Paul J. Fox

A tectonic model for ridge-transform-ridge plate boundaries: Implications for the structure of oceanic lithosphere

Abstract The first-order geologic and morphologic relationships at, along and proximal to ridge-transform-ridge plate boundaries are used to construct an empirical and speculative tectonic model. The distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere (slowly-slipping ridge-transfrom-ridge plate boundaries) to extremely high rates of strain associated with very thin edges of lithosphere (fast-slipping ridge-transform-ridge plate boundaries). The geometry of a ridge-transform intersection necessitates the juxtaposition of a relatively cold, thick edge of lithosphere against the truncated end of an accreting plate boundary. The cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time. The shallow level manifestation of this cold edge effect is a thinner oceanic crust. At depth, the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere. The ridge-transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis, at some distance from the ridge-transform intersection, to an oblique angle near the boundary. The brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique, dip-slip faults. The model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge. At very slow rates of accretion ( 100 km) juxtapose thick (30–50 km) edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere. In this environment, a relatively strong mantle weld will form at the ridge-transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike-slip tectonism to a relatively narrow and temporally stable principal transform displacement zone. In contrast, at very fast rates of accretion large offset transforms ( >100 km) place relatively thin edges of lithosphere (

The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions

Abstract The axis of the East Pacific Rise (EPR) undulates up and down hundreds of meters over distances of 30–200 km along strike, the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers (OSCs). We have suggested that systematic variations in depth and cross-sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment [1]. A comparison of recently collected multichannel seismic (MCS) data [2] with our Sea Beam and SeaMARC II data have allowed us to test and advance this hypothesis. Along the EPR from 9° to 13°N there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment: the cross-sectional shape of the rise, the presence or absence of an axial summit graben, and the presence or absence of a shallow axial magma chamber (as interpreted from MCS data). Where the axial magma chamber is present, the cross-sectional shape of the ridge is broad and an axial summit graben is recognized along the axis. In contrast, where the cross-sectional shape of the rise is narrow and triangular, an axial magma chamber is not detected and an axial summit graben is absent. These ridge axis characteristics tend to occur along deeper portions of a given ridge segment, often near ridge axis discontinuities. We suggest that these systematic variations in ridge axis morphology (cross-sectional shape) and structure (presence or absence of an axial graben) reflect spatial and temporal variations in the magmatic budget of the ridge axis. Where the magmatic budget is waxing, shallow-level magma reservoirs in the crust and underlying upper mantle swell, creating a broad axial bulge with a summit graben. Where the magmatic budget is diminished, the crustal magma chamber is small ( This proposed correlation of shape, structure and magmatic parameters fails along only two short portions of the ridge. In these areas there is evidence for an axial magma chamber and the rise has a broad cross-sectional shape, but there is no summit graben. Bottom photographs and submersible results, however, show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting, suggesting that the summit graben has been recently filled in by lava flows, and the development of a summit graben (or a linear caldera) by volcano-tectonic collapse has not yet occurred.

The East Pacific Rise and its flanks 8–18° N: History of segmentation, propagation and spreading direction based on SeaMARC II and Sea Beam studies

SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr−1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.There has been a counter-clockwise change in the direction of spreading on the EPR between 8 and 18° N during the last 1 Ma. The cumulative change has been 3°–6°, producing opening across the Orozco and Siqueiros transform faults and closing across the Clipperton transform. The instantaneous present-day Cocos-Pacific pole is located at approximately 38.4° N, 109.5° W with an angular rotation rate of 2.10° m.y.−1 This change in spreading direction explains the predominance of right-stepping discontinuities of orders 2–4 along the Siqueiros-Clipperton and Orozco-Rivera segments, but does not explain other aspects of segmentation which are thought to be linked to patterns of melt supply to the ridge axis.There are 23 significant seamount chains in the mapped area and most are created very near the spreading axis. Nearly all of the seamount chains have trends which fall between the absolute and relative plate motion vectors.

Second-order ridge axis discontinuities in the south Atlantic: Morphology, structure, and evolution

Continuous along-axis Sea Beam coverage of the slow-intermediate spreading (34–38 mm yr−1 full rate) southern Mid-Atlantic Ridge (25°–27°30′S and 31°–38° S) shows that the ridge axis is segmented by both rigid and non-rigid discontinuities. Following the model of Macdonald et al. (1988b), a hierarchy of four orders is proposed for ridge axis discontinuities based on a continuum of relative age and distance offset across the discontinuites. This paper discusses the characteristics associated with five second-order discontinuities found in the areas surveyed. First-order discontinuities represent rigid offsets, transform faults, whereas non-rigid discontinuities fall into the second, third and fourth orders. Like transform fault boundaries, second-order discontinuities have distinctive morphologic signatures both on and off-axis-discordant zones — and therefore are better defined than third- or fourth-order discontinuities. Second-order discontinuities are offsets that range in distance from less than 10 km to approximately 30 km and vary in age offset from 0.5 to approximately 2.0 m.y. The variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike-slip tectonism and that the geometries are unstable in time. Three characteristic geometries are recognized: (1)en echelon jog in the plate boundary where ridge axis tips overlap slightly, (2)en echelon jog in the plate boundary where ridge axes are separated by an extensional basin whose long axis is oriented parallel to the strike of the adjoining ridge axes, and (3) oblique offset characterized by a large extensional basin that is oriented approximately 45° to the strike of the ridge axes. In the case of the third type, evidence for short strands of strike-slip tectonism that link an obliquely oriented extensional basin flanking ridge tips is often apparent. Analysis of the detailed bathymetric and magnetic data collected over the second-order discontinuities and their off axis terrain out to 5–7 m.y. documents that second-order discontinuities can follow several evolutionary paths: they can evolve from transform fault boundaries through prolonged asymmetric spreading, they may migrate along strike leaving a V-shaped wake, and they may remain in approximately the same position but oscillate slightly back and forth. In addition, a small change in the pole of relative motion occurring 4–5 Ma is thought to have resulted in the initiation of at least one second-order discontinuity in the survey area. A geologic model is proposed which involves the interplay of lithospheric thickness, asymmetric spreading, temporal and spatial variability of along-axis magmatic input and changes in the poles of relative motion to explain the origin, morphology and evolution of second-order ridge axis discontinuities.

The geology of the Oceanographer Transform: The ridge-transform intersection

Seven dives in the submersible ALVIN and four deep-towed (ANGUS) camera lowerings have been made at the eastern ridge-transform intersection of the Oceanographer Transform with the axis of the Mid-Atlantic Ridge. These data constrain our understanding of the processes that create and shape the distinctive morphology that is characteristic of slowly-slipping ridge-transform-ridge plate boundaries. Although the geological relationships observed in the rift valley floor in the study area are similar to those reported for the FAMOUS area, we observe a distinct change in the character of the rift valley floor with increasing proximity to the transform. Over a distance of approximately ten kilometers the volcanic constructional terrain becomes increasingly more disrupted by faulting and degraded by mass wasting. Moreover, proximal to the transform boundary, faults with orientations oblique to the trend of the rift valley are recognized. The morphology of the eastern rift valley wall is characterized by inward-facing scarps that are ridge-axis parallel, but the western rift valley wall, adjacent to the active transform zone, is characterized by a complex fault pattern defined by faults exhibiting a wide range of orientations. However, even for transform parallel faults no evidence for strike-slip displacement is observed throughout the study area and evidence for normal (dip-slip) displacement is ubiquitous. Basalts, semi-consolidated sediments (chalks, debris slide deposits) and serpentinized ultramafic rocks are recovered from localities within or proximal to the rift valley. The axis of accretion-principal transform displacement zone intersection is not clearly established, but appears to be located along the E-W trending, southern flank of the deep nodal basin that defines the intersection of the transform valley with the rift floor.

The Mid-atlantic Ridge (31°S–34°30′S): Temporal and spatial variations of accretionary processes

The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr−1, a transitional velocity between the very slow (≤20 mm yr−1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr−1) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies the segment.

Tectonic evolution of ridge-axis discontinuities by the meeting, linking, or self-decapitation of neighboring ridge segments

Recent deep-tow, Sea Beam, and SeaMARC II studies indicate that ridge axis discontinuities along the East Pacific Rise evolve in several different ways. Where magmatic pulses along the spreading center happen to meet head-on, a low point or “saddle point” occurs along the axial depth profile of the spreading center. Where the magmatic pulses misalign, overlapping spreading centers develop and two distinct evolutionary paths are possible. One spreading center tip may cut through to and link with the adjacent en echelon ridge, chopping off the opposing ridge tip, as proposed by Macdonald and Fox (1983). In addition, either spreading center may cut inside or outside of itself, repeatedly decapitating its own ridge tip. Crack-propagation studies show that the crack-propagation force, G , drops significantly when the ratio of crack overlap to crack offset ( L / W ) exceeds 3. Applying this relation to spreading centers, we suggest that propagation of the individual spreading center tips may stall when L / W is greater than 3. If linkage has not yet occurred, then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip, decapitating the ridge tip in a process we call self-decapitation. Subsequent magmatic pulses may be deflected or “derailed” from the existing path of the ridge because freezing of the axial magma chamber near ridge-axis discontinuities creates a core of coherent, unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber. In addition, the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge. For propagating rifts, which represent first-order changes in plate-boundary geometry, L / W remains <1.5 so that G is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced.

Further evidence of contour currents in the Western North Atlantic

A study of compass-oriented sea-floor photographs, echograms and sediment cores on the Atlantic continental margin of North America has been made in order to evaluate the role of deep-sea contour currents in the shaping of the continental rise. The sediments on the upper continental rise consist of lutites which are being deposited on the sea floor in an environment devoid of strong bottom currents. Below an abrupt change in regional slope, that marks the boundary between the upper and lower continental rise, a swift bottom current is observed which flows to the southwest parallel to the contours. Beneath this current the surface sediments are distinctly coarser grained and long cores show many quartz silt laminations in the sedimentary column. Further downslope on the lower continental rise the currents are variable in direction and weaker. Measurements of northerly directions indicate that at certain locations the Gulf Stream may intermittently scour the sea floor. In the area of the Lower Continental Rise Hill a swift southwesterly current is again observed. Hyperbolic echo traces on echograms, prolonged multiple echo sequences, and wedging of sub-bottom reflecting interfaces can be mapped as distinct zones. These zones of sea floor micromorphology parallel the regional contours of the continental rise, and are produced by erosional and depositional processes of bottom currents. We conclude that the continental rise is a large sediment wedge which owes its shape to deep geostrophic contour currents.

Tectonic reconstruction of the Clipperton and Siqueiros Fracture Zones: Evidence and consequences of plate motion change for the last 3 Myr

An electronic circuit is described that adapts the output of a position-sensing detector, designed to determine the position of an incident CW laser beam, to determine the position of a fast pulsing laser incident on the detectors surface.

Tectonics of a fast spreading center: a Deep-Tow and Sea-Beam survey on the East Pacific Rise at 19°30'S

Sea Beam and Deep-Tow were used in a tectonic investigation of the fast-spreading (151 mm yr-1) East Pacific Rise (EPR) at 19°30′ S. Detailed surveys were conducted at the EPR axis and at the Brunhes/Matuyama magnetic reversal boundary, while four long traverses (the longest 96 km) surveyed the rise flanks. Faulting accounts for the vast majority of the relief. Both inward and outward facing fault scarps appear in almost equal numbers, and they form the horsts and grabens which compose the abyssal hills. This mechanism for abyssal hill formation differs from that observed at slow and intermediate spreading rates where abyssal hills are formed by back-tilted inward facing normal faults or by volcanic bow-forms. At 19°30′ S, systematic back tilting of fault blocks is not observed, and volcanic constructional relief is a short wavelength signal (less than a few hundred meters) superimposed upon the dominant faulted structure (wavelength 2–8 km). Active faulting is confined to within approximately 5–8 km of the rise axis. In terms of frequency, more faulting occurs at fast spreading rates than at slow. The half extension rate due to faulting is 4.1 mm yr-1 at 19°30′ S versus 1.6 mm yr-1 in the FAMOUS area on the Mid-Atlantic Ridge (MAR). Both spreading and horizontal extension are asymmetric at 19°30′ S, and both are greater on the east flank of the rise axis. The fault density observed at 19°30′ S is not constant, and zones with very high fault density follow zones with very little faulting. Three mechanisms are proposed which might account for these observations. In the first, faults are buried episodically by massive eruptions which flow more than 5–8 km from the spreading axis, beyond the outer boundary of the active fault zone. This is the least favored mechanism as there is no evidence that lavas which flow that far off axis are sufficiently thick to bury 50–150 m high fault scarps. In the second mechanism, the rate of faulting is reduced during major episodes of volcanism due to changes in the near axis thermal structure associated with swelling of the axial magma chamber. Thus the variation in fault spacing is caused by alternate episodes of faulting and volcanism. In the third mechanism, the rate of faulting may be constant (down to a time scale of decades), but the locus of faulting shifts relative to the axis. A master fault forms near the axis and takes up most of the strain release until the fault or fault set is transported into lithosphere which is sufficiently thick so that the faults become locked. At this point, the locus of faulting shifts to the thinnest, weakest lithosphere near the axis, and the cycle repeats.