Plate tectonic processes

Dept. Geological Sciences, University of Durham, Durham DH1 3LE, U.K.

g.r.foulger@durham.ac.uk

The ideas described herein arise from several years of brainstorming with Don Anderson, Warren Hamilton, Anders Meibom, Jim Natland, Mike O'Hara, Dean Presnall, Hetu Sheth, Alan Smith, Seth Stein, Peter Vogt and Jerry Winterer.

Later work modelled convection using fluids in tanks and numerical methods and predicted that the arrival at the surface of a new plume would be preceded by 0 – 3 km of uplift, followed by the eruption of a large igneous province (LIP) (representing the “head” of a mushroom-shaped thermal upwelling). The formation of a time-progressive volcanic chain would follow, and was caused by the “mushroom (plume) stem”.

Subsequent work has shown that the volumes of magma produced are highly variable and may be modest, e.g., at the Louisville chain. Volcanic lineations are often not time-progressive, or are intermittently so, e.g., the Line Islands, and “hot spots” are not fixed relative to one another between plates. OIB geochemistry is widespread throughout the Pacific, occurs on normal spreading ridges and in back arc environments, and may be entirely explained as coming from shallow depths. High ³He/⁴He isotope ratios may or may not be observed at “hot spots”.

Evidence for uplift preceeding LIP emplacement is variable (e.g., Deccan and the Siberian Traps), few LIPs have time-progressive volcanic chains emanating from them (e.g., Ontong Java does not), and few chains have LIPs at their oldest end (e.g., the Emperor-Hawaiian chain does not).

Tectonic associations occur at individual “hot spots” that are not predicted or explained by the plume hypothesis, and are not persuasively attributed to coincidence, e.g., the presence of parallel spreading ridges near Easter island and in Iceland, the location of the Yellowstone-Eastern Snake River Plain province between the North American craton and the Basin Range province, and the location of many “hot spots” on spreading ridges or at triple junctions. In short, there is great variability amongst “hot spots”.

The prediction shortcomings and inability of the plume hypothesis to explain auxiliary observations have required immunisation of the theory against the absence of almost every predicted primary characteristic, and a special case for almost every individual “hot spot”. This progressive elaboration of plume theory has reached a point where many scientists feel that plumes have become a data-independent a-priori assumption rather than a valid, testable scientific hypothesis (see also “Plumes, Plates and Popper”). Dissatisfaction with this, and with the attribution of remarkable tectonic associations to coincidence, have led scientists in recent years to question the hypothesis and investigate alternatives.

A plume-like thermal upwelling is expected to rise from a layer that is heated from below - a “thermal boundary layer”. The only such layer known in the deep mantle is the core-mantle boundary and this is thus expected to be the origin of plumes. However, there is little evidence for f) (Dziewonski, 2003), though seismic anomalies extending down to the base of the upper mantle at ~ 660 km are observed beneath some “hot spots” [Ritsema & Allen, 2003]. This has led to the suggestion that plumes rise from the 660-km discontinuity. However, this is a mineralogical phase change and there is no evidence that it is a thermal boundary layer. Such a source location would not anyway satisfy the requirement for approximate relative fixity that motivated the original sourcing of plumes in the “deep mantle” below the level of relatively rapid convection assumed to be associated with plate tectonics. g) is not confirmed by heat flow measurements, and while some petrological evidence has been cited for a hot source below some “hot spots” (e.g., Gudfinnsson & Presnall, 2002), most erupted material is similar to normal mid-ocean ridge basalt (N-MORB) and provides no evidence for a source with elevated temperature. The term “hot spot” thus implies an underlying assumption that is not, as yet, convincingly confirmed, and “hot spots” are more accurately described as volcanic anomalies.

Any alternative hypothesis should be able to explain a) – e), along with the seismic, heat flow and geothermometry observations that bear on structure, flow and temperature beneath volcanic anomalies. Furthermore, if it is to do better than the plume hypothesis an alternative should also be able to explain the variability of volcanic anomalies and the tectonic associations currently attributed to coincidence in the plume model.

The theory of Plate Tectonic Processes (PTP) (also called “Platonics”; see also “If not plumes – what else?”) proposes that volcanic anomalies are “by products” of plate tectonics. The most important elements are:

Simply put, volcanism occurs where lithospheric extension allows melt to leak up to the surface. The location of the volcanism is governed by the stress field in the plate and the amount of melt is governed by the fusibility of the mantle beneath. This theory views volcanism as resulting from lithospheric processes rather than from a heat influx from below, at the core-mantle boundary. It predicts that volcanic anomalies and their geochemistry are shallow sourced and related structures do not extend very deep into the mantle.

Earth’s tectonic plates are in reality, not rigid. They move coherently, but deform internally in response to changes in stress that may result from changes in the plate boundary configuration. The Basin Range Province in the western USA and the East African Rift are examples. Basin Range extension in the western USA onset in the late Cenozoic when the North American plate overrode the East Pacific Rise and the plate boundary there changed from being subduction to transform type (see animations). The East African rift formed at ~ 30 Ma, when the eastern boundary of the African plate changed as a result of a ridge jump in the Indian Ocean [Burke, 1996]. Intraplate deformations are most likely to occur along old sutures or other lines of weakness, e.g., at Yellowstone.

Plate tectonics dehomogenises the mantle by extracting fusible melt at ridges and returning it, in the form of basalt, back into the mantle at subduction zones, along with sub-adjacent melt-depleted abyssal peridotite, lithospheric mantle, accumulated pelagic and terrigenous sediments and volatiles including H₂O and CO₂. Subducting slabs probably do not sink deep into the lower mantle, which is density stratified, and geochemical recycling is thus largely confined to the shallow mantle. Continental collisions trap young, hot, buoyant slabs, back-arc basins, and arc and mantle wedge material in sutures. Subsequent continental breakup often occurs along old sutures and the trapped material, along with delaminated continental mantle lithosphere, is recycled into the mantle beneath the new ocean.

Eclogite – the high-pressure form of basalt, has a much lower solidus and liquidus than peridotite and may be completely molten at temperatures below the peridotite solidus. Even a 30-70 mixture of eclogite and peridotite can produce several times more melt than peridotite alone [Yaxley, 2000]. Subducted slabs warm up to ambient mantle temperatures on a time-scale of tens of millions of years by conduction of heat from the surrounding mantle and radiogenic decay, after which time they may exist in the upper mantle in a state of nascent melting, or even containing a small degree of melt. Slabs may be imbricated or have a steeply dipping aspect, which increases the melt extraction potential at a given location. The tapping of such fertile mantle domains can thus potentially explain the large volumes of melt produced at some volcanic anomalies at temperatures similar to those beneath ridges.

It is unknown how much melt could pond beneath the lithosphere prior to eruption but the huge volumes (up to ~ 6 x 10⁷ km³) and magma production rates (possibly exceeding 6 km³/yr for millions of years) [Coffin & Eldholm, 1994; Gladczenko et al., 1997] of some LIPs are difficult to explain by any mechanism without appealing to significant ponding.

Such source materials can potentially explain OIB geochemistry [e.g., Hofmann & White, 1982]. Compared with MORB, OIB geochemistry is characterised by enrichment in light-rare-earth-elements and incompatible elements such as U and Sr, and isotope ratios indicative of an older source. These features could result from recycling aged subducted oceanic crust or continental lithosphere with their diverse lithologies, associated sediments and mantle wedge material. The wide distribution of OIB geochemistry throughout the Pacfic and its occasional occurrence at ridges, suggests that it may also be widespread at the lithosphere-asthenosphere boundary, where it has been termed the “perisphere” [Anderson, 1995]. Recycling in the shallow mantle is less astonishing than the requirement of plume theory that fusible subducted crust is transported, unmelted, all the way to the 3,000-km-deep core-mantle boundary, where it is swept up in plumes and restored to the surface. High ³He/⁴He isotope ratios may arise from helium that has been stored in an environment low in U+Th relative to [He]. Old dunite-rich cumulates in subducted crust, or mantle residuum could provide such an environment (see Helium Fundamentals page).

How does this model account for the apparent fixity of volcanic anomalies? First, what are the observations? Few long volcanic chains are well dated, the Emperor-Hawaiian chain being perhaps the best. Many other chains are constrained by only a few dates or none. For example, the Iceland volcanic anomaly has no time-progressive track. The available data show that the apparent relative fixity of volcanic anomalies is, to a first order, confined to individual plates or plate boundaries. Some Pacific volcanic anomalies move coherently relative to some on the African plate or its boundary at a rate of ~ 3 cm/year [Raymond et al., 2000; Tarduno & Gee, 1995]. The requirement for this is readily understandable from the fact that a disproportionate number of volcanic anomalies lie on or very close to plate boundaries (e.g., Iceland, Azores, Tristan, Easter, Samoa, Galapagos and Afar), and plate boundaries must migrate with respect to one another since some plates are growing and others shrinking.

PTP theory predicts that volcanic anomalies occur where stress in the lithosphere is extensional. Extensional regions may exist in the interiors of plates as a result of their boundary configurations, e.g., encircling subduction zones or differential lithospheric cooling. They are expected to remain quasi-fixed relative to the overall plate stress field, which will remain relatively constant while the boundary configuration remains stable. In this way, intraplate volcanic anomalies in a single plate may remain approximately fixed relative to one another through their stablilty relative to the overall plate stress field. Time progression then results from motion of the plate relative to its boundary as a whole.

This model does not require that a set of volcanic anomalies on one plate remains fixed relative to a set on another plate. It predicts, however, re-organisations of volcanic anomalies when plate boundary configurations change radically, e.g., when continents collide, subduction zones change into transform faults with changes in plate motion direction, and ridges are subducted. Global plate boundary reorganisation occurred, for example, at the end of the Mesozoic. Gradual changes in plate boundary configuration are predicted to result in gradual evolution of the intraplate stress field and gradual migration of loci of extension. This would be manifest by melt anomalies migrating at speeds different from the plate rate. Stresses induced by local loading may be responsible for local volcanic anomalies [e.g., Hieronymus & Bercovici, 1999] that do not exhibit time progression, large volumes, longevity or fixity, e.g., the Pukapuka ridge. The common occurrence of volcanic anomalies on or close to spreading ridges is predicted, since these are zones of extension.

PTP theory predicts that many aspects of volcanic anomalies will be distributed over a wide spectrum, e.g., the volumes of melt produced, the spatial and temporal distributions of volcanism and the lateral and vertical extents of the sources and underlying structures detectable by seismic tomography. This contrasts with the bimodal distribution predicted by the plume hypothesis which attributes volcanism to two separate, unrelated sources – shallow plate tectonics on the one hand and deep mantle plumes on the other.

PTP theory views magmatic fecundity as being a function of lithospheric extension and mantle fertility. This amounts to a continuum view of Earth in contrast to a bimodal one. Instead of viewing Earth as being surfaced by rigid plates that move relative to one another only at plate boundaries, deformation is recognised to be a continuum with intraplate deformation sometimes exceeding the rates of motion at slow plate boundaries. Instead of assuming Earth’s mantle to be essentially homogenous, it is viewed as having varying physical properties, including composition, fusibility, fertility, temperature and state. Earth is maintained in this state by stresses imposed on the interiors of plates, for example, by plate boundaries whose configurations may change temporally, and by processes at plate boundaries that continually de-homogenise the upper mantle.

A criticism leveled at PTP theory is that it is more complex than simple, elegant plume theory. However, volcanic anomalies are not all simple and alike – there is great variability between them. Hawaii, Yellowstone and Iceland, for example, are fundamentally different in many respects. This variability is problematic to explain in plume theory.

PTP theory offers a unifying theory of volcanism and convection in Earth’s mantle. In the framework of plume theory, two separate, independent modes of convection are proposed – plate tectonics on the one hand, accounting for volcanism at subduction zones and spreading plate boundaries, and plumes on the other, to which intraplate volcanism is attributed. On-ridge volcanic anomalies, with their spectrum of variability from large-volume, long-lived features to small-volume, short-lived phenomena, or even mere isolated ridge segments with slightly more OIB-like geochemistry, sit uneasily in between, along with the plethora of mid-ocean seamounts, many unassociated with time-progressive lineations but nevertheless also capped with OIB. The present theory suggests that there is only one mode of convection in Earth – that driven by plate tectonics, and that this can explain most volcanism on Earth, including that currently attributed to plumes.