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Magma supply likely exerts primary control on seafloor morphology of oceanic crust, but most studies have related morphology to spreading rate. Here we examine global patterns of morphology on mid‐ocean ridge (MOR) flanks in relation to magma supply derived from residual mantle Bouguer gravity anomaly (proxy for relative crustal thickness) and spreading rate. We use multibeam bathymetry to characterize morphology using both qualitative (descriptive) and quantitative approaches, and we compare results to both magma supply and spreading rate. Morphology becomes more isotropic and abyssal hills are more irregular and discontinuous as magma supply decreases, while roughness, area of steeper slopes, and anomalous fabric orientation increase. We interpret these changes to reflect changing magma distribution along‐axis, from large‐volume and spatially extensive to progressively reduced, increasingly localized, and more irregularly emplaced. Observed relations between crustal thickness and morphology imply that average thickness of purely magmatic crust in the Atlantic and parts of the Indian ridge system is significantly less than average seismically determined crust. Thus seismically defined crustal thickness in those regions likely includes significant non‐magmatic components such as serpentinized mantle. Excepting regions of extensive mantle exposure, most morphologic parameters that we examined are sensitive to estimated magma supply but not necessarily to spreading rate alone. We summarize our results in schematic models that relate morphologic variations to changes in magma supply and mantle serpentinization throughout the global MOR system. Finally, we note that combined qualitative and quantitative results of our study may be useful for developing automated morphologic classification schemes. Plain Language Summary: Mid‐ocean ridges encircle the globe and spread apart along their axes to form ocean crust. Seafloor morphology of crust on the flanks of different ridges can be highly variable. The variations classically have been attributed to differences in spreading rate but there are notable exceptions to this correlation. Here we use changes in gravity over different mid‐ocean ridges (MORs) together with spreading rate to infer magma supply to the ridges, and we examine how morphology varies with changes in the magma supply. We find that features such as long, linear, and structurally continuous abyssal hills correlate well with high magma supply while progressively rougher, steeper, and more irregular morphology forms as magma supply is reduced; this is observed irrespective of spreading rate, indicating that magma supply better predicts morphologic style. Observed relations between crustal thickness and morphology imply that seismically determined crustal thickness in the Atlantic and parts of the Indian Ocean MOR system includes significant components of non‐magmatic crust (e.g., altered mantle). Our study characterizes crustal morphology both qualitatively (visually) and quantitatively; correlations between these characterizations may be useful for developing automated morphologic classification schemes. Key Points: Variable morphology of global mid‐ocean ridge flanks is examined qualitatively and quantitatively using multibeam bathymetric dataChange in magma supply as derived from gravity and spreading rate, plus mantle serpentinization, explains observed morphologic variationsMorphologic changes indicate thinner magmatic crust than seismically determined crust in the Atlantic and parts of the Indian Ocean [ABSTRACT FROM AUTHOR]

Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identified on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%-50% of total extension is accommodated by magmatic accretion and that there is significant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges. Keywords: mid-ocean ridge, detachment fault, megamullion, oceanic core complex, oceanic magmatism, numerical modeling.

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3-4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to Keywords: continental margin, seafloor spreading, extension tectonics, continental breakup.

Long before the plate-tectonic revolution began in the 1960s, scientists envisioned drilling into the ocean crust to investigate Earth's evolution. As early as 1881, Darwin suggested drilling Pacific atolls for [...]

Bathymetric, magnetic, gravity, and morphologic data from the flank of the slow-spreading Mid-Atlantic Ridge reveal obliquely oriented features that offset magnetic isochrons and morphological patterns within individual ridge segments. These features form angles of [approximately] 10 [degrees]-40 [degrees] with the isochrons and are inferred to result from rift propagation at rates several times the spreading rate, representing the fastest propagators yet observed at a slow-spreading ridge. These fast propagators appear to have formed as a result of tectonic extension migrating along ridge segments as the segments change from more magmatic to less magmatic periods of spreading.

Seismic reflection profiles, basement structure-contour maps, and bathymetric data are used to analyze sedimentation patterns over a distance of approximately 2,300 km along the Kane Fracture Zone and to interpret uplift history of the Bermuda Rise. Sediments within the fracture valley thin seaward on younger crust and with distance from continental sources. Superposed on this pattern are the effects of local and regional basement structure, variations in surface-water productivity, influence of bottom currents, and local effects of sediment redistribution. Preservation of a thicker and more complete sedimentary record within the Kane fracture valley facilitates identification and eastward correlation of seismic reflections, and we have cross-correlated these reflections to synthesize an along-axis transect of the fracture zone. Stratigraphic correlations of key reflections to Deep Sea Drilling Project drill sites determine the composition and ages of the lithologic sequences. Observed pinch-outs of seismic horizons J(sub 1), Beta, and A(super *) on fracture-valley crust extend significantly seaward of previously mapped limits, and the younger pinch-out ages accord better with ages defined by deep-sea drilling. Paleosedimentation patterns in successive stratigraphic intervals document the growth of the continental margin/basin sedimentary prism from Late Jurassic time and the uplift of the Bermuda Rise beginning in middle Eocene time. Middle Eocene cessation of turbidite deposition atop the Rise, as identified by Horizon A(super t), marks initiation of regional uplift. A seismic discontinuity in turbidite ponds east of the Rise records a probable pulse of uplift during early Oligocene time. Structure of the overlying seismic reflection sequence suggests continued uplift, albeit at decreasing rates, through the late Oligocene and possibly through Miocene time. The anomalous elevation of the Rise subsequently has been maintained, but there is no clear stratigraphic evidence for further growth. Turbidite ponds fill the Kane fracture valley east of the Bermuda Rise and terminate against a prominent structural dam at 55 degrees 20 minutes W. These ponds began to fill with turbidites in middle Eocene to early Oligocene time, and the sediments were derived from local sources. Within the past 10-12 m.y., turbidity currents began to enter the ponds through connections with the encroaching Nares Abyssal Plain, and they probably now constitute the dominant sediment source for the fracture valley.

We studied long‐term evolution of nontransform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0‐ to ~20‐ to 25‐Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (<2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3‐D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle. Key Points: Propagation of non‐transform discontinuities on the Mid‐Atlantic Ridge was analyzed in reconstructions of bathymetric and gravity dataRapid and slow propagation occurs and is controlled by magma distribution within the mantle and crust of spreading segmentsWe attribute long‐wavelength cross‐axis asymmetry in gravity anomalies to asymmetric distribution of density anomalies in the upper mantle [ABSTRACT FROM AUTHOR]

Drilling during ODP Leg 210 penetrated two post-rift sills (dated as ∼105.3 and ∼97.8 Ma) in the deep sediments overlying basement of the continent–ocean transition zone on the magma-poor Newfoundland margin. The sill emplacement post-dated the onset of seafloor spreading by at least 7–15 Myr. The shallower of the two sills coincides with the high-amplitude U reflection observed throughout the deep Newfoundland Basin, and strong reflectivity in the sub-U sequence suggests that a number of other sills are present there. In this paper, we use multichannel seismic reflection data and synthetic seismograms to investigate the nature, magnitude and extent of this post-rift magmatism in the deep basin. Features observed in seismic profiles that we attribute to sill injection include high-amplitude reflections with geometries characteristic of intrusions such as step-like aspect; abrupt endings, disruptions and junctions of reflections; finger-like forms; differential compaction around possible loci of magma injection and disruption of overlying sediments by apparent fluid venting. Interpreted sills occur only over transitional basement that probably consists of a mixture of serpentinized peridotite and highly thinned continental crust, and they cover an area of ∼80 000 km2. From analysis of synthetic seismograms, we estimate that sill intrusions may comprise ∼26 per cent of the sub-U high-reflectivity sequence, which yields a crude estimate of ∼5800 km3 for the total volume of sills emplaced by post-rift magmatism. This is significant for a margin usually described as ‘non-volcanic’. We discuss competing hypotheses about the source of the magmatism, which is still uncertain. [ABSTRACT FROM AUTHOR]

A P-wave velocity model along a 565-km-long profile across the Grand Banks–Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8–6.25 km s−1), middle (6.3–6.53 km s−1) and lower crust (6.77–6.9 km s−1), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6–7 km thick occur in two primary layers within the Jeanne d'Arc and the Carson basins (∼3 km s−1 in upper layer; ∼5 km s−1 in lower layer). Abrupt crustal thinning (Moho dip ∼35°) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5–1.5 s−1) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6–7.9 km s−1) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5–6.3 km s−1) and layer 3 (6.3–7.2 km s−1) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side. [ABSTRACT FROM AUTHOR]

Seismic reflection and refraction data were acquired along the southeast margin of Flemish Cap at a position conjugate to drilling and geophysical surveys across the Galicia Bank margin. The data document first-order asymmetry during final break-up between Newfoundland and Iberia. An abrupt necking profile of continental crust observed off Flemish Cap contrasts strongly with gradual tapering on the conjugate margin. There is no evidence beneath Flemish Cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong ‘S’-like reflection, which indicates that this mode of extension occurred only on the Galicia Bank margin. Compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the Flemish Cap margin. Instead, anomalously thin, 3–4-km-thick oceanic crust is observed. This crust is highly tectonized and broken up by high-angle normal faulting. The thin crust and rift structures that resemble the abandoned spreading centre in the Labrador sea suggest that initial seafloor spreading was affected by processes observed in present-day ultra-slow spreading environments. Landwards, Flemish Cap is underlain by a highly reflective lower crust. The reflectivity most likely originates from older Palaeozoic orogenic structures that are unrelated to extension and break-up tectonics. [ABSTRACT FROM AUTHOR]

Seventeen piston cores up to 13 m long were recovered from representative acoustic and lithologic environments of the Hatteras and Nares Abyssal Plains in the western North Atlantic. Compressional-wave velocities (corrected to in situ conditions) and bulk physical properties measured on the cores are used to characterize the acoustic framework of these areas. For correlation with conventional seismic data, whole-core averages of properties are a better index to the acoustic nature of abyssal plain sediments than properties of the upper few centimeters of the seafloor because (1) strong changes in lithofacies (and acoustic properties) occur over depth scales of tens of centimeters to meters in the sediment column, and (2) conventional seismic frequencies of 3.5 kHz or less sample these variations to subbottom depths of tens of meters and more. Whole-core properties are a function of the thickness and distribution of high-velocity silt and sand layers in the core; they vary in a complex fashion with proximity to the source of turbidity currents, distance from axial paths of turbidity-current flows, local and regional basin geometry, and seafloor slope. Thus strongly reflective seabed regions with numerous high-velocity layers are not restricted simply to near-source areas nor are weakly reflective seabed regions (clay sediments only) limited to ''distal'' areas. Whole-core properties show a good qualitative correlation to variations in 3.5-kHz reflection profiles, and 3.5-kHz echo character therefore provides a useful means of mapping general acoustic properties over large regions of abyssal plains. [ABSTRACT FROM AUTHOR]