Gravity anomalies and crustal thickness variations along the Mid‐Atlantic Ridge between 33°N and 40°N

116

Abstract. We have determined the three-dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid-Atlantic Ridge (MAR) near 35øN. The best fitting one-dimensional model is -0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three-dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak-to-peak variation at 1 km depth) in both the along-and cross-axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3-5 km in lateral extent, is located beneath a near-axis seamount. We interpret this feature as a region of near-solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1-1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high-velocity, ridge-parallel feature commonly observed at the axis of fast spreading ridges and attributed to high-velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.

Section: Mid-atlantic Ridge Near 35ønmentioning

confidence: 81%

Section: Mid-atlantic Ridge Near 35ønmentioning

confidence: 93%

See 1 more Smart Citation

Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

Barclay¹,

Toomey²,

Solomon³

1998

116

“…The duration of the thermal reequilibration was probably influenced by the neighboring Cape Verde Hot Spot. Gravity and bathymetric anomalies observed along spreading centers influenced by prominent hot spots suggest that crustal accretion processes are affected along segments hundreds of kilometers long [Detrick et al, 1995;Ito and Lin, 1995].…”

Section: Implications For Crustal Accretion Processes At Slow Spreadimentioning

confidence: 99%

The crustal structure of the Canary Basin: Accretion processes at slow spreading centers

Ranero¹,

Banda²,

Buhl³

1997

Abstract. Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower-crustal reflectivity is almost absent and reflections from the crust-mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust-mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid-Atlantic Ridge.

“…Magde et al [1999] have mapped similar low velocities at shallow crustal levels along the rift valley of OH-l, while in the lower crust these low velocities are confined to the center of the segment. Segment OH-1 shows a large (40 mGal peak to trough) bull'seye gravity low at its center and a distinct gravity asymmetry between the inside and outside corners [Detrick et al, 1995]. From a gravity analysis, Detrick et al [1995] inferred that the crust is 8-9 km thick at the axial center of OH-1 and that the crustal thickness variation along the eastern flank may be 2-3 km from the inside to the outside corner.…”

mentioning

confidence: 99%

Crustal and upper mantle seismic structure beneath the rift mountains and across a nontransform offset at the Mid‐Atlantic Ridge (35°N)

Canales¹,

Detrick²,

Lin³

et al. 2000

Self Cite

119

130

Abstract. We present new results on the crustal and upper mantle structure beneath the rift mountains along two segments of the Mid-Atlantic Ridge and across a nontransform offset (NTO). Our results were obtained from a combination of forward modeling and twodimensional tomographic inversion of wide-angle seismic refraction data and gravity modeling. The study area includes two segments: OH-1 between the Oceanographer fracture zone and the NTO-1 at 34ø35'N and OH-2 between NTO-1 and the NTO at 34ø10'N. The center of OH-1 is characterized by anomalously thick crust (-8 km) with a thick Moho transition zone with Vp=7.2-7.6 km/s. This transition zone, coincident with a gravity low, is probably composed of gabbro sills alternating with dunites, as observed in some ophiolites. OH-1 has larger alongaxis crustal thickness variations than OH-2, but average crustal thicknesses are similar (6.0+1.2 km at OH-l, 6.1+0.7 at OH-2). Thus we do not find significant differences in magma supply between these segments, in contrast to what has been inferred from morphological and gravity studies. At both segments the shoaling of the Moho is more rapid at the inside than at the outside corners, consistent with models in which the inside-corner crust is tectonically modified. The structural differences between inside-and outside-corner crust are more apparent at OH-2, suggesting that the extrusive layer is thinner at the inside corner of OH-2 than at the inside corner of OH-1, probably due to differences in axial morphology and along-axis magma transport. NTO-1 is characterized by a nearly constant velocity gradient within the upper 5 km and low upper mantle velocities (7.4-7.8 km/s). The anomalous structure beneath NTO-1 is interpreted as fractured mafic crust. The P wave velocities and densities required to match the gravity data suggest that serpentinites are common beneath the NTO-1 and possibly beneath the inside corners. Serpentinization could be as much as 40% at-3.8 km below seafloor and probably does not occur at subseafloor depths greater than -6.2 km at the NTO-1. Our results indicate that in a slow spreading environment where magmatism and tectonism are equally important, the seismic Moho cannot be correlated with an unique geological structure. At the center of a segment the seismic Moho may represent the lower boundary of an interlayered grabbro-dunite transition zone, while beneath the inside corner and NTO where the crust is thinner, it may correspond to an alteration front.