Structure and origin of the J Anomaly Ridge, western North Atlantic Ocean

Abstract

The J Anomaly Ridge is a structural ridge or step in oceanic basement that extends southwest from the eastern end of the Grand Banks. It lies beneath the J magnetic anomaly at the young end (M-4 to M-0) of the M series magnetic anomalies. Its structural counterpart beneath the J anomaly in the eastern Atlantic is the Madeira-Tore Rise, but this feature has been overprinted by post-middle Cretaceous deformation and volcanism. In order to study the origin and evolution of the J Anomaly RidgeMadeira-Tore Rise system, we obtained seismic refraction and multichannel reflection profiles across the J Anomaly Ridge near 39øN latitude. The western ridge flank consists of a series of crustal blocks downdropped along west-dipping normal faults, but the eastern slope to younger crust is gentle and relatively unfaulted. The western flank also is subparallel to seafloor isochrons, becoming younger to the south. Anomalously smooth basement caps the ridge crest, and it locally exhibits internal, eastward-dipping reflectors similar in configuration to those within subaerially emplaced basalt flows on Iceland. When isostatically corrected for sediment load, the northern part of the J Anomaly Ridge has basement depths about 1400 m shallower than in our study area, and deep sea drilling has shown that the northern ridge was subaerially exposed during the middle Cretaceous. We suggest that most of the system originated under subaerial conditions at the time of late-stage rifting between the adjacent Grand Banks and Iberia. The excess magma required to form the ridge may have been vented from a mantle plume beneath the Grand Banks-Iberia rift zone and channelled southward beneath the rift axis of the abutting Mid-Atlantic Ridge. Resulting edifice-building volcanism constructed the ridge system between anomalies M-4 and M-0, moving southward along the ridge axis at about 50 mm/yr. About M-0 time, when true drift began between Iberia and the Grand Banks, this southward venting rapidly declined. The results were rapid return of the spreading axis to normal elevations, division of the ridge system into the separate J Anomaly Ridge and Madeira-Tore Rise, and unusually fast subsidence of at least parts of these ridges to depths that presently are near normal. This proposed origin and evolutionary sequence for the J Anomaly Ridge-Madeira-Tore Rise system closely matches events of uplift and unconformity development on the adjacent Grand Banks.

Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect

“…Initial continental stretching started in the Late Triassic and was followed by three main rifting phases up to the Early Cretaceous. The first unequivocal seafloor spreading anomaly in this region is the J anomaly (Figure 2), which lies between magnetic chrons M0 and M2 [e.g., Tucholke and Ludwig, 1982]. However, Whitmarsh and Miles [1995] and Whitmarsh et al [1996] matched additional anomalies to the reversal timescale up to 20 km landward of the J anomaly, suggesting that seafloor spreading began west of the peridotite ridges (Figure 2) LG [Krawczyk et al, 1996].…”

A Seismic Comparison of the Early Stratigraphic Evolution of Conjugate Passive Continental Margins: The Newfoundland/Flemish Basin and the Eastern Iberian Abyssal Plain South of Galicia Bank

“…The J anomaly appears to be associated with basement topo graphic features on both sides of the North Atlantic, that is, the Madeira-Tore Rise, a basement ridge trending along the foot of the Iberian continental margin (Pitman and Talwani, 1972;Ra binowitz et al, 1979;Sibuet et al, 1987), and the J anomaly ridge south of the Grand Banks (Tucholke and Ludwig, 1982;Fig. 3).…”

Origin of the Bermuda volcanoes and the Bermuda Rise: History, observations, models, and puzzles

“…Bathymetry, residual depth contours, and boreholes reproduced from Fig. 1-top; western (after Sundvik et al, 1984) and approximate eastern limits (thick solid lines) of rough oceanic basement topography, created by slow spreading; eastern limits of seismic reflector A t (dotted lines; Tucholke and Mountain, 1979), turbidites that covered part of the region prior to uplift of Bermuda Rise and spread eastwards inside the Fracture Zone valley (Jaroslaw and Tucholke, 1994); JAR, J-Anomaly Ridge and associated basement ecarpment (Tucholke and Vogt, 1979;Tucholke and Ludwig, 1982); small open circles, earthquake epicenters (NEIC database as of 12/2005; with additional older events from Zoback et al, 1986); A, B and C denote epicenters whose first motion and/or depth could be determined. For events with aftershocks, only the location of the main shock is plotted.…”