTDs impeding dyke propagations\,
  amagamtic extensions with little magma emplacements at any depth are also
  observed. The wide spectrum of melt supply leads to a complex interaction
  between magma emplacement\, tectonic and hydrothermal processes\, which c
 reates a diversity of seafloor morphology and extremely heterogeneous crus
 t at ultraslow spreading ridges.
GEO:41.423116;-71.457042
LOCATION:Narragansett Bay Campus\, Challenger Room\, OSEC Building
SUMMARY:Geological Oceanography Seminar: Dr. Hanchao Jian (WHOI)
URL;VALUE=URI:https://events.uri.edu/event/geological_oceanography_seminar_
 dr_hanchao_jian_whoi
CATEGORIES:Lectures / Presentations
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