Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest

Wang, Xuejin; Cochran, James R.

doi:10.1029/93jb01551

Cited by 86 publications

(93 citation statements)

References 69 publications

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“…The "bull's eye" gravity lows, however, are of lesser amplitude at segments of fast-spreading ridges such as the East Pacific Rise (EPR) [Madsen et al, 1990;Wang and Cochran, 1993;Cormier et al, 1995] Morgan, 1990; suggest a transition from three-dimensional plumelike mantle upwelling to two-dimensional sheetlike upwelling with increasing spreading rate which is consistent with the observed spreading rate dependence of ridge axis gravity structure . The range of MBA amplitudes decreases with increasing spreading rate (Figure 2).…”

Section: Introductionsupporting

confidence: 69%

“…The existence of distinctive lows in mantle Bouguer gravity anomalies (MBA) at the Mid-Atlantic Ridge (MAR) (Figure 1) indicate that the crust is thicker and the upper mantle is hotter at segment centers than at distal ends [Kuo and Forsyth, 1988;Blackman and Forsyth, 1991;Morris and Detrick, 1991;. These residual gravity lows suggest that magmatic accretion occurs in the form of discrete, buoyancydriven upwelling centers [Kuo and Forsyth, 1988;, thus supporting earlier hypotheses on the existence of melt or buoyant mantle diapirs in the underlying asthenosphere [e.g., Rabinowicz et al, 1984;.The "bull's eye" gravity lows, however, are of lesser amplitude at segments of fast-spreading ridges such as the East Pacific Rise (EPR) [Madsen et al, 1990;Wang and Cochran, 1993;Cormier et al, 1995] Morgan, 1990; suggest a transition from three-dimensional plumelike mantle upwelling to two-dimensional sheetlike upwelling with increasing spreading rate which is consistent with the observed spreading rate dependence of ridge axis gravity structure . The range of MBA amplitudes decreases with increasing spreading rate (Figure 2).…”

supporting

confidence: 59%

“…We examine digital multibeam bathymetric and residual gravity data collected over four areas of the slow-spreading MAR Morris and Detrick, 1991; (Figure 1), two areas of the fast-spreading EPR between 7°-9"30'S [Wang and Cochran, 1993] and 8050 '-9°50*N [Madsen et al, 1990], and the very slow-spreading Southwest Indian Ridge (SWIR) near the Atlantis II Transform [Dick et al, 1991; Dick, unpublished data, 1990]. These data sets encompass a total of 8 transforms and 14 nontransform offsets with age offsets from 0.1 to 23 Ma and half-spreading rates from 0.8 to 7.7 cm/yr.…”

Section: Discussionmentioning

confidence: 99%

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Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Escartı́n¹

1996

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Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments.In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments.Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis.Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable.Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting. Thesis supervisor:Jian Lin Title:Associate...

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Section: Introductionsupporting

confidence: 69%

supporting

confidence: 59%

Section: Discussionmentioning

confidence: 99%

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Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Escartı́n¹

1996

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“…[72] Fast-spreading ridges have the smallest along-axis topographic relief, as low as 0.1 m per kilometer [Wang and Cochran, 1993], but it is not clear whether fast-spreading segments are largely fed by magma from a central magma chamber. Ridge morphology, lava flow style and magma chemistry may indicate central supply [e.g., MacDonald, 1998], however, magma lens are imaged seismically along most fast-spreading segments [e.g., Hooft et al, 1997].…”

Section: Applications To Other Settingsmentioning

confidence: 99%

Tectonic stress and magma chamber size as controls on dike propagation: Constraints from the 1975–1984 Krafla rifting episode

2006

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[1] The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from [1975][1976][1977][1978][1979][1980][1981][1982][1983][1984]. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the ''breakout'' pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this ''stopping'' pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.Citation: Buck, W. R., P. Einarsson, and B. Brandsdóttir (2006), Tectonic stress and magma chamber size as controls on dike propagation:

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“…However, the variation in the MBA gravity is not smooth and continuous (Figure 7). Segment scale MBA anomalies are related to the upwelling and melt distribution pattern within individual segments [Macdonald, 1989;Wang and Cochran, 1993;Tolstoy et al, 1993]. There are also a number of distinct steps in the regional level of the MBA gravity.…”

Section: Gravity Anomalies and Transitions In Axial Morphologymentioning

confidence: 99%

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

Cochran¹,

Sempéré²

1997

J. Geophys. Res.

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Abstract. Although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical survey of the intermediate spreading rate Southeast Indian Ridge between 88øE and 118øE, a 2300-km-long section of the ridge located between the Amsterdam hot spot and the AustralianAntarctic Discordance where satellite gravity data suggest that the Southeast Indian Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103ø30'E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well-developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifted high that is lower and pervasively faulted, sometimes with significant (> 50 m throw) faults within a kilometer of the axis. A shallow axial valley (< 700 m deep) is observed from 104øE to 114øE with a sudden change to a deep (>1200 m deep) valley across a transform at 114øE. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near-axis ridge flank depth from 2800 m near 88øE to 3300 m near 114øE and by a 50 mGal increase in the regional level of mantle Bouguer gravity anomalies over the same distance. The regional changes in depth and mantle Bouguer anomaly (MBA) gravity can be both explained by a 1.7-2.4 km change in crustal thickness or by a mantle temperature change of 50øC-90øC. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneously and these estimates serve as upper bounds. The along-axis MBA gradient is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103ø30'E-104øE and at 114øE -116øE and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them. The change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly. The presence or absence of a quasi-steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have an MBA low located at a depth minimum near the center of the segment. At EPR-like segments, the MBA low is about 10 mGal with along-axis gradients of 0.15-0.25 mGal/km, similar to those observed at the EPR. Rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply. Segments with ...

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Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest

Cited by 86 publications

References 69 publications

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Ridge segmentation, tectonic evolution and rheology of slow-spreading oceanic crust

Tectonic stress and magma chamber size as controls on dike propagation: Constraints from the 1975–1984 Krafla rifting episode

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

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