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Effects of off-axis melt supply at f...

Durant, Douglas Troy.

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Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise.

Record Type:	Language materials, printed : Monograph/item
Title/Author:	Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise./
Author:	Durant, Douglas Troy.
Description:	117 p.
Notes:	Source: Dissertation Abstracts International, Volume: 72-10, Section: B, page: .
Contained By:	Dissertation Abstracts International72-10B.
Subject:	Geophysics. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3466331
ISBN:	9781124793542

Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise.
Durant, Douglas Troy.

Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise. - 117 p.

Source: Dissertation Abstracts International, Volume: 72-10, Section: B, page: .

Thesis (Ph.D.)--University of Oregon, 2011.

Results from a recent mid-ocean ridge tomography study along the fast-spreading, northern East Pacific Rise (EPR) reveal that the axis of mantle upwelling beneath the ridge is skewed with respect to the spreading axis, giving rise to regions of both rise-centered and off-axis mantle melt accumulation. Here, we investigate the effects of off-axis melt accumulation on the architecture of overlying crust as well as off-axis melt delivery on crustal construction along the ridge axis. We first present evidence for off-axis magmatism 20 km from the spreading center in 300-ka-old crust overlying a region of off-axis melt supply. Seismic data reveal an intrusive complex &sim;2 km beneath the seafloor that is limited in lateral extent (<5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust, which provides the necessary conditions to drive off-axis volcanic and hydrothermal activity. We next present results from thermodynamic modeling that show systematic, along-axis variations in the depth of crystallization and degree of differentiation of magma produce crustal density variations of &sim;0.1 g/cm3. These density anomalies are on the order inferred from a recent study that shows increasing axial depth along the northern EPR correlates with an increase in crustal density and offset of mantle upwelling with respect to the ridge axis. Our results, along with geophysical and geochemical data from the 9°--10°N region of the EPR, suggest that along-axis deeps correspond with magmatic systems that have significant near-Moho (i.e., crust-mantle transition) crystallization, which we attribute to off-axis delivery of mantle melt. As this investigation is motivated by the EPR tomography results, we conclude with a numerical study that examines the travel time sensitivity of Pn, a sub-crustal head wave commonly used in local travel time tomography, to crustal and mantle heterogeneity. Our results indicate that Pn travel times and Fresnel zones are insensitive to normal sub-axial crustal thickness anomalies, mantle velocity gradients and crust-mantle velocity contrast variations and that mantle low-velocity zones must be at least 3 km thick to produce significant, near-constant Pn delay times. Our data support the validity and interpretation of the EPR tomography results.

ISBN: 9781124793542Subjects--Topical Terms:

535228
Geophysics.

Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise.
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100 1 $a Durant, Douglas Troy. $3 1683794
245 1 0 $a Effects of off-axis melt supply at fast-spreading mid-ocean ridges: A study of the 9-10n region of the East Pacific Rise.
300 $a 117 p.
500 $a Source: Dissertation Abstracts International, Volume: 72-10, Section: B, page: .
500 $a Adviser: Douglas R. Toomey.
502 $a Thesis (Ph.D.)--University of Oregon, 2011.
520 $a Results from a recent mid-ocean ridge tomography study along the fast-spreading, northern East Pacific Rise (EPR) reveal that the axis of mantle upwelling beneath the ridge is skewed with respect to the spreading axis, giving rise to regions of both rise-centered and off-axis mantle melt accumulation. Here, we investigate the effects of off-axis melt accumulation on the architecture of overlying crust as well as off-axis melt delivery on crustal construction along the ridge axis. We first present evidence for off-axis magmatism 20 km from the spreading center in 300-ka-old crust overlying a region of off-axis melt supply. Seismic data reveal an intrusive complex &sim;2 km beneath the seafloor that is limited in lateral extent (<5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust, which provides the necessary conditions to drive off-axis volcanic and hydrothermal activity. We next present results from thermodynamic modeling that show systematic, along-axis variations in the depth of crystallization and degree of differentiation of magma produce crustal density variations of &sim;0.1 g/cm3. These density anomalies are on the order inferred from a recent study that shows increasing axial depth along the northern EPR correlates with an increase in crustal density and offset of mantle upwelling with respect to the ridge axis. Our results, along with geophysical and geochemical data from the 9°--10°N region of the EPR, suggest that along-axis deeps correspond with magmatic systems that have significant near-Moho (i.e., crust-mantle transition) crystallization, which we attribute to off-axis delivery of mantle melt. As this investigation is motivated by the EPR tomography results, we conclude with a numerical study that examines the travel time sensitivity of Pn, a sub-crustal head wave commonly used in local travel time tomography, to crustal and mantle heterogeneity. Our results indicate that Pn travel times and Fresnel zones are insensitive to normal sub-axial crustal thickness anomalies, mantle velocity gradients and crust-mantle velocity contrast variations and that mantle low-velocity zones must be at least 3 km thick to produce significant, near-constant Pn delay times. Our data support the validity and interpretation of the EPR tomography results.
520 $a This dissertation includes both previously published and unpublished co-authored material.
590 $a School code: 0171.
650 4 $a Geophysics. $3 535228
650 4 $a Marine Geology. $3 1674685
650 4 $a Petrology. $3 535210
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690 $a 0584
710 2 $a University of Oregon. $b Department of Geological Sciences. $3 1683795
773 0 $t Dissertation Abstracts International $g 72-10B.
790 1 0 $a Toomey, Douglas R., $e advisor
790 1 0 $a Wallace, Paul J. $e committee member
790 1 0 $a Humphreys, Eugene $e committee member
790 1 0 $a Isenberg, James $e committee member
790 $a 0171
791 $a Ph.D.
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856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3466331