Brian E. Tucholke

Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid-Atlantic Ridge

In a study of geological and geophysical data from the Mid-Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside-corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low-angle detachment faults, and they provide the best evidence to date for the common development and longevity (∼1–2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift-valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside-corner highs.

A geological model for the structure of ridge segments in slow spreading ocean crust

First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside-corner sides, both first-and second-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside-corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.

Role of melt supply in oceanic detachment faulting and formation of megamullions

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 identifi ed 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 signifi cant 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.

Breakup of the Newfoundland–Iberia rift

Abstract The Newfoundland–Iberia rift is considered to be a type example of a non-volcanic rift. Key features of the conjugate margins are transition zones (TZs) that lie between clearly continental crust and presumed normal (Penrose-type) oceanic crust that appears up to 150–180 km farther seaward. Basement ridges drilled in the Iberia TZ consist of exhumed, serpentinized peridotite of continental affinity, consistent with seismic refraction studies. Although the boundaries between continental crust and the TZs can be defined with relative confidence, there are major questions about the position and nature of the change from rifting to normal sea-floor spreading at the seaward edges of the TZs. Notably, drilling of presumed oceanic crust in the young M-series anomalies (<M5) has recovered serpentinized peridotite, and this basement experienced major extension up to approximately 15 million years after it was emplaced. In addition, existing interpretations place the ‘breakup unconformity’ (normally associated with the separation of continental crust and simultaneous formation of oceanic crust) near the Aptian–Albian boundary, which is also some 15 million years younger than the oldest proposed oceanic crust (anomaly M5–M3) in the rift.  To investigate and potentially resolve these conflicts, we analysed the tectonic history and deep (pre-Cenomanian) stratigraphy of the rift using seismic reflection profiles and drilling results. Rifting occurred in two main phases (Late Triassic–earliest Jurassic and Late Jurassic–Early Cretaceous). The first phase formed continental rift basins without significant thinning of continental crust. The second phase led to continental breakup, with extension concentrated in three episodes that culminated near the end of Berriasian, Hauterivian and Aptian time. The first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift, respectively, suggesting that the rift opened from south to north in a two-step process. The third episode persisted through Barremian and Aptian time. We suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary, and that elevated in-plane tensile stress throughout the rift caused intraplate extension, primarily within the exhumed mantle. This rifting may have been interrupted for a time during the Barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the Southeast Newfoundland Ridge and J Anomaly Ridge, and the conjugate Madeira–Tore Rise. We propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest Aptian–earliest Albian time, initiating sea-floor spreading. This resulted in relaxation of in-plane tensile stress (i.e. a pulse of relative compression) that caused internal plate deformation and enhanced mass wasting. This ‘Aptian event’ produced a strong, rift-wide reflection that is unconformably onlapped by post-rift sediments that were deposited as a stable sea-floor-spreading regime was established. Although previously considered to be a breakup unconformity associated with separation of continental crust, the event instead marks the final separation of the subcontinental mantle lithosphere. Our analysis indicates that interpretation of tectonic events in a non-volcanic rift must consider the rheology of the full thickness of the continental lithosphere, in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift.

Segmentation and crustal structure of the western Mid‐Atlantic Ridge flank, 25°25′–27°10′N and 0–29 m.y.

We conducted a detailed geological-geophysical survey of the west flank of the Mid-Atlantic Ridge between 25°25′N and 27°10′N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep multibeam bathymetry, HAWAII MR1 sidescan-sonar imagery, gravity, magnetics, and single-channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform, right-stepping discontinuities which are subparallel to flow lines but which migrated independently of one another. Some discontinuities alternated between small right- and left-stepping offsets or exhibited zero offset for up to 3–4 m.y. Despite these changes, the spreading segments have been long-lived and extend 20 m.y. or more across isochrons. A large shift (∼9°) in relative plate motion about 24–22 Ma caused significant changes in segmentation pattern. The nature of this plate-boundary response, together with the persistence of segments through periods of zero offset at their bounding discontinuities, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt. Within segments, there are distinct differences in seafloor depth, morphology, residual mantle Bouguer gravity anomaly, and apparent crustal thickness between inside-corner and outside-corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low-angle, detachment faulting near segment ends. Cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal-thickness changes of at least 1–2 km every 2–3 m.y. These are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes, creating crustal domes analogous to “turtleback” metamorphic core complexes that are formed by low-angle, detachment faulting in subaerial extensional environments.

Development of Cenozoic Abyssal Circulation South of the Greenland-Scotland Ridge

Seismic, lithostratigraphic, faunal, and isotopic evidence from the western and northern North Atlantic indicates that formation of northern sources for strongly circulating bottom water began in the late Eocene to early Oligocene. The widely distributed reflector R4 correlates with an unconformity eroded along basin margins at the Eocene/Oligocene boundary. This change in abyssal regime also correlates with a major benthic foraminiferal turnover in the deep southern Labrador Sea (DSDP Site 112) and with a faunal reorganization in the Bay of Biscay. The principal bottom-water source probably was of Arctic origin; it entered the Norwegian Sea following separation of Greenland and Spitsbergen and flowed south across the Greenland-Scotland Ridge through the Faeroe-Shetland Channel and possibly across a sill east of Greenland. This flow may have been supplemented by dense Arctic water entering the basin via Nares Strait and Baffin Bay, and by cooling and sinking of saline surface water south of the Greenland-Scotland Ridge and in the Labrador Sea. Current-controlled sedimentation and erosion, often of a chaotic nature, continued through the Oligocene above reflector R4, but the general intensity of abyssal circulation is thought to have decreased. Above reflector R2 (upper lower Miocene) current-controlled sedimentation became more coherently developed, and a major phase of sedimentary drift development was initiated. We interpret this to be a result of a further general reduction and especially a stabilization of the abyssal circulation, possibly linked with degeneration of numerous fracture-zone conduits that previously funnelled bottom water across the Reykjanes Ridge. The gross nature of the circulation has not changed substantially since the middle Miocene, although it has been punctuated by further climatic and tectonic events.

Gas-Hydrate Horizons Detected in Seismic-Profiler Data from the Western North Atlantic

Reflecting horizons which have anomalously high amplitude and which are conformable to the seafloor at about 500 to 600 m subbottom have been reported in two locations off the United States east coast--one along the crest of the Blake Outer Ridge, and another beneath the upper continental rise off New Jersey and Delaware. Detailed mapping of these horizons shows that: (1) the horizons cut across bedding planes in the sediment; (2) subbottom depth of the horizons increases with increasing seafloor depth and thus with decreasing seafloor (bottom water) temperature; and (3) the horizons are restricted to areas where sediment strata dip landward; such anomalous horizons are uncommon within the normal seaward-dipping continental-rise strata. Deep-sea drilling into or close to the anomalous horizon on the Blake Outer Ridge (Sites 102, 103, 104) recovered methane-rich sediment. Pressure/temperature conditions within the sediment column in both areas of anomalous horizons are appropriate for formation of gas hydrates to several hundred meters depth, thus suggesting that a zone of gas-hydrates overlies the anomalous horizons. A plot of the hydrate/gas phase transformation of the methane/seawater system in the sedimentary column, using geologically reasonable values for seafloor temperature and for thermal gradient and sound-velocity in the sediment, shows a good correlation between the depth of the phase change and the minimum depth of the anomalous reflecting horizons. The horizons therefore are thought to represent an imped nce contrast caused by the downward change from gas hydrate to gas in the sediment. Landward-dipping strata and the gas-hydrate layers in the areas exhibiting anomalous horizons appear to form traps for free gas, whereby the gas-hydrate layer blocks seaward gas migration and the dipping strata restrict landward migration.

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin

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 <1.3 km and exhibits an unusual, highly reflective layering. We propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep-marine sheet flows to form this layering. Subsequent seafloor spreading formed normal, ∼6-km-thick oceanic crust. This interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow-spreading system was established.

Crustal Evolution of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone in the last 5 Ma

Author Posting.

Kane Fracture Zone

The Kane Fracture Zone probably is better covered by geophysical survey data, acquired both by design and incidentally, than any other fracture zone in the North Atlantic Ocean. We have used this data to map the basement morphology of the fracture zone and the adjacent crust for nearly 5700 km, from near Cape Hatteras to the middle of the Mesozoic magnetic anomalies west of Cap Blanc, northwest Africa. We use the trends of the Kane transform valley and its inactive fracture valley to determine the record of plate-motion changes, and we interpret the basement structural data to examine how the Kane transform evolved in response to changes in plate motion. Prior to about 133 Ma the Kane was a small-offset transform and its fracture valley is structurally expressed only as a shallow ( < 0.5 km) trough. In younger crust, the offset may have increased to as much as 190 km (present offset 150 km) and the fracture valley typically is up to 1.2 km deep. This part of the fracture valley records significant changes in direction of relative plate motion (5°–30°) near 102 Ma, 92 Ma, 59 Ma, 22 Ma, and 17 Ma. Each change corresponds to a major reorganization of plate boundaries in areas around the Atlantic, and the fracture-zone orientation appears to be a sensitive recorder of these events.The Kane transform has exhibited characteristic responses to changes in relative plate motion. Counterclockwise plate-motion changes put the left-lateral transform offset into extension, and the response was for ridge tips at the ridge-transform intersections to propagate across the transform valley and against the truncating lithosphere. Heating of this lithosphere appears to have produced uplift and formation of a well developed transverse ridge that bounds the inactive fracture valley on its older side. The propagating ridge tips also rotated toward the transform fault in response to the local stress field, forming prominent hooked ridges that now extend into or across the inactive fracture valley. Clockwise (compressional) changes in relative plate motion produced none of these features, and the resulting fracture valleys typically have a wide-V shape.The Kane transform experienced severe adaptions to the changes in relative plate motion at about 102 Ma (compressional shift) and 92 Ma (extensional shift), and new transform faults were formed in crust outside the contemporary transform valley. Subsequently, the transform offset has been smaller and the rates of change in plate motion have been more gradual, so transform-fault adjustment has been contained within the transform valley. The fracture-valley structure formed during extensional and compressional changes in relative plate motion can be decidedly asymmetrical in conjugate limbs of the fracture zone. This asymmetry appears to be related to the ‘absolute’ motion of the plate boundary with respect to the asthenosphere.