Martin C. Kleinrock

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.

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.

Quantitative analysis of abyssal hills in the Atlantic Ocean: A correlation between inferred crustal thickness and extensional faulting

A recent cruise to the Office of Naval Research Atlantic Natural Laboratory obtained ∼100% Hydrosweep bathymetrie coverage, >200% Hawaii MRl (HMRl) side scan coverage, gravity and magnetics over an area spanning three ridge segments along axis (∼25°25′N to ∼27°10′N), and crustal ages from 0 to 26–30 Ma (∼400 km) on the west flank of the Mid-Atlantic Ridge. This data set represents a first opportunity for an extensive regional analysis of abyssal hill morphology created at a slow spreading ridge. The primary purpose of this work is to investigate the relationship between abyssal hill morphology and the properties of the ridge crest at which they were formed. We apply the method of Goff and Jordan [1988] for the estimation of two-dimensional statistical properties of abyssal hill morphology from the gridded Hydrosweep bathymetry. Important abyssal hill parameters derived from this analysis include root-mean-square (rms) height, characteristic width, and plan view aspect ratio. The analysis is partitioned into two substudies: (1) analysis of near-axis (< 7 Ma) abyssal hills for each of the three segments and (2) analysis of temporal variations (∼2–29 Ma) in abyssal hill morphology along the run of the south segment. The results of this analysis are compared and correlated with analysis of the gravity data and preliminary determination of faulting characteristics based on HMRl side scan data. Principal results of this study are: (1) Abyssal hill morphology within the study region is strongly influenced by the inside-outside corner geometry of the mid-ocean ridge segments; abyssal hills originating at inside corners have larger rms height and characteristic width and smaller plan view aspect ratio than those originating at outside corners. (2) The residual mantle Bouguer gravity anomaly is positively correlated with intersegment and along-flow-line variations in rms height and characteristic width, and it is negatively correlated with plan view aspect ratio. From this result, we infer that lower-relief, narrower, and more elongated abyssal hills are produced when the crust being generated is thicker. (3) Intersegment variations in near-axis rms height negatively correlate with average fault density as determined from analysis of HMRl side scan imagery.

Faulting and fault scaling on the median valley floor of the trans-Atlantic geotraverse (TAG) segment, ∼26°N on the Mid-Atlantic Ridge

A quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high-resolution DSL-120 sidescan sonar and coregistered bathymetric data from the TAG segment near 26°N on the Mid-Atlantic Ridge. At this location, faulting is exposed within a 3–5 km wide ridge-parallel zone lying asymmetrically on the eastern half of the median valley floor. Mapped faults have a normal sense of displacement, are <2 km in length, and accommodate ∼1.5% brittle extension. Evidence of fault linkage within the fault population includes kinked and bent fault traces in map view, the development of overlapping fault segments or relay ramps, and the presence of multiple local maxima in the displacement-distance profiles of some faults. Faults have a slight tendency to dip to the east, or outward relative to the valley axis, and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge-parallel line. Faults exhibit a roughly linear relationship between maximum fault throw and fault length, with a mean ratio of 0.030 for the population. Regression of length-frequency data indicates a power-law distribution, with an exponent of 1.64–1.96, demonstrating that fractal populations can exist in the mid-ocean ridge environment. The fractal nature of this length-frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment, where the distribution is exponential and the throw-to-length ratio is ∼5 times lower. These results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and/or fault size.

Detailed morphology of the TAG Active Hydrothermal Mound: Insights into its formation and growth

The detailed structure and morphology of the TAG active hydrothermal mound (Mid-Atlantic Ridge) are investigated using near-bottom, high-resolution bathymetry data and digital photographic imagery. The mound comprises two distinct circular platforms with one superposed asymmetrically on the other. Areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales (years). We propose a model for the growth of the TAG active mound in which changes in the near-surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed.

Long-term denudation of ocean crust in the central North Atlantic Ocean

Near-bottom sidescan, bathymetric, and visual observations of fault scarps in 4–24 Ma sea floor show that basaltic ocean crust undergoes extensive denudation that creates wide, complex scarp zones. Most denudation is caused by mass wasting, probably accompanied by abrasion by disaggregated debris. In 11–24 Ma crust, trellis drainage patterns are formed by deep cross-scarp canyons and intersecting scarp-parallel gullies that appear to follow weak zones of fracturing and intracrustal weathering. Until stabilized (e.g., by sediment burial), fault scarps will continue to degrade with time, yielding increasingly distorted representations of apparent fault geometry in remotely sensed morphological data.

Transform zone migration: Implications of bookshelf faulting at oceanic and Icelandic propagating ridges

At a propagating ridge a migrating transform zone (MTZ) offsets the propagating and retreating spreading axes. We examine several kinematic models of MTZ deformation to determine which models can generate observed seafloor fabric orientations, vertical tectonics, and faulting patterns within the 95.5°W Galapagos propagating ridge (PR) system. Models that involve transform-parallel simple shear within the MTZ can fit observed seafloor fabric rotation patterns within the 95.5°W MTZ. However, these models predict pervasive transform-parallel (strike-slip) faults within the MTZ that are not observed and do not predict the dramatic shear-concurrent deepening and post-shear uplift that is seen in this propagating ridge system. Since we only measure the amount of structural rotation of the abyssal hill fabric within the MTZ and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills, this structural information can only determine the rotational component of the finite strain tensor. Thus this simple shear family of kinematic deformation models is actually only a small subset of MTZ deformation patterns that can produce the same finite rotation patterns, but with different amounts of shortening and/or extension parallel or normal to abyssal hills. There is strong seismic and surface fault-break evidence in Iceland that the South Iceland Seismic Zone MTZ propagates along an array of en echelon, “transform”-perpendicular strike-slip faults. A likely reason for this is that if deformation were accommodated through transform-parallel strike-slip faults, then migration of the transform zone would require the creation of new transform-parallel strike-slip faults for each increment of transform migration. Instead, if transform migration can be accommodated through slip on preexisting faults, then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration. We examine the kinematic consequences of finite MTZ deformation within this generalized “bookshelf faulting” deformation scenario. We find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95.5°W Galapagos PR system. Vertical tectonics are an almost inevitable by-product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation. Thus local extension within the abyssal hills of De Steiguer Deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated. We conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a MTZ since it can explain not only the structural rotations observed at the 95.5°W propagating ridge but can also relate these rotations to the uplift history recorded there.

Evidence for spreading-rate dependence in the displacement-length ratios of abyssal hill faults at mid-ocean ridges

New data from the eastern flank of the Mid-Atlantic Ridge (~25‐27°N) show that slow-spreading abyssal hill faults maintain maximum displacement-length ratios that are systematically greater than those reported on the fast-spreading East Pacific Rise. Lower displacement-length ratios in the fast-spreading environment may reflect the importance of fault linkage (rather than lateral propagation) in determining the lengths of abyssal hill faults and the limited ability of fault systems that evolve within an extremely thin lithosphere to acquire additional displacement during or following linkage.

Reduced crustal magnetization beneath Relict Hydrothermal Mounds: TAG Hydrothermal Field, Mid‐Atlantic Ridge, 26°N

Submersible magnetic profiles over the MIR and ALVIN relict hydrothermal zones of the TAG hydrothermal field on the Mid-Atlantic Ridge at 26°N show magnetic anomalies that are consistent with the presence of reduced crustal magnetization. These magnetic anomalies are similar to the anomaly found associated with the TAG active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones. Magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems.

Fast rift propagation at a slow-spreading ridge

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 ∼10°–40° 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.