V. Courtillot

Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails

Continental flood basalt eruptions have resulted in sudden and massive accumulations of basaltic lavas in excess of any contemporary volcanic processes. The largest flood basalt events mark the earliest volcanic activity of many major hot spots, which are thought to result from deep mantle plumes. The relative volumes of melt and eruption rates of flood basalts and hot spots as well as their temporal and spatial relations can be explained by a model of mantle plume initiation: Flood basalts represent plume heads and hot spots represent continuing magmatism associated with the remaining plume conduit or tail. Continental rifting is not required, although it commonly follows flood basalt volcanism, and flood basalt provinces may occur as a natural consequence of the initiation of hot-spot activity in ocean basins as well as on continents.

Magnetic Field Reversals, Polar Wander, and Core-Mantle Coupling

True polar wander, the shifting of the entire mantle relative to the earths spin axis, has been reanalyzed. Over the last 200 million years, true polar wander has been fast (approximately 5 centimeters per year) most of the time, except for a remarkable standstill from 170 to 110 million years ago. This standstill correlates with a decrease in the reversal frequency of the geomagnetic field and episodes of continental breakup. Conversely, true polar wander is high when reversal frequency increases. It is proposed that intermittent convection modulates the thickness of a thermal boundary layer at the base of the mantle and consequently the core-to-mantle heat flux. Emission of hot thermals from the boundary layer leads to increases in mantle convection and true polar wander. In conjunction, cold thermals released from a boundary layer at the top of the liquid core eventually lead to reversals. Changes in the locations of subduction zones may also affect true polar wander. Exceptional volcanism and mass extinctions at the Cretaceous-Tertiary and Permo-Triassic boundaries may be related to thermals released after two unusually long periods with no magnetic reversals. These environmental catastrophes may therefore be a consequence of thermal and chemical couplings in the earths multilayer heat engine rather than have an extraterrestrial cause.

40Ar/39Ar dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan traps

Abstract There has been an ongoing debate for almost two decades on the age and duration of Deccan trap volcanism. Some of the best available ages have been determined using the 40 Ar/ 39 Ar method, but differences in sample location, laboratory procedure, type of analyzed material, and ages of monitors may have been responsible for much of the apparent spread in the data. We have collected and analyzed samples from close to the bottom and top of the main lava pile in the Western Ghats near Bombay. We confirm previous findings that alteration and Ar recoil strongly affect these rocks, particularly measurements made on whole rocks. When mineral separates (mostly plagioclase) are analyzed, the spread is much reduced. We find a mean age of 65.4±0.7 Ma for five flows near the base of the section (Jahwar and Igatpuri formations) and 65.2±0.4 Ma for a dyke cross-cutting the Poladpur formation, below the topmost Mahabaleshwar formation. This implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval, less than 1 Ma long and close to 65.5 Ma, i.e. close to the Cretaceous–Tertiary boundary. We find that older ages (near 67 Ma) found by several authors are essentially based on whole rock samples, which display disturbed spectra (when this can be checked, i.e. when the spectra are published).

Propagation of an accreting plate boundary: a discussion of new aeromagnetic data in the Gulf of Tadjurah and southern Afar

Abstract A detailed aeromagnetic survey of the Republic of Djibouti and immediate surroundings was performed in 1977. This paper summarizes the reduction techniques which are used in order to produce a magnetic anomaly map and discusses the accuracy of this map, which is presented as an insert at a scale of 1/250,000. Two distinct magnetic styles are recognized: linear anomalies with both large amplitude and short wavelength, considered to be typical of oceanic lithosphere, contrast with areas of lower-amplitude longer-wavelength anomalies, which are found mostly in the northern part of the survey. This quiet zone of subdued magnetic style is thought to have undergone major tectonic deformation in the last millions of years. The general morphology of magnetic anomalies is interpreted in terms of a propagating crack model, as proposed by Courtillot [23]. The crack propagates westwards at approximately 3 cm/yr and the crack tip is thought to lie close to Lake Asal, both on the basis of the magnetic data and of other geophysical evidence. The land section of the survey is a central topic of this paper and is interpreted in terms of the crack propagation model in the light of other available geological, geochemical and geophysical data.

Cosmic markers, 40Ar/39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province

Abstract Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo–French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13–28]. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313–319; Bajpai, J. Geol. Soc. London 157 (2000) 257–260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within uppermost Maastrichtian normal chron C30N at ∼66.5–67 Ma in the Anjar area. Volcanism then stopped at least locally, and lacustrine sediments were deposited over a period that could be in the order of 1–2 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels. This was also responsible for the partial remagnetisation of the underlying flows. These findings generally confirm and complement those of Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424], and are compatible with the occurrence of the K/T impact at the paleontological K/T boundary, and of Deccan trap volcanism straddling the boundary and starting before the impact. Anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors, though still within normal chron C30N. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet. Sci. Lett. 80 (1986) 361–374; Vandamme et al., Rev. Geophys. 29 (1991) 159–190].

Opening of the Gulf of Aden and Afar by progressive tearing

Abstract A new detailed aeromagnetic survey of the Republic of Djibouti and surrounding area reveals a wealth of new information which can be correlated with other data, in particular geologic and tectonic maps. Oceanic magnetic anomalies are identified from the Gulf of Aden westward to the Ghoubbet-Asal rift in Afar. Identification of the anomalies, together with a reinterpretation of earlier magnetic profiles, indicates that rifting started earlier in the east and provides clear evidence for the westward propagation of a crack through the lithosphere at an approximate velocity of 3 cm y−1. The crack tip is now thought to lie somewhere close to lake Asal and should continue its motion further to the northwest. Some first consequences of this non-rigid model of plate opening are discussed and independent support from a number of sources is obtained.

Surface features associated with transform faults: A comparison between observed examples and an experimental model

Abstract In several tectonic provinces where active ridge segments are offset, transform faults are expected but not observed. This paper discusses the evolution of the surface expression of some transform faults with the help of a few geological examples and a simple experimental clay model in which the importance of en-echelon fault systems is assessed. We conclude that the azimuth of observed fault traces may not coincide with the direction of movement, but be oblique to it. Thus we must be cautious when using a fieldobserved fault direction to infer a transform-fault direction for use in plate-tectonics models. This study also suggests the scale at which the assumption of rigid plates fails.

The late 1960's secular variation impulse: Further constraints on deep mantle conductivity☆

Abstract Analysis of geomagnetic secular variation data from world observatories demonstrates the occurrence of a secular variation impulse in the late 1960s. When a simple mantle conductivity model is used in conjunction with the impulse data, it can be shown that the average conductivity of the lower mantle does not exceed 150 ω −1 m −1 . This is orders of magnitude smaller than recently published values.

Core motions, electromagnetic core-mantle coupling and variations in the earth's rotation: New constraints from geomagnetic secular variation impulses

Recently observed secular acceleration impulses (SAI) of the geomagnetic field are interpreted in terms of organized motions of the outer core layers. Such motions have planetary dimensions (5000 km) and a large amplitude (3 × 10−4 m s−1) and are established in very short times (less than one year). The correlation of SAI observed in the Northern Hemisphere with minima in the Earths rotation rate (around 1840, 1905 and 1970) is shown to be consistent with a simple model involving electromagnetic coupling of the weakly conducting (of the order of 100 ω−1 m−1) mantle, of a coherent outer core layer (thickness 100 to a few hundred kilometres) and of the rest of the core. The magnitude of the torque which acts suddenly on both parts of the core at the time of the impulses is estimated.

A model for the evolution of the axial zone of mid-ocean ridges as suggested by icelandic tectonics*

Abstract In Iceland tectonic activity in the neovolcanic zone leads to the formation of three kinds of parallel structures: open fissures, emissive fissures, and normal faults. This observation is used to built a kinematic model which is based on the superposition of lava flows generated from an active central belt of finite width. The results are in good agreement with the recent results in magnetism and tectonic observations of both subaerial and underwater active ridges.