Tucholke Brian E.

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Brian E.

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Record of seamount production and off-axis evolution in the western North Atlantic Ocean, 25°25′–27°10′N

(American Geophysical Union, 2000-02-10) Jaroslow, Gary E. ; Smith, Deborah K. ; Tucholke, Brian E.

Using multibeam bathymetry, we identified 86 axial and 1290 off-axis seamounts (near-circular volcanoes with heights ≥70 m) in an area of 75,000 km2 on the western flank of the Mid-Atlantic Ridge (MAR), 25°25′N to 27°10′N, extending ∼400 km from the inner rift valley floor to ∼29 Ma crust. Our study shows that seamounts are a common morphological feature of the North Atlantic seafloor. Seamount-producing volcanism occurs primarily on the inner rift valley floor, and few, if any, seamounts are formed on the rift valley walls or the ridge flank. The high abundance of off-axis seamounts is consistent with 1–3 km wide sections of oceanic crust being transferred intact from the axial valley to the ridge flank on crust >4 Ma. Significant changes in seamount abundances, sizes, and shapes are attributed to the effects of faulting between ∼0.6 and 2 m.y. off axis in the lower rift valley walls. Few seamounts are completely destroyed by (inward facing) faults, and population abundances are similar to those on axis. However, faulting reduces the characteristic height of the seamount population significantly. In the upper portions of the rift valley, on 2–4 Ma crust, crustal aging processes (sedimentation and mass wasting), together with additional outward facing faults, destroy and degrade a significant number of seamounts. Beyond the crest of the rift mountains (>4 Ma crust) faulting is no longer active, and changes in the off-axis seamount population reflect crustal aging processes as well as temporal changes in seamount production that occurred at the ridge axis. Estimates of population density for off-axis seamounts show a positive correlation to crustal thickness inferred from analysis of gravity data, suggesting that increased seamount production accompanies increased magma input at the ridge axis. We find no systematic variations in seamount population density along isochron within individual ridge segments. Possible explanations are that along-axis production of seamounts is uniform or that seamount production is enhanced in some regions (e.g., segment centers), but many seamounts do not meet our counting criteria because they are masked by younger volcanic eruptions and low-relief flows.
A geological model for the structure of ridge segments in slow spreading ocean crust

(American Geophysical Union, 1994-06-10) Tucholke, Brian E. ; Lin, Jian

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

(American Geophysical Union, 2003-03-08) Fujiwara, Toshiya ; Lin, Jian ; Matsumoto, Takeshi ; Kelemen, Peter B. ; Tucholke, Brian E. ; Casey, John F.

The Mid-Atlantic Ridge around the Fifteen-Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along-axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid-Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high-amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low-amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long-lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.
Acoustic environment of the Hatteras and Nares Abyssal Plains, western North Atlantic Ocean, determined from velocities and physical properties of sediment cores

(Acoustical Society of America, 1980-11) Tucholke, Brian E.

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.
Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

( 2006-03-03) Lau, K. W. Helen ; Louden, Keith E. ; Deemer, Sharon ; Hall, Jeremy ; Hopper, John R. ; Tucholke, Brian E. ; Holbrook, W. Steven ; Larsen, Hans Christian

New multi-channel seismic (MCS) reflection data were collected over a 565km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350-km of the profile: (1) continental crust; (2) transitional basement; (3) oceanic crust. Continental crust thins over a wide zone (~160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastward beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landward by a basement high that may consist of serpentinized peridotite and seaward by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landward of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ~138Ma (Valanginian) in the south (southern Newfoundland Basin) to ~125Ma (Barremian-Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.
Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid-Atlantic Ridge

(American Geophysical Union, 1998-05-10) Tucholke, Brian E. ; Lin, Jian ; Kleinrock, Martin C.

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.
Acoustic environment of the Hatteras and Nares Abyssal Plains, western North Atlantic Ocean, determined from velocities and physical properties of sediment cores

(Woods Hole Oceanographic Institution, 1981-06) Tucholke, Brian E.

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, wholecore 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 (I) 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. Wholecore 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.
Comparison of laboratory and in situ compressional-wave velocity measurements on sediment cores from the western North Atlantic

(American Geophysical Union, 1979-02-10) Tucholke, Brian E. ; Shirley, Donald J.

Laboratory and in situ velocity measurements have been made on six piston cores taken in the western North Atlantic Ocean. Sediments from the southwestern Bermuda Rise and Greater Antilles Outer Ridge are clays having velocities ranging mostly from 1500 to 1530 m/s and velocity gradients near 1 s−1. In cores from the Nares Abyssal Plain, the clayey sediments have comparable velocities, but interbedded silty turbidites exhibit much higher values (up to 1690 m/s). Velocity gradients are slightly higher in the abyssal-plain cores. After the laboratory measurements are corrected to in situ conditions, they show reasonable agreement in average velocity and velocity gradient with in situ measurements, although the in situ velocities average 10–12 m/s higher in the clayey cores and 15–20 m/s higher in the turbidites. This difference may be caused by reduction in the dynamic frame bulk modulus and/or the dynamic shear modulus due to visually undetected coring disturbance. The profilometer used to obtain the in situ measurements does not record the fine-scale variations in velocity that were measured in the laboratory, but it accurately determines average velocities and velocity gradient. Where cores were closely spaced (2–12 km apart), inter-core correlations in lithology, velocity, and bulk properties are possible. Fluctuations in the latter two parameters are very similar in position and magnitude from core to core, suggesting either that effects of coring disturbance are small or that they are uniform in a given kind of sedimentary bed. Inter-core comparison also shows that some beds are laterally discontinuous as a result of local (less than a few kilometers) patterns of seafloor erosion and deposition.
Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

(American Geophysical Union, 2006-11-17) Van Avendonk, Harm J. A. ; Holbrook, W. Steven ; Nunes, Gregory T. ; Shillington, Donna J. ; Tucholke, Brian E. ; Louden, Keith E. ; Larsen, Hans Christian ; Hopper, John R.

We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide-angle data recorded on a linear array of 24 ocean-bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35-km-thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ~6 km thickness. In the distal margin we find a 60-km-wide zone with seismic velocities between 5.0 and 6.5 km/s that thins to the southeast from 6 km to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km/s and higher beneath this thin crustal layer in the continent-ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater, or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km-wide zone where seismic velocities increase gradually from 6.3 km/s just below the top of acoustic basement to 7.7 km/s at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seawards, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km/s in basement ridges to 7.0 km/s at its base, overlying mantle velocities between 7.8 and 8.2 km/s. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magmastarved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent-ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.
Evidence for age and evolution of Corner Seamounts and Great Meteor Seamount Chain from multibeam bathymetry

(American Geophysical Union, 1990-10-10) Tucholke, Brian E. ; Smoot, N. Christian

The Comer seamounts in the western North Atlantic and Great Meteor seamount “chain” in the eastern North Atlantic are thought to progress in age from Late Cretaceous through late Cenozoic. They both presumably formed by volcanism above the New England hotspot when first the North American plate, and then the Mid-Atlantic Ridge axis and African plate, moved over the hotspot. High-resolution, multibeam bathymetry of the seamounts shows geomorphic features such as guyots, terraces, and a base level plateau (Cruiser plateau) that we interpret to have formed at sea level. We have backtracked these features to sea level along the North Atlantic crustal age-depth curve in order to estimate their ages. The derived age pattern of volcanism indicates formation of the Comer seamounts at ca. 80 Ma to 76 Ma, with migration of the Mid-Atlantic Ridge plate boundary over the hotspot and formation of the Cruiser plateau about 76 Ma. Seamount ages suggest that subsequent volcanism on the African plate moved first northward, in the Late Cretaceous to early Cenozoic (Plato, Tyro, and Atlantis seamount groups), then southward to Great Meteor Seamount in the late Cenozoic. Recurrent volcanism appears to have occurred at some seamounts up to 20–30 m.y. after their initial passage over the hotspot. It would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away.
Segmentation and crustal structure of the western Mid-Atlantic Ridge flank, 25°25′–27°10′N and 0–29 m.y.

(American Geophysical Union, 1997-05-10) Tucholke, Brian E. ; Lin, Jian ; Kleinrock, Martin C. ; Tivey, Maurice A. ; Reed, Thomas B. ; Goff, John A. ; Jaroslow, Gary E.

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.
Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic

(Elsevier, 2017-01-10) Gardner, Wilford D. ; Tucholke, Brian E. ; Richardson, Mary Josephine ; Biscaye, Pierre

Benthic storms are episodic periods of strong abyssal currents and intense, benthic nepheloid (turbid) layer development. In order to interpret the driving forces that create and sustain these storms, we synthesize measurements of deep ocean currents, nephelometer-based particulate matter (PM) concentrations, and seafloor time-series photographs collected during several science programs that spanned two decades in the western North Atlantic. Benthic storms occurred in areas with high sea-surface eddy kinetic energy, and they most frequently occurred beneath the meandering Gulf Stream or its associated rings, which generate deep cyclones, anticyclones, and/or topographic waves; these create currents with sufficient bed-shear stress to erode and resuspend sediment, thus initiating or enhancing benthic storms. Occasionally, strong currents do not correspond with large increases in PM concentrations, suggesting that easily erodible sediment was previously swept away. Periods of moderate to low currents associated with high PM concentrations are also observed; these are interpreted as advection of PM delivered as storm tails from distal storm events. Outside of areas with high surface and deep eddy kinetic energy, benthic nepheloid layers are weak to non-existent, indicating that benthic storms are necessary to create and maintain strong nepheloid layers. Origins and intensities of benthic storms are best identified using a combination of time-series measurements of bottom currents, PM concentration, and bottom photographs, and these should be coupled with water-column and surface-circulation data to better interpret the specific relations between shallow and deep circulation patterns. Understanding the generation of benthic nepheloid layers is necessary in order to properly interpret PM distribution and its influence on global biogeochemistry.
Magnetization of 0–29 Ma ocean crust on the Mid-Atlantic Ridge, 25°30′ to 27°10′N

(American Geophysical Union, 1998-08-10) Tivey, Maurice A. ; Tucholke, Brian E.

A sea-surface magnetic survey over the west flank of the Mid-Atlantic Ridge from 0 to 29 Ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust. We find that magnetization decays rapidly within the first few million years, although the filtering effect of water depth on the sea-surface data and the slow spreading rate (<13 km/m.y.) preclude us from resolving this decay rate. A distinctly asymmetric, along-axis pattern of crustal magnetization is rapidly attenuated off-axis, suggesting that magnetization dominated by extrusive lavas on-axis is reduced off-axis to a background value. Off-axis, we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust, indicative of induced magnetization in thin inside corner (IC) crust. In general, we find that off-axis segment ends show an induced magnetization component regardless of polarity and that IC segment ends tend to have slightly more induced component compared with outside corner (OC) segment ends, possibly due to serpentinized uppermost mantle at IC ends. We find that remanent magnetization is also reduced at segment ends, but there is no correlation with inside or outside corner crust, even though they have very different crustal thicknesses. This indicates that remanent magnetization off-axis is independent of crustal thickness, bulk composition, and the presence or absence of extrusives. Remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in OC crust and a combination of crustal thinning and alteration in IC crust. From all these observations, we infer that the remanent magnetization of extrusive crust is strongly attenuated off-axis, and that magnetization of the lower crust may be the dominant source for off-axis magnetic anomalies.
The western boundary undercurrent as a turbidity maximum over the Puerto Rico Trench

(American Geophysical Union, 1974-09-20) Tucholke, Brian E. ; Eittreim, Stephen

Nephelometer measurements in the Puerto Rico trench record a midwater light scattering maximum at the depth of the near-bottom nepheloid layer found in the deep Atlantic basin to the northwest. This midwater maximum is best developed near the south slope of the trench and is interpreted as a southeasterly continuation of the western boundary undercurrent, which has been documented along the continental rise of eastern North America. The eastward-advecting core of the flow overrides clearer colder antarctic bottom water that enters the trench from the east. A near-bottom nepheloid layer, best developed in the eastern part of the trench, appears to be associated with the westward-flowing antarctic bottom current.
Problematic plate reconstruction

( 2012-10) Tucholke, Brian E. ; Sibuet, Jean-Claude

As has been previously proposed, Bronner et al. suggest that opening of the rift between Newfoundland and Iberia involved exhumation of mantle rocks until 112 million years ago, subsequent seafloor spreading, and crustal thickening along the high-amplitude J magnetic anomaly by magma that propagated from the Southeast Newfoundland Ridge area. Conventionally, the anomalous magnetism and basement ridges associated with the J anomaly north of the Newfoundland-Gibraltar Fracture Zone are thought to have formed about 125 million years ago at chron M0 (Fig. 1a), although the crust probably experienced some later magmatic overprinting. The M0 age would make their formation simultaneous with that of the similar J anomaly and basement ridges (the J Anomaly Ridge and Madeira Tore Rise) along the Mid-Atlantic Ridge to the south and place them within a zone of exhumed mantle in the Newfoundland-Iberia rift. In contrast, Bronner et al. propose that the J anomaly and associated basement ridges were formed by later magmatism (about 112 million years ago) that marked the end of mantle exhumation in the rift. We argue here that constraints from plate tectonic reconstructions render this possibility untenable.
The history of sedimentation and abyssal circulation on the Greater Antilles outer ridge

(Woods Hole Oceanographic Institution, 1974-01) Tucholke, Brian E.

The Greater Antilles Outer Ridge is an 1800 km long, submarine sedimentary ridge which lies below 5000 m in the southwestern North Atlantic Ocean. Seismic reflection profiles and core data indicate that the ridge is composed of more than 6 x 104 km3 of acoustically transparent sediment which has accumulated above a sequence of acoustically stratified sediments deposited before late Eocene time. The sediments consist of low-carbonate, homogeneous, terrigenous lutites which have accumulated at rates of up to 30 cm/1000 yr since the middle Eocene. Clay-mineral analyses indicate that the chlorite-enriched sediment is derived from the northeastern continental margin of North America. Abyssal contour-following currents which flow around the Greater Antilles Outer Ridge are interpreted as an extension of the Western Boundary Undercurrent (WBUC) found along the continental rise of eastern North America. This current system is proposed to be the agent which has transported sediment southward for more than 2500 km and deposited it on the Greater Antilles Outer Ridge. Sediment is presently carried in concentrations up to 65 ug/liter in the currents flowing around the outer ridge, and mineral analyses show that the suspended sediment has a northern provenance; it is similar in composition to the bottom sediment and is interpreted as the source of sediment deposited on the Greater Antilles Outer Ridge. The Puerto Rico Trench began to form in middle Eocene time, and it cut off direct downslope sedimentation to the Greater Antilles Outer Ridge. At the same time, the newly formed WBUC interacted with existing sea-floor topography and the Antarctic Bottom Water (AABW) flowing in from the South Atlantic, and it began to deposit acoustically transparent sediment on the eastern outer ridge. This depositional pattern persisted until the middle or late Miocene, when increased AABW flow diverted the WBUC to the northwest and initiated deposition of the western sector of the Greater Antilles Outer Ridge. Shortly thereafter, decreased AABW flow and lower current speeds allowed rapid deposition of sediment on the Greater Antilles Outer Ridge and on the Caicos Outer Ridge to the west. The bottom topography has controlled the abyssal current pattern, and current-controlled deposition has continued to construct the Greater Antilles Outer Ridge since early Pliocene time.
Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin

(American Geophysical Union, 2006-09-22) Shillington, Donna J. ; Holbrook, W. Steven ; Van Avendonk, Harm J. A. ; Tucholke, Brian E. ; Hopper, John R. ; Louden, Keith E. ; Larsen, Hans Christian ; Nunes, Gregory T.

Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: (1) extended continental crust, (2) transitional basement, and (3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first-order differences between this margin and its well-studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: (1) slow spreading oceanic basement produced by unstable early seafloor spreading, (2) exhumed, serpentinized mantle with different properties from that off Iberia, and (3) thinned continental crust, likely emplaced by one or more detachment or rolling-hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide-angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.
Submersible study of an oceanic megamullion in the central North Atlantic

(American Geophysical Union, 2001-08-10) Tucholke, Brian E. ; Fujioka, Kantaro ; Ishihara, Takemi ; Hirth, Greg ; Kinoshita, Masataka

Recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long-lived detachment faults operating near the ends of spreading segments in slow spreading crust. We conducted five submersible dives on one of these features just east of the rift valley in the Mid-Atlantic Ridge at 26°35′N and obtained visual, rock sample, gravity, and heat flow data along a transect from the breakaway zone (where the fault is interpreted to have first nucleated in ∼2.0–2.2 Ma crust) westward to near the termination (∼0.7 Ma). Our observations are consistent with the detachment fault hypothesis and show the following features. In the breakaway zone, faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone; the youngest normal faults in this sequence are interpreted to have evolved into the long-lived detachment fault. In younger crust the interpreted detachment surface rises as monotonously flat seafloor in a pair of broad, gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment. The detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall. The domes coincide with a gravity high that continues along isochrons within the spreading segment. Modeling of on-bottom gravity measurements and recovery of serpentinites imply that mantle rises steeply and is exposed within ∼7 km west of the breakaway but that rocks with intermediate densities prevail farther west. Within ∼5 km of the termination, small volcanic cones appear on the detachment surface, indicating melt input into the footwall. We interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment, with mantle being exhumed by the detachment fault <0.5 m.y. after its initiation. Increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between ∼1.3 m.y. and 0.7 m.y. Identifiable but low-amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component. We infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes.
Spatial and temporal variations in crustal production at the Mid-Atlantic Ridge, 25°N–27°30′N and 0–27 Ma

(John Wiley & Sons, 2015-04-21) Wang, Tingting ; Tucholke, Brian E. ; Lin, Jian

We use high-resolution multibeam bathymetry, shipboard gravity, side-scan sonar images, and magnetic anomaly data collected on conjugate flanks of the Mid-Atlantic Ridge at 25°N–27°30′N and out to ~27 Ma crust to investigate the crustal evolution of the ridge. Substantial variations in crustal structure and thickness are observed both along and across isochrons. Along isochrons within spreading segments, there are distinct differences in seafloor morphology and gravity-derived crustal thickness between inside and outside corners. Inside corners are associated with shallow depths, thin crust, and enhanced normal faulting while outside corners have greater depths, thicker crust, and more limited faulting. Across-isochrons, systematic variations in crustal thickness are observed at two different timescales, one at ~2–3 Myr and another at >10 Myr, and these are attributed to temporal changes in melt supply at the ridge axis. The shorter-term variations mostly are in-phase between conjugate ridge flanks, although the actual crustal thickness can be significantly different on the two flanks at any given time. We observe no correlation between crustal thickness and spreading rate. Thus, during periods of low melt supply, tectonic extension must increase to accommodate the full plate separation rate. This extension commonly is concentrated in long-lived faults on only one side of the axial valley, resulting in strong across-axis asymmetries in crustal thickness and seafloor morphology. The thin-crust flank has few volcanic features and exhibits elevated, blocky topography with large-offset, often irregular faults, while the conjugate thicker-crust flank shows shorter-offset, regular faulting, and common volcanic features. The variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle.
Correction to “Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin”

(American Geophysical Union, 2006-12-09) Shillington, Donna J. ; Holbrook, W. Steven ; Van Avendonk, Harm J. A. ; Tucholke, Brian E. ; Hopper, John R. ; Louden, Keith E. ; Larsen, Hans Christian ; Nunes, Gregory T.

Tucholke Brian E.

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