Vincent Courtillot

Three distinct types of hotspots in the Earth's mantle

The origin of mantle hotspots is a controversial topic. Only seven (‘primary’) out of 49 hotspots meet criteria aimed at detecting a very deep origin (three in the Pacific, four in the Indo-Atlantic hemisphere). In each hemisphere these move slowly, whereas there has been up to 50 mm/a motion between the two hemispheres prior to 50 Ma ago. This correlates with latitudinal shifts in the Hawaiian and Reunion hotspots, and with a change in true polar wander. We propose that hotspots may come from distinct mantle boundary layers, and that the primary ones trace shifts in quadrupolar convection in the lower mantle.

Revised and synthetic apparent polar wander paths of the African, Eurasian, North American and Indian Plates, and true polar wander since 200 Ma

We have reviewed paleomagnetic data available for the Eurasian, African, North American and Indian plates over the last 200 Ma. Selection criteria are those generally accepted, with an emphasis on evidence for lack of remagnetization, accurate dating and proper structural analysis. This results in 23, 35, 51 and 2 poles for Eurasia, Africa, North America and India, respectively. We believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander (APW) paths than averaging of larger data sets involving less stringent selection criteria. We propose and describe revised APW paths, but more importantly, we next use relative motion models to transfer all data in a common reference frame. We find good agreement between transferred data, when they are averaged separately for each plate in independent 20-Ma windows. This is a check on consistency of paleomagnetic data, kinematic models, and of the geocentric dipole hypothesis. Transferred data from all plates are averaged in 20-Ma windows to generate synthetic APW paths for all plates studied. These synthetic paths are in agreement with the original (revised) APW paths that use only data from a single plate. Moreover, both geographic and time resolution are improved (spatial confidence intervals are of the order of 5°). The synthetic paths display interesting features, such as a previously ill-recognized APW loop for Eurasia. Paleomagnetic and hotspot APW are next compared, and a determination of true polar wander (TPW) is derived. We find significant TPW, amounting to over 20° in the last 200 Ma. TPW appears to be episodic, with a standstill between 180 and 110 Ma. There is general agreement with a previous study of Livermore et al. (1984). The TPW standstill appears to correlate with a time of decreasing reversal frequency, ending with the Cretaceous Long Normal Superchron. Other periods of fast TPW would seem to correspond to increasing reversal frequency. However, it is suggested that the major TPW hairpin at 50 Ma might correspond to the collision of India with Eurasia. Tentative correlations with core or mantle indicators can be understood as a result of couplings between the core, mantle and lithosphere, we believe primarily related to episodic evolution of the D″ layer.

Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present

Paleomagnetic study of China and its environs has been the center of a major international effort for the last 10 years. In this paper, we critically review all available paleomagnetic poles of Upper Permian to Tertiary age from the main blocks of China with the goal of placing constraints on models of the formation and the subsequent deformation of the region. After selecting “reliable” poles by applying objective criteria, we divide our analysis into first-order (motions of blocks) and second order (deformation within blocks). For first order analysis, apparent polar wander paths are constructed for the major blocks. We discuss the compatibilities and contradictions between the geological and paleomagnetic records. A sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented. In general, the major blocks were probably in contact throughout the Permian and Triassic, but the Jurassic was the key age during which most of the movement toward Chinas present configuration took place. Our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events. During the Cretaceous, Chinese poles agree with poles from other continents transferred onto Eurasia. At the second order, we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region, indicating that much of China has been affected by small (< 20°) differential rotations. This we interpret as deformation caused in part by the extrusion of the Chinese blocks away from the Indian collision. The complete annotated list of poles is given as an appendix.

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Abstract Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.

Paleontological view of the ages of the Deccan Traps, the Cretaceous/Tertiary boundary, and the India-Asia collision

Volcanism in the Deccan Traps of India occurred over at most three magnetic chrons, centered on a main reversed chron. Paleontological data indicate that this reversed chron must coincide with 29R, which contains the Cretaceous/Tertiary (K/T) boundary. Recent 40 Ar- 39 Ar data may therefore provide one of the best estimates of the absolute age of the K/T boundary, 65.7 ±2.0 Ma. Moreover, the Eurasian character of K/T boundary terrestrial faunas in India suggests that this was also close to the time of India-Asia collision. This is significantly earlier than generally recognized, and implies about 4000 km of continental shortening in the collision zone, probably absorbed jointly by crustal thickening and strike-slip extrusion.

Tectonic evolution of the Tancheng‐Lujiang (Tan‐Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data

The north-striking Tancheng-Lujiang (Tan-Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2-E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan-Lu during the North China Block (NCB)-SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3-E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North-South China collision and account for Tan-Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling-Dabieshan and Sulu UHP belts while Tan-Lu acted as a transform fault that connected the two subduction zones, or (2) Tan-Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan-Lu. After North and South China impacted near Dabieshan, the Tan-Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan-Lu, with both right lateral strike-slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India-Asia collision.

Propagation of rifting along the Arabia-Somalia plate boundary: The Gulfs of Aden and Tadjoura

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large-scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia-Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia-Somalia plate. The longest pause was at the Shukra-El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia-Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra-El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en echelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large-scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small-scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia-Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea-Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self-sustaining, plate-scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

Paleomagnetism and age determinations of the Deccan Traps (India): Results of a Nagpur-Bombay Traverse and review of earlier work

Review of available radiometric age determinations of the Deccan traps (India) shows a spectrum of K-Ar ages that is highly polluted by argon loss. Stepwise 40Ar-39Ar age determinations include estimates of data quality and thus avoid contaminated results. The absolute age of the Deccan traps determined using 22 40Ar-39Ar plateau age spectra is 65.5 ± 2.5 Ma. Paleontological data on infratrappean and intertrappean sediments constrain Deccan age to between the A. mayaroensis zone, in the Upper Maestrichtian (about 67 Ma), and the P2 foraminifer zone, in the Lower Paleocene (about 60.5 Ma). Paleomagnetic study of a Nagpur-Bombay traverse (preliminary results of which were used by Courtillot et al. (1986a, b) for a general discussion about Deccan volcanism and the Cretaceous-Tertiary boundary) is presented in detail. All available paleomagnetic results from the Deccan traps (563 flows) are then compiled. Results considered to be transitional or to come from suspicious sites are removed leaving 485 flow results. This extensive data set from a single geological unit allowed us to look in some detail at its statistical distribution. The virtual geomagnetic poles (VGP) are approximately Fisher distributed but present a complex asymmetry. No regional variation can be seen (to within paleomagnetic uncertainties). Although the 3.5° angular difference between the separate normal (pole) and reversed (antipole) data is not statistically significant, it can be explained by either a 2.1 m.y. drift along the apparent polar wander path (APWP) of the Indian plate assuming a normal-reverse-normal (N-R-N) magnetostratigraphy, or a 3.5% contamination by a present field overprint, or a slight nondipole field component. A quality coefficient has been assigned to each result on the basis of existence and value of published 95% confidence angle. Because the normal and reversed mean poles become more precisely antipodal with higher-quality data and with more recent publication date (as a consequence of the evolution of paleomagnetic techniques), we favor the overprint hypothesis. The angular standard deviation of our VGP set is 20% larger than the value predicted by the paleosecular variation model of McFadden and McElhinny (1984). Finally, our best estimate of the overall mean pole is located at 281.3°E, 36.9°N, A95 = 2.4° (calculated from the 163 highest-quality flow VGPs). It is in remarkable agreement with the reference APWP of Besse and Courtillot (1991), leading to an independent estimate of the age of Deccan volcanism at 67.2 ± 6.6 Ma. The same comparison following Acton and Gordon (1989) provides an estimated age at 67 ± 5 Ma. These estimates which are based on paleomagnetic data only are in perfect agreement with both radiometric and paleontological ages.

Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar

It is generally accepted that the Aden ridge has propagated westward from ∼58°E to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal-Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW-SE striking subrifts, all of which appear to have propagated northwestward. In Asal-Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia-Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, σ3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from ∼58°E to Asal-Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia-Somalia (or Danakil-Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N-S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en echelon subrifts, and a high-angle, smaller, nested, “subrifts within subrifts.” The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia-Somalia and Arabia-Nubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large-scale bookshelf faulting.

Resolving the problem of shallow magnetizations of Tertiary age in Asia: insights from paleomagnetic data from the Qiangtang, Kunlun, and Qaidam blocks (Tibet, China), and a new hypothesis

We present new paleomagnetic results obtained at 39 sampling sites from five sections of Tertiary red bed formations: two Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8°N, 96.6°E) and the Xining basin of Qaidam (Xining locality; 36.5°N, 102.0°E) and three Neogene formations from the Xining basin (Jungong locality; 34.7°N, 100.7°E) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3°N, 98.6°E and 33.2°N, 96.7°E, respectively). Thermal demagnetization of the rocks isolated a high-temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6°N/352°E (dp/dm = 6.0°/10.7°) for Xialaxiu, 61.6°N/211.3°E (dp/dm = 9.7°/16.1°) for Xining, 66.0°N/228.6°E (dp/dm = 3.6°/6.9°) for Jungong, and 53.9°N/205.4°E (dp/dm = 5.6°/10.0°) for West Yushu. As in previous studies of Tertiary formations from Asia, the inclinations we obtained are shallower (by 18° to 26°) than the magnetic field computed from the Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene rocks and at 40 and 60 Ma for Eocene rocks. On the basis of a compilation of Eocene data from the South China Block, Tibet, central Asia and Kyrgyzstan, we conclude that this inclination anomaly reflects erroneous predictions of positions of the Siberian craton when based on the APWP of Eurasia. The main reason for this discrepancy might be nonrigid behavior of the Eurasian plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Verification of this new interpretation of the “inclination anomaly” will require new geologic and paleomagnetic data from the northern parts of these remote regions in Mongolia and Siberia.