On the roughness of Mesozoic oceanic crust in the western North Atlantic

S U M M A R YWe present depth images, from portions of profiles that are close to flow-lines, of Cretaceous oceanic crust in the eastern Central Atlantic. Compared with post-stack time migrations, the images illustrate the improvement resulting from the application of pre-stack depth migration. The images document the scale and geometry of normal faulting in oceanic crust formed over 25 Myr at a half-spreading rate of less than 10 mm yr −1 , and the variation in extensional style with position within the spreading segment. Away from major fault zones (FZs), most faults are subplanar, dip more than 35 • , are associated with moderate basement relief (0.2-1 km relief) and may penetrate to deep crustal levels. These faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains. Also observed away from FZs are gently dipping to subhorizontal reflections in the upper crust, which resemble detachment faults. In contrast, the inside corner crust is more rugged, with basement highs rising up to 2 km above the intervening basins. This larger-scale topography is associated with a different style of faulting: the depth images reveal gently dipping (<35 • ) faults that are rooted in the deep crust and that project to the ridgeward flank of the dome-shaped large basement highs (1-2 km vertical relief). The faults seem to continue as the ridge-facing flank of these highs and some may extend over the crest of the high to breakaways beyond. In this case, the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes. These controlling faults are up to 20 km long and have a heave of ∼10 km, sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks. We suggest that similar faults can explain the structure and lithologies found at megamullion structures (oceanic core complexes) at inside corners near the present-day spreading ridge.

Section: Discussionmentioning

confidence: 65%

Section: Discussionmentioning

confidence: 65%

Section: Discussionmentioning

confidence: 82%

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Geometry of extensional faults developed at slow-spreading centres from pre-stack depth migration of seismic reflection data in the Central Atlantic (Canary Basin)

Reston¹,

Ranero²,

Ruoff³

et al. 2004

“…In Fig. 6, published roughness values (Ranero et al 1970; Goff 1991, 1997; Bird & Pockalny 1994; Henstock & White 1996; Minshull 1999; Weigelt & Jokat 2001) are shown in a global context, highlighting the relationship between crustal roughness and half spreading rates. The new roughness values supplement the global data for the ultra‐slow end of the spectrum.…”

Section: Discussionmentioning

confidence: 98%

Subsidence and crustal roughness of ultra-slow spreading ridges in the northern North Atlantic and the Arctic Ocean

Ehlers¹,

Jokat²

2009

S U M M A R YFive basin-wide seismic reflection profiles of up to 550 km each were acquired in the Arctic Ocean and the northern North Atlantic in 2001 and 2002. The main objective was to investigate the depth to the basement and to analyse the crustal structure, morphology and roughness of ultra-slow spreading ridges of the Gakkel, Molloy and Knipovich ridges. To date, little is known to date of the ultra-slow spectrum of such spreading ridges. The seismic profiles of all investigated ridges show similar morphological characteristics with deep axial valleys and rough basement topography. Magnetic data compilation and interpretation suggests that the ultra-slow spreading systems are fairly stable and existed during the entire evolution of the basins to the north of the Greenland Fracture Zone. The thermal subsidence curve was calculated and corrected for sediment loads, and crustal roughness values are estimated for all five profiles. The resulting roughness values append the global roughness data set for ultraslow spreading systems. The results are higher than those predicted by interpolating existing global roughness.This study confirms the presence of a global relationship between crustal roughness, ridge morphology and spreading rates. New curve fits, supporting the global relationship, are discussed. Data on present spreading rates, ridge morphology, subsidence and roughness provide a better insight into the development of the axial ridge morphology in the study area. The results show that the basins to the north of the Greenland Fracture Zone were formed at ultraslow spreading axial rift valleys and continued spreading at ultra-slow rates to the present day configuration.

“…Empirical relationships have been developed between spreading rate and basement relief and allow comparison with the roughness of this region, calculated as the rms misfit between the depth migrated basement topography and a best‐fit line through these data to account for thermal subsidence (e.g. Malinverno 1991; Minshull 1999). Intermediate zone (ii) has a roughness of 128 m (Fig.…”

Section: Discussionmentioning

confidence: 99%

From continental extension to seafloor spreading: crustal structure of the Goban Spur rifted margin, southwest of the UK

Bullock

Minshull

2005

S U M M A R YWe present a 169-km wide-angle velocity model across the Goban Spur rifted margin, southwest of the UK. A 120-km-wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly (anomaly 34r) and thinned continental crust, where velocities increase from 4.5 km s −1 to 6.8 km s −1 in the top 4 km beneath acoustic basement. At depth it can be divided into a region where a 1.5-km-thick high-velocity layer (7.2-7.6 km s −1 ) exists and a region where this layer is absent but velocities of ∼7 km s −1 are present. Wide-angle PmP arrivals are observed across the whole of the intermediate region but a normal incidence reflection Moho is not present.Based on these velocities and combined with seismic reflection, gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high Poisson's ratio at top basement (>0.34), an extremely low topographic expression and a high-velocity deep layer. Empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to <25 per cent from 5-7 km into basement. The observed magnetic anomaly is best fit by a thin magnetized layer (1 km) of magnetization 2-3 A m −1 . The intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water. Between this exhumed mantle and anomaly 34r is a 50-km-wide region raised 400 m above the adjacent exhumed mantle, consisting of a series of basement ridges. Poisson's ratio is not well defined in this region, which may be composed of anomalous oceanic crust or exhumed mantle.