R. van der Voo

A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion

Abstract We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.

PALEOMAGNETISM AND THE OROCLINE HYPOTHESIS

Eldredge, S., Bachtadse, V. and Van der Voo, R., 1985. Paleomagnetism and the orocline hypothesis. In: N.L. Carter and S. Uyeda (Editors), Collision Tectonics: Deformation of Continental Lithosphere. Tectonophysics, 119: 153-179. Oroclines were originally defined by Carey as curved mountain belts which initially were straight. or at feast straighter than they are today. In the last few years, the definition has been broadened to include any curved mountain belt, regardless of its original shape. Since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting. paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt. The arcuate belts investigated are: the Sicilian-Calabr~~n Arc and the Umbrian Arc of Italy. the Swiss portion of the Jura Mountains, the central portion of the Appalachian Mountains (from Pennsylvania to Virginia, U.S.A.). the Wyoming-Idaho overthrust belt of western North America and the Hercynides of Western and Central Europe. The Jura Mountains and the Pennsylvania-Virginia portion of the Appalachians fail to show significant oroclinal bending. ‘The Wyoming-Idaho belt shows a combination of rotated (possibly oroclinai) and unrotated thrust sheets. In the Sicilian-Calabrian Arc significant oroclinat bending caused by the impingement of the Calabria-Peloritani nappes in the Late Tertiary can be demonstrated. while the Umbrian Arc of similar age, in the Northern Apennines, also shows oroclinal bending on a smaller scale. Hercynian Europe (the only belt included in which deformation of basement rocks can be demonstrated) shows oroclinal bending (at least SO”) as well as a marked original curvature (70”) in its western part. Common to all the orocIines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny, causing subsequent deformation and bending of the fold belt.

The formation of an orocline by multiphase deformation: a paleomagnetic investigation of the Cantabria-Asturias Arc (northern Spain)

The Variscan orogeny in the Cantabria‐Asturias Arc (CAA) of northern Spain represents a multiphase deformation history associated with Pangea’s amalgamation. To determine the timing and kinematic history of deformation, a paleomagnetic investigation was carried out on Devonian limestones in three structural domains of the CAA’s hinge zone. Two characteristic remagnetizations in the CAA are distinguished: an Early Permian (B) component that postdates initial Westphalian‐Stephanian aged folding (F1 and F2), and a Late Carboniferous (C) component that postdates F1 deformation. Both the B and C magnetizations experienced a later F3 folding phase. The kinematics and geometry of post-magnetization deformation are determined by bringing measured magnetic directions into agreement with reference directions. These structural corrections also allow separation of deformation events in time. We conclude that generally east‐west-trending, but variably plunging, fold axes characterize F3 folding in the hinge zone of the CAA. This post-magnetization deformation involved significant amounts of tilting and (sub)vertical axis rotation, which together produced clockwise rotation in the north of the arc and counterclockwise rotation in the south of the arc, to form the horseshoe-shaped orocline that is observed today. F3 fold axes change from near-vertical to moderately inclined between and within structural domains, due to the structural fabric imposed by F1 and F2. Based on calculated F3 fold properties, we can reconstruct pre-existing F1 and F2 structural geometries of the hinge zones. This analysis shows that F1 and F2 structures are the result of Late Carboniferous deformation, as part of the same east‐west but temporally discrete compression regimes. On the other hand, F3 is controlled by Permian, north‐south

Structural deformation of the Idaho-Wyoming overthrust belt (U.S.A.), as determined by Triassic paleomagnetism

Grubbs, K.L. and Van der Voo, R., 1976. Structural deformation of the Idaho-Wyoming overthrust belt (U.S.A.), as determined by Triassic paleomagnetism. In: J.C. Briden (editor), Ancient Plate Margins, Tectonophysics, 33: 321-336. A paleomagnetic study of Triassic redbeds from fourteen sites within the IdahoWyoming overthrust belt and three sites from the adjacent stable foreland was undertaken in an attempt to decipher the origin of the arcuate shape of the thrust-fold belt in northwestern Wyoming. This belt rises from beneath the Snake River lava plain in eastern Idaho and trends east-southeast into Wyoming. South of Jackson, Wyoming, the belt swings 45 to 70 degrees to trend almost due south into west-central Wyoming. A total of 193 samples were paleomagnetically analyzed using spinner and cryogenic magnetometers. Thermal demagnetization up to 681” C reveals that the natural remanent magnetization consists characteristically of two directional components. A small secondary (chemical) magnetization is revealed at temperatures below 500” C, whereas a proportionally large characteristic magnetization appears to be primary and conforms to the expected Triassic latitudes of the North American craton. The site-mean directions have Fisher precision h ranging from 14 to 149, with confidence cones of 95%.probability ranging between 4.8” and 16.2”. The characteristic (primary) directions of magnetization indicate that the structural bend in the overthrust belt originated through tectonic rotations of the thrust sheets in the horizontal plane, as the paleomagnetic declinations show a systematic change corresponding to the structural configuration mentioned above. We propose that as overthrusting progressed from west to east in the Late Mesozoic and Early Tertiary, the thrust sheets experienced an increasing resistance to movement due to the proximity of the ancient arcuate foreland margin. The buttressing effect of the foreland hinge on the overthrust plates caused the frontal edges of the thrusts to assume the configuration of the foreland margin resulting in horizontal rotations of the thrust sheets. The easternmost Prospect-Cliff Creek-Jackson thrust sheet came into actual contact with the ancestral Teton-Gros Ventre Precambrian block on the north and the Game Hill reverse fault on the east; as a result the northern sections of the thrust sheet rotated by an amount of up to 60 degrees in a counter-clockwise sense and the southern and eastem sections rotated in a clockwise direction by an amount of up to 30 degrees.

Silurian continental distributions, paleogeography, climatology, and biogeography

Abstract Continental orientations during the Silurian Period have been determined using paleoclimatic in addition to paleomagnetic data. The influence of climate on lithology is particularly marked during periods like the Silurian when epeiric seas were widespread and sedimentation was dominantly autochthonous (evaporites, carbonates, reefs, authigenic minerals) and therefore reflective of climate at the depositional site. During such times, with few large land areas in low latitudes, one would expect climatic patterns to have been more zonal than cellular, and also that long river systems (capable of transporting clastic sediments from wet to dry belts) would not have existed. Therefore, even allochthonous deposits, particularly thick sequences of coarse elastics can be added to the list of paleoclimatic indicators. Silurian northern hemisphere atmospheric circulation can be modeled on present patterns in the southern hemisphere because of the lack of significant land influence on climate. The wet-hot (10°N—10°S), dry-warm (10°—30°), wet-cool (30°-60°), dry-cold (60°—pole) pattern is amply confirmed by Silurian sediment distribution on those paleocontinents whose orientations have been established from paleomagnetic measurements (Laurentia, Baltica, Siberia). Paleozoic sedimentation in these zones is as follows: 10°N—10°S, thick elastics and reefs associated with leading plate margins, and carbonates and reefs in the epeiric seas; 10°—30°, evaporites, carbonates and reefs; 30°—60°, clastics, coals and tillites; 60°—pole mostly tillites. The other paleo-continents (Kazakhstania, North China, South China, Gondwana) can be oriented by using the above lithologic associations in ways consistent with their known convergent and collision patterns in the late Paleozoic. All were in relatively low latitudes with the exception of Gondwana which was over the South Pole. A large north polar ocean existed which must have had an ameliorating effect on climate in the northern hemisphere, while the opposite was true of the southern hemisphere. The conclusion that most paleocontinents had extensive epeiric seas and were positioned in low latitudes accounts for the cosmopolitan nature of Silurian faunas. Only Gondwana in the south ( Clarkeia fauna), and Mongolia in the north ( Tuvaella fauna) shows signs of provincialism and low faunal diversities. This situation can be contrasted with the Devonian, when the collision of Laurentia and Baltica resulted in land barriers and marked provincialism.

Relative hotspot motions versus True Polar Wander

Abstract The fixity of hotspots and mantle plume locations has long been axiomatic. If the assumption of fixed hotspots is granted, ‘absolute’ plate motions and movements of the spin axis with respect to the hotspot framework, defined by some as True Polar Wander (TPW), can be determined. However, this assumption can be tested by paleomagnetic data, and such tests are gradually raising some doubts about the fixity of hotspots. The result is that discrepancies between Cretaceous and Tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle. In the Indo–Atlantic, hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr. However, the Pacific hotspots, notably Hawaii, appear to have undergone large-scale southward drift with respect to the spin axis during the Early Tertiary. Global paleomagnetic data do not indicate that any TPW occurred during the Late Cretaceous or Tertiary. Although the Early Cretaceous paleomagnetic and hotspot frames for the Indo–Atlantic realm can be interpreted as slow TPW, direct estimates of paleolatitude and hotspot motion, in particular the Kerguelen hotspot, challenge TPW as a global phenomenon. At present, we consider that the large Early Cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the Atlantic hotspots prior to ∼95 Ma.

Analysis of Variscan dynamics; early bending of the Cantabria^Asturias Arc, northern Spain

Calcite twinning analysis in the Cantabria^Asturias Arc (CAA) of northern Spain provides a basis for evaluating conditions of Variscan stress and constrains the arc’s structural evolution. Twinning typically occurs during earliest layer-parallel shortening, offering the ability to define early conditions of regional stress. Results from the Somiedo^ Correcilla region are of two kinds: early maximum compressive stress oriented layer-parallel and at high angles to bedding strike (D1c1) and later twin producing compression oriented sub-parallel to strike (D2c1). When all D1 compressions are rotated into a uniform east^west reference orientation, a quite linear, north^south trending fold^ thrust belt results showing a slight deflection of the southern zone to the south^southeast. North^south-directed D2c1 compression was recorded prior to bending of the belt. Calcite twinning data elucidate earliest structural conditions that could not be obtained by other means, whereas the kinematics of arc tightening during D2 is constrained by paleomagnetism. A large and perhaps protracted D2c1 is suggested by our results, as manifested by approximately 50% arc tightening prior to acquisition of paleomagnetic remagnetizations throughout the CAA. Early east^west compression (D1c1) likely resulted from the Ebro^Aquitaine massif docking to Laurussia whereas the north-directed collision of Africa (D2c1) produced clockwise bending in the northern zone, radial folding in the hinge, and rotation of thrusts in the southern zone. fl 2000 Elsevier Science B.V. All rights reserved.

Reply to a comment on ‘‘A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion’’ by G.R. Dickens and J.M. Francis

Contrary to Dickens and Francis’s claim [1] that we ‘challenge the idea of a massive CH4 release during the PETM (Paleocene/Eocene thermal maximum)’, our consideration of an extraterrestrial carbon contribution to the carbon isotope excursion (CIE) is speci¢cally limited to the initial and most rapid decrease in N13C, which accounts for less than half of the full magnitude of the CIE [2]. Thermal dissociation in response to the warming at the PETM is explicitly allowed in our hypothesis, as reiterated in our conclusions that the impact ‘may have triggered a more gradual thermal dissociation of sea£oor methane hydrates’ [2]. We directly challenge only that portion of the hydrate dissociation hypothesis that relies on gradual warming intrinsic to Earth’s climate system as the triggering mechanism [3]. Such a mechanism is not consistent with the documented essentially synchronous and instantaneous warming and decrease in N13C values at the onset of the event [3,4] and is also at odds with the occurrence of the CIE during an interval of low amplitude orbital forcing of climate [5]. Instead, we postulate a comet impact as an explanation for the rapid onset of the event. Dickens and Francis state that the ‘primary di⁄culty with invoking a comet T is that there is no supporting evidence’ and then list four points from our paper that, taken out of context, are construed as damaging to our hypothesis : (1) ‘There is no crater’. If this were to be taken as a fatal problem with hypothesizing an impact then the idea of a K/T (Cretaceous/Tertiary) impact would never have gained any traction ^ it took 10 years to identify the smoking gun at Chicxulub crater [6^8]. In addition, it is hardly ‘contrived’ to acknowledge that the P/E (Paleocene/ Eocene) impact may have occurred on oceanic crust, which constitutes more than half of Earth’s surface area and where impact craters of any age have been very di⁄cult to ¢nd. (2) ‘The remarkable fossil turnovers T strongly contrast to those across the Cretaceous/Tertiary Boundary T.’ In fact, although we noted that the two events are ‘clearly diierent’ [2] and that an a priori assumption that big impacts should be

Near-Laurentian paleogeography of the Lawrence Head volcanics of central Newfoundland, northern Appalachians

Abstract Paleomagnetic analyses were completed on two volcanic units of the Exploits Group in Newfoundlands Central Mobile Belt, which are part of an Ordovician arc-back-arc system. The Tea Arm Volcanics display scattered and unstable characteristic directions that cannot be interpreted. However, stable end-points in six sites supported by great-circle analysis of seven sites in the mid-Arenigian to Llanvirnian Lawrence Head Volcanics yield a tilt- and strike-corrected characteristic direction of D = 56°, I = 23°, (α95 = 19°), k = 14, N = 6). The magnetization is carried by magnetite and passes a tilt test. The corresponding paleolatitude of 12° ± 10° is interpreted as southerly, and is similar to the paleolatitude of 11° ± 4° for arc volcanics of the nearby Moretons Harbour Group. However, this near-Laurentian paleolatitude is distinctly different from the paleolatitude of 31° ± 8° reported for the Roberts Arm-(Cottrells Cove)-Chanceport-Summerford volcanic terrane. The low paleolatitude of the Lawrence Head Volcanics supports the location of a major Ordovician subduction system near the Laurentian margin of Iapetus, whereas the present-day juxtaposition of latitudinally distinct elements in the area represents a complex accretionary history involving Early Silurian or older thrusting and younger strike-slip faulting of several Iapetan island-arc terranes to Laurentia.