Jenny S. Collier

Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250–4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties. Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14° 00′ S is characterized by three 2–4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80–90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Catastrophic flooding origin of shelf valley systems in the English Channel.

Megaflood events involving sudden discharges of exceptionally large volumes of water are rare, but can significantly affect landscape evolution, continental-scale drainage patterns and climate change. It has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the English Channel—the narrow seaway between England and France. This hypothesis has remained untested through lack of direct evidence, and alternative non-catastrophist ideas have been entertained for valley formation. Here we analyse a new regional bathymetric map of part of the English Channel derived from high-resolution sonar data, which shows the morphology of the valley in unprecedented detail. We observe a large bedrock-floored valley that contains a distinct assemblage of landforms, including streamlined islands and longitudinal erosional grooves, which are indicative of large-scale subaerial erosion by high-magnitude water discharges. Our observations support the megaflood model, in which breaching of a rock dam at the Dover Strait instigated catastrophic drainage of a large pro-glacial lake in the southern North Sea basin. We suggest that megaflooding provides an explanation for the permanent isolation of Britain from mainland Europe during interglacial high-sea-level stands, and consequently for patterns of early human colonisation of Britain together with the large-scale reorganization of palaeodrainage in northwest Europe.

A seismic study of lithospheric flexure in the vicinity of Tenerife, Canary Islands

Seismic data have been used to determine the crustal and upper mantle structure of Tenerife, Canary Islands, a volcanic island of Tertiary age located on > 140 Ma oceanic crust. Reflection data show that oceanic basement dips gently towards the island, forming a flexural moat which is infilled by 2–3 km of well stratified material. The moat is characterised by a major angular unconformity, which we attribute to volcanic loading of pre-existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived, at least in part, from the islands. Refraction data show that the flexed oceanic crust has a mean thickness of 6.41 ± 0.42 km and upper and lower crustal velocities of 4.8–5.4 km s−1 and 6.7–7.3 km s−1 respectively. The flexure, which has been verified by gravity modelling, can be explained by a model in which Tenerife and adjacent islands have loaded a lithosphere with a long-term (> 106 yr) elastic thickness of approximately 20 km. Seismic and gravity data suggest that up to 1.5 × 105 km3 of magmatic material has been added to the surface of the flexed oceanic crust which, assuming an age of 6–16 Ma for the shield building stage on Tenerife, implies a magma generation rate of about 0.006 to 0.02 km3 a−1. This rate is similar to estimates from other African oceanic islands (e.g., Reunion and Cape Verdes), but is significantly less than that which has been calculated at Hawaii. There is no evidence in either the seismic or gravity data that any significant amount of magmatic material has “underplated” the flexed oceanic crust. The crustal and upper mantle structure at Tenerife therefore differs from other oceanic islands such as Hawaii and Marquesas where > 4 km of underplated material have been reported.

Seismic mapping of a magma chamber beneath the Valu Fa Ridge, Lau Basin

We analyze a dense grid of seismic normal incidence and wide-angle data collected over an active back arc spreading center in the Lau Basin of the southwest Pacific. Our survey area covers the whole of a 35-km-long morphological segment and a small overlapping spreading center. A bright reflector, which is coincident with a velocity inversion at a depth of 3.2 ± 0.2 km below the seafloor, is observed on every one of the 40 across-axis profiles. We interpret this reflector as being the roof of a crustal magma chamber. The widest magma chamber reflector occurs beneath the overlapping spreading center, where it extends up to 4 km, being imaged beneath both ridges and, in places, beneath the overlap basin. Elsewhere the width of the reflector varies between 0.6 and 2.3 ±0.4 km. The narrowest reflectors are observed beneath deviations (devais) of the ridge axis. In addition to being observed on each of the kilometer-spaced across-axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis. The reflection coefficient of the event is between −0.34 and −0.65. This requires the melt at the top of the chamber to have a P wave velocity less than 2.7 km/s if the interface is planar or less than 3.8 km/s if the interface is layered.

Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50--60-m-thick roof is overlain by a 150--200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

The importance of rift history for volcanic margin formation

Rifting and magmatism are fundamental geological processes that shape the surface of our planet. A relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial. Largely on the basis of detailed observations from the North Atlantic Ocean, mantle temperature was identified as the primary factor controlling magmatic production, with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break-up. However, as more detailed observations have been made at other rifted margins worldwide, the validity of this interpretation and the importance of other factors in controlling break-up style have been much debated. One such observation is from the northwest Indian Ocean, where, despite an unequivocal link between an onshore flood basalt province, continental break-up and a hot-spot track leading to an active ocean island volcano, the associated continental margins show little magmatism. Here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension. Our approach allows us to directly compare break-up magmatism generated at different locations and so isolate the key controlling factors. We show that the volume of rift-related magmatism generated, both in the northwest Indian Ocean and at the better-known North Atlantic margins, depends not only on the mantle temperature but, to a similar degree, on the rift history. The inherited extensional history can either suppress or enhance melt generation, which can explain previously enigmatic observations.

Morphology of the Valu Fa Spreading Ridge in the southern Lau Basin

Multibeam echosounder surveys and seismic reflection profiling were conducted in the southern Lau back arc basin (SW Pacific). The bathymetric survey covered a 130-km-long section of the active spreading ridge (3 cm/yr half spreading rate), the N-S trending Valu Fa Ridge (VFR), while seismic profiles concentrated on a 35-km-long ridge section. Analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber. The ridge shows similarities, but also differences, to the segmentation model developed for mid-ocean ridges. Analogous to this model the section of the VFR studied displays third- and fourth-order segmentation. The amplitudes of the along-axis undulations, however, are a factor of 5 to 10 greater than at mid-ocean ridges. Additionally, the VFR has numerous small volcanic cones along the axis, which have previously only been observed at much slower spreading ridges. In cross section the VFR has a very steep, narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas. The axial magma chamber appears to vary in width (1–4 km) and location relative to the axis beneath different morphological segments. Perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity, a small overlapping spreading center (OSC). This is counter to observations from mid-ocean ridges. Off-axis traces of ridge segments allow us to reconstruct the evolution of this OSC for the past 220 kyr. We conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid-ocean ridges, but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc.

Factors influencing magmatism during continental breakup: New insights from a wide-angle seismic experiment across the conjugate Seychelles-Indian margins

We present a model of the northern Seychelles continental margin derived from controlled source, wide-angle seismic traveltime inversion and teleseismic receiver functions. This margin has been widely cited as a classic example of rifting in association with a continental flood basalt province, the Deccan Traps. However, we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts, such as those of the north Atlantic. The oceanic crust formed immediately after breakup and throughout the first 3 Ma of seafloor spreading is just 5.2 km thick, less than half that typically seen at other volcanic margins. The continent-ocean transition zone is narrow and while two packages of seaward-dipping reflectors are imaged within this transition they are weakly developed. Beneath the thinned continental crust there is an approximately 4 km thick layer of high-velocity material (7.5–7.8 km/s) that we interpret as mafic material intruded and underplating the lower crust. However, we believe that this underplating most likely happened prior to the breakup. Overall the observations show that the rifting of India from the Seychelles was characterized by modest magmatism. The spatial extent of the Deccan flood basalt province is therefore smaller than previously thought. We speculate that either the lateral flow of Deccan-related hot material beneath the breakup region was hampered, perhaps as the rifted margins did not intersect the center of the Deccan source, or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates. If the latter explanation is correct, then the rate of plate separation, as indicated by the initial seafloor-spreading rate, is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized.

Deformation at plate boundaries around the Gulf of Oman

Abstract Seismic studies during Charles Darwin cruise 18/86 focused on three zones of deformation: the Makran accretionary prism, the Murray Ridge-Owen Fracture Zone system, and the continental margin of east Oman. A multichannel seismic reflection profile across the Makran margin clearly imaged the imbricate thrust slices and their bounding faults. A “bottom simulating reflector” (BSR) 500–800 m beneath the seabed, marking the base of a gas hydrate stability zone, may be traced across most of the profile. The heat flow variation estimated from changes in the depth of this reflector shows evidence for localised fluid expulsion. Reflection profiles across the Murray Ridge show evidence for active extension along a series of major normal faults. The rift structure is segmented, with the fault polarity changing between segments. A combined reflection and wide-angle study of the east Oman margin shows a deep offshore basin, underlain by crust only 5 km thick, and bounded by a basement ridge interpreted as a tilted fault block. The wide-angle data suggest a sleeply-dipping Moho beneath the margin. The large lateral changes in crustal thickness may be explained by episodes of margin-parallel shear.

A calibration of the Bagnold beach equation

Abstract A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m −4 s −2 for the calibration coefficient in the Bagnold beach equation.