John C. Mutter

Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise

A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.

A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica

We propose that magnetic anomalies south of Australia and along the conjugate margin of Antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34. The original anomaly identification has two troublesome aspects: (1) it does not account for an “extra” anomaly between anomalies 20 and 21, and (2) it provides no explanation for the rough topography comprising the Diamantina Zone. With our revised identification there is no “extra” anomaly and the Diamantina Zone is attributed to a period of very slow spreading (∼4.5mm/yr half rate) between 90 and 43 m.y. The ages bounding the interval of slow spreading (90 and 43 m.y.) correspond to times of global plate reorganizations. Our revised identification opens up the possibility that part of the magnetic quiet zone south of Australia formed during the Cretaceous long normal polarity interval. Breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P. The “breakup unconformity” identified by Falvey in the Otway Basin may correspond to a eustastic sea level change.

SEISMIC STRUCTURE OF THE SOUTHERN EAST PACIFIC RISE

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.

Origin of seaward-dipping reflectors in oceanic crust off the Norwegian margin by “subaerial sea-floor spreading”

A remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the Norwegian continental margin. It comprises a seaward-dipping complex of reflectors in the form of a wedge. We suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean-basin genesis.

STRUCTURAL PROCESSES AT SLOW-SPREADING RIDGES

Slow-spreading (<35 millimeters per year) mid-ocean ridges are dominated by segmented, asymmetric, rifted depressions like continental rifts. Fast-spreading ridges display symmetric, elevated volcanic edifices that vary in shape and size along axis. Deep earthquakes, major normal faults, and exposures of lower crustal rocks are common only along slow-spreading ridges. These contrasting features suggest that mechanical deformation is far more important in crustal formation at slow-spreading ridges than at fast-spreading ridges. New seismic images suggest that the nature and scale of segmentation of slow-spreading ridges is integral to the deformational process and not to magmatic processes that may control segmentation on fast-spreading ridges.

Variations in thickness of layer 3 dominate oceanic crustal structure

Abstract Seismic refraction studies have determined that, while much of the crust underlying the worlds oceans is about 7 km thick, it also occurs in lesser and considerably greater thicknesses. Here we show that vastly different thicknesses of oceanic crust (formed at ridges by seafloor spreading) are generally characterized by the two-gradient velocity structure recognized as typical of 6–7 km thick oceanic crust. Adopting a standard, we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km. Several structures formed in midplate settings (ocean islands and seamounts) are also considered for comparison. Regardless of origin (ridge or midplate) we find that the percentage of the whole crust formed by Layer 2 systematically decreases with increasing total crustal thickness. While the average velocity of Layer 2 varies widely for all crust, the average velocity of Layer 3 increases systematically with increases in the average whole crustal thickness and velocity. Crust formed in midplate settings differs in that the thickness of Layer 2 velocity crust is generally two to three times that of crust formed at ridges. Residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling. Most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle. The crustal sections were restored to ‘zero-age’, assuming the average depth of emplacement for 7 km thick oceanic crust is 2.5 km below sea level. From this restoration it appears that the depth to the top of Layer 3 at the time it was formed is 4.25 (± 1.25) km below sea level regardless of whole crustal thickness or spreading rate. These results suggest that the mechanism of construction of oceanic crust is, in many ways, remarkably uniform despite very large changes in the total thickness of the crust produced. Very basic questions remain. It is, for instance not at all clear why Layer 2 and Layer 3 exist in the proportions they do. While neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the Layer 2/3 boundary, they do not seem adequate to account for the global phenomenon.

Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second-order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along-strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off-axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long-term reductions in magma supply. Second-order differences in the style of extrusive thickening may be related to structural differences within the low-velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross-axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise

Multichannel seismic reflection data acquired between 8°50′ and 9°50′N and between 12°30′ and 13°30′N along the East Pacific Rise provide a three-dimensional view of the young oceanic crust. Seafloor-to-Moho reflection travel times vary by up to 0.9 s within our study areas; the total range of crustal travel times in the 9°N area is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05 s. The variation is systematic, indicating thinner crust locally associated with overlapping spreading centers (OSCs) and, in the 9°N area, segment-scale variation along crustal isochrons. Crustal travel time is found to be a valid proxy for oceanic crustal thickness. Outside of the axial low-velocity volume, thickness can be calculated from time to ∼500 m. Even in the axial region thickness can be calculated to <1 km, if low-velocity zone position is known. Crustal thicknesses calculated from travel times vary by 2.6 km in the 9°N area, and by 1.5 km in the 13°N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment-scale variation of ∼1.8 km (∼5.5 to 7.3 km thickness) is observed in the 9°N area, with thinnest crust formed between ∼9°40′ and 9°50′N and thickest formed between ∼9°15 and 9°20′N. Results imply a three-dimensional pattern of magma supply to the 9°N segment. The OSC at 9°03′N is associated with major disruptions of the segment-scale pattern, in the form of local thin areas within the discordant zone; the smaller OSC at 12°54′N is not associated with dramatic changes in thickness of the surrounding crust. In the absence of OSCs, the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust-mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. We hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low. Such a diapiric upwelling center appears to be now located beneath the East Pacific Rise near 9°40′ to 9°50′N.

Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17°10′–17°40′S, from a study of layer 2A geometry

Multichannel seismic images from the East Pacific Rise 17°10′–17°40′S are used to study the geometry of seismic layer 2A in the near-axis region. Wave-equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2A event. Processed stacks are compared with seafloor morphology and tectonic fabric imaged in side-scan sonar data. Assuming layer 2A corresponds with the extrusive crust, these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge-crest and ridge-flank morphology. We find variations in axial thickening of layer 2A which imply twofold variation in the width of the extrusive layer accumulation zone, as well as systematic changes in the pattern of accumulation of this layer. On the ridge flanks the layer 2A horizon mimics abyssal hill relief, consistent with a horst and graben origin for this topography. Short-wavelength variations in the thickness of layer 2A are superimposed on this relief, which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone. The integrated off-axis thickness of layer 2A is correlated with ridge cross-sectional area and indicates persistent (100,000 years) spatial gradients in extrusive layer thickness. Systematic changes in the cross-axis shape of the ridge, observed over distances of 5–10 km, cannot be attributed to the extrusive layer, and it appears that axial structure beneath the volcanics governs the cross-axis morphology of the ridge.

Breakup between Australia and Antarctica: a brief review in the light of new data

Abstract The arguments justifying the revised timing of breakup between Australia and Antarctica (Cande and Mutter, 1982) and the reconstruction of Broken Ridge and Kerguelen Plateau (Mutter and Cande, 1983) are reviewed and considered with respect to new subsidence data. The age of breakup was revised from anomaly 22 time (55 My B.P.) to anomaly 34 time (85 My B.P.). The rough topography of the Diamantina Zone can be attributed to very slow spreading (−5 mm/yr.) beginning between the times of anomaly 34 and anomaly 19. The reconstruction of Broken Ridge and Kerguelen Plateau at anomaly 34 time shows overlap of these two features, but the overlap problem is nearly resolved by anomaly 18 time ( ~ 42 My B.P.). Normal seafloor spreading rates (22 mm/yr.) commenced at anomaly 19 time ( ~ 43 My B.P.). Subsidence patterns calculated from biostratigraphic data from wells drilled along Australias southern margin are interpreted as more consistent with the revised age of Australia-Antarctic breakup. Subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally-controlled subsidence during the drift phase. The timing of the rift-to-drift transition is believed to coincide with the age of breakup ( ~ 60 to 110 My B.P.). In addition, the subsidence curves indicate a west-to-east propagation of breakup along the southern margin. Magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis.