Jean-Christophe Sempere

Stochastic analysis of seafloor morphology on the flank of the Southeast Indian Ridge: The influence of ridge morphology on the formation of abyssal hills

In this study we estimate the statistical properties of abyssal hill morphology adjacent to the Southeast Indian Ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate. We explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate. Two cruises aboard the R/V Melville collected Sea Beam 2000 multibeam data along the Southeast Indian Ridge, providing continuous multibeam coverage of the axis from ∼89°W to ∼118°W, and ∼100% coverage within four survey regions extending out to ∼45 km (∼1.2 Ma) from the axis [Sempere et al., 1997; Cochran et al., 1997]. We apply the statistical modeling method of Goff and Jordan [1988] to gridded data from the four survey areas, examining in particular estimates of abyssal hill rms height, characteristic width and length, aspect ratio, and skewness. Two analyses are performed: (1) comparison of the along-axis variation in abyssal hill characteristics to ridge segmentation, and (2) a calculation of population statistics within axial high, intermediate, and axial valley data populations of this study, and comparison of these results to population statistics derived from studies adjacent to the Mid-Atlantic Ridge and East Pacific Rise. We find that abyssal hills generated along axial high mid-ocean ridges are very different from those generated along axial valley mid-ocean ridges, not only with respect to size and shape, but also in their response to such factors as spreading rate and segmentation.

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.

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

Although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical survey of the intermediate spreading rate Southeast Indian Ridge between 88°E and 118°E, a 2300-km-long section of the ridge located between the Amsterdam hot spot and the Australian-Antarctic Discordance where satellite gravity data suggest that the Southeast Indian Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103°30′E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well-developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifted high that is lower and pervasively faulted, sometimes with significant (>50 m throw) faults within a kilometer of the axis. A shallow axial valley ( 1200 m deep) valley across a transform at 114°E. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near-axis ridge flank depth from 2800 m near 88°E to 3300 m near 114°E and by a 50 mGal increase in the regional level of mantle Bouguer gravity anomalies over the same distance. The regional changes in depth and mantle Bouguer anomaly (MBA) gravity can be both explained by a 1.7–2.4 km change in crustal thickness or by a mantle temperature change of 50°C–90°C. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneously and these estimates serve as upper bounds. The along-axis MBA gradient is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103°30′E–104°E and at 114°E–116°E and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them. The change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly. The presence or absence of a quasi-steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have an MBA low located at a depth minimum near the center of the segment. At EPR-like segments, the MBA low is about 10 mGal with along-axis gradients of 0.15–0.25 mGal/km, similar to those observed at the EPR. Rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply. Segments with a shallow axial valley have very flat along-axis MBA profiles with little correspondence between axial depth and axial MBA gravity.

The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid-ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian-Antarctic Discordance (88°E–118°E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69–75 mm yr−1), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid-Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102°45′E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR-like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian-Antarctic Discordance may be explained by the thinning of the oceanic crust by ∼4 km and the change in axial topography. The long-wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian-Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along-axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.

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.

Thermal convection in a vertical permeable slot: Implications for hydrothermal circulation along mid‐ocean ridges

Hydrothermal vents along unsedimented portions of mid-ocean ridges are fed by flow confined to a 1–3 km high percolating system of fissures subparallel to the spreading center. We have solved the coupled thermal and flow equations in such a system. We assume that hydrothermal circulation occurs in a vertical porous slot 1.5 km high, 2.3 km long, and at most 100 m wide. In this model we take into account the heat transported by the hydrothermal flow inside the porous slot as well as the heat conducted through the surrounding impermeable crustal layer. The fluid is free to enter or leave the top of the slot. The fluid enters the slot at a temperature of 2°C, while the bottom of the porous slot is held at a constant temperature of 420°C. We first consider a vertical slot with a horizontal base. Our calculations show the development of tall and narrow unsteady convective cells. Because of the nonlinear relationship of the viscosity and density of seawater with temperature, the upper thermal boundary layer is much thicker than the lower one. The fluid advected in the hot plumes tends to remain confined inside the slot because the upper boundary is thick and viscous. In high Rayleigh number experiments, some very active hot plumes are able to thin and pierce the upper cold boundary layer, and vent hot fluids at the seafloor. The maximum temperature at the exit of these plumes reaches a value at most equal to 270°C. The presence of an axial magma chamber along most of the length of the East Pacific Rise may justify the use of a horizontal base to model hydrothermal systems along fast spreading ridges. However, where the hydrothermal systems develop above a short-lived magmatic intrusion a few kilometers long, the lower interface of the percolating system of fissures is inclined. In that case our calculations show that the convective flow tends to be unicellular and steady state with a wide region of downwelling and a narrow upwelling zone. The temperature of the fluid exiting the slot at the top reaches a maximum value of 360°C. These results lead us to suggest that hydrothermal plumes generated inside a fracture with a horizontal base give rise to vents which wander along the fracture plane with a maximum temperature of about 270°C. However, when the base of the fracture plane is inclined, a large hydrothermal steady plume is generated which can give rise, at its exit, to a black smoker with fluid temperatures in excess of 350°C.

Temporal and spatial variations in crustal accretion along the Mid-Atlantic Ridge (29°–31°30′N) over the last 10 m.y.: Implications from a three-dimensional gravity study

We have conducted a three-dimensional gravity study of the Mid-Atlantic Ridge near the Atlantis Transform to study the evolution of accretionary processes at this slow-spreading center over the last 10 m.y. We have removed from the free-air gravity anomaly the gravity contribution of the density contrast at the seafloor and the gravity contribution of the lateral density variations associated with the cooling of the lithosphere. The resulting residual gravity anomaly exhibits substantial variation along and across the ridge axis. The residual gravity anomaly can be accounted for by variations in crustal thickness of up to 3 km. For the first two segments south of the Atlantis Transform, the midportions of the segments have been associated with thick crust and the segment discontinuities have been associated with thin crust for the last 10 m.y., suggesting the segment discontinuities act as long-term boundaries in the delivery of melt to the individual segments. In contrast, our calculations indicate that for the segments north of the fracture zone, thick crust is associated with the midportions of segments and thin crust is associated with segment discontinuities only in crust less than ∼3 m.y. This result suggests that focused mantle upwelling has only recently developed north of the fracture zone. The onset of focused mantle upwelling at approximately 2–3 m.y. may be related to a change in the spreading direction which occurred between magnetic anomalies 5 and 3 (Figure 1) and resulted in changes in the geometry of the plate boundary north of the fracture zone. Cross sections of crustal thickness extracted along the midpoint traces of paleosegments show that, for a few segments, up to 2 km of gradual crustal thinning is observed. We suggest that the “apparent” crustal thinning is a result of lateral changes in mantle density associated with buoyant upwelling not predicted by the passive flow model used in our study. Variations in computed crustal thickness are observed across axis in all of the paleosegments in our study area, but are not correlated between individual segments. If these computed crustal thickness variations are due to temporal variations in melt production, this implies that there is little interdependence in the amount of melt supplied to adjacent segments.

Detailed study of the Brunhes/Matuyama reversal boundary on the east Pacific rise at 19°30′ S: Implications for crustal emplacement processes at an ultra fast spreading center

We have conducted the first detailed survey of the recording of a geomagnetic reversal at an ultra-fast spreading center. The survey straddles the Brunhes/Matuyama reversal boundary at 19°30′ S on the east flank of the East Pacific Rise (EPR), which spreads at the half rate of 82 mm yr-1. In the vicinity of the reversal boundary, we performed a three-dimensional inversion of the surface magnetic field and two-dimensional inversions of several near-bottom profiles including the effects of bathymetry. The surface inversion solution shows that the polarity transition is sharp and linear, and less than 3–4 km wide. These values constitute an upper bound because the interpretation of marine magnetic anomalies observed at the sea surface is limited to wavelengths greater than 3–4 km. The polarity transition width, which represents the distance over which 90% of the change in polarity occurs, is narrow (1.5–2.1 km) as measured on individual 2-D inversion profiles of near-bottom data. This suggests a crustal zone of accretion only 3.0–4.2 km wide. Our method offers little control on accretionary processes below layer 2B because the pillow and the dike layers in young oceanic crust are by far the most significant contributors to the generation of marine magnetic anomalies. The Deep-Tow instrument package was used to determine in situ the polarity of individual volcanoes and fault scarps in the same area. We were able to make 96 in situ polarity determinations which allowed us to locate the scafloor transition boundary which separates positively and negatively magnetized lava flows. The shift between the inversion transition boundary and the seafloor transition boundary can be used to obtain an estimate of the width of the neovolcanic zone of 4–10 km. This width is significantly larger than the present width of the neovolcanic zone at 19°30′ S as documented from near-bottom bathymetric and photographic data (Bicknell et al., 1987), and also larger than the width of the neovolcanic zone at 21° N on the EPR as inferred by the three-dimensional inversion of near-bottom magnetic data (Macdonald et al., 1983). The eruption of positively magnetized lava flows over negatively magnetized crust from the numerous volcanoes present in the survey area and episodic flooding of the flanks of the ridge axis by extensive outpourings of lava erupting from a particularly robust magma chamber may result in a widened neovolcanic zone. We studied the relationship between spreading rate and polarity transition widths obtained from 2-D inversions of the near-bottom magnetic field over various spreading centers. The mean transition width corrected for the time necessary for the reversal to occur decreases with increasing spreading rate but our data set is still too sparse to draw firm conclusions from these observations. Perhaps more interesting is the fact that the range of the measured transition widths also decreases with spreading rate. In the light of these results, we propose a new model for the spreading rate dependency of polarity transition widths. At slow spreading centers, the zone of dike injection is narrow but the locus of crustal accretion is prone to small lateral shifts depending on the availability of magmatic sources, and the resulting polarity transition widths can be narrow or wide. At intermediate spreading centers, the zone of crustal accretion is narrow and does not shift laterally, which leads to narrower transition widths on the average than at slow spreading centers. An intermediate, or even a slow spreading center, may behave like a fast or hot-spot dominated ridge for short periods of time when its magmatic budget is increased due to melting events in the upper mantle. At fast spreading centers, the zone of dike injection is narrow, but the large magmatic budget of fast spreading centers may result in occasional extensive flows less than a few tens of meters thick from the axis and off-axis volcanic cones. These thin flows will not significantly contribute to the polarity transition widths, which remain narrow, but they may greatly increase the width of the neovolcanic zone. Finally the gabbro layer in the lower section of oceanic crust may also contribute to the observed polarity transition widths but this contribution will only become significant in older oceanic crust (≈50–100 m.y.).

Magnetic properties of some young basalts from the East Pacific Rise

We report the results of a study of the magnetic properties of basalts recovered from the axis and from 0.7 m.y. old crust at 21° N and 19°30′ S on the East Pacific Rise as well as from the 9°03′ N overlapping spreading centers. The natural remanent magnetization of the samples from 21° N and 19°30′ S decreases from the axis to 0.7 m.y. old crust as a result of low-temperature oxidation. In addition, the magnetic properties of the samples from the 21° N sites indicate that: (1) the magnetic susceptibility and the Koenigsberger ratio decrease with low-temperature alteration, (2) the Curie temperature, the median demagnetizing field and the remanent coercivity increase with maghemitization, (3) the saturation magnetization measured at room temperature does not change significantly with age. The magnetic properties of the basalt samples from the 9°03′ N overlapping spreading centers indicate the presence of a high magnetization zone at the tip of the eastern spreading center. This high magnetization zone is the result of the high percentage of unaltered, fine-grained titanomagnetites present in the samples. These measurements are consistent with the results of the three-dimensional inversion of the magnetic field over the 9°03′ N overlapping system [Sempere et al., 1984] as well as with detailed tectonic and geochemical investigations of overlapping spreading centers (Sempere and Macdonald, 1986a; Langmuir et al., 1986; Natland et al., 1986). The high magnetization zone appears to be the result of the eruption of highly fractionated basalts enriched in iron associated with the propagation of one of the limbs of the overlapping system into older lithosphere and not just to rapid decay, due to low-temperature oxidation, of the initially high magnetization of pillows extruded in the neovolcanic zone.

Morphology of the transition from an axial high to a rift valley at the Southeast Indian Ridge and the relation to variations in mantle temperature

The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise-like axial high near 100°E to a Mid-Atlantic Ridge-like rift valley near 116°E but spreads at a nearly constant rate of 74–76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity-derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102°45′E; rift valleys are observed east of a transform fault at 114°E; and an intermediate-style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107° and 108°30′E, forms an exception to the geographical boundaries. Gravity-derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107°E and 108°30′E. The long-wavelength variations in these properties correlate with the above morphological classification. Gravity-derived crustal thickness varies on average ∼2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25°C–50°C, depending on the model used. The average depth of ridge flanks varies by ∼550 m over the study area. For a variation of 25°–50°C, thermal models of the mantle predict depth variations of 75–150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate-style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high-rifted high/intermediate-style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber.