Rob Van der Voo

Paleozoic Base Maps

This paper contains 50 maps which have been designed for use by the geologic community in preparing paleogeographic, biogeographic, climatologic, and tectonic reconstructions of the Paleozoic periods. Seven maps for each of seven Paleozoic intervals are included, plus a suture map showing the outlines of the Paleozoic continents in their present positions. The intervals chosen are the Late Cambrian (Franconian), Middle Ordovician (Llandeilian-earliest Caradocian), Middle Silurian (Wenlockian), Early Devonian (Emsian), Early Carboniferous (Visean), Late Carboniferous (Westphalian CD), and Late Permian (Kazanian). The paleomagnetic information used to orient the continents is given. For each interval, three types of maps are included, one locality map with place names labelled, four paleogeographic maps with our interpretation of the distribution of mountains, lowlands, shallow seas, and deep oceans, and two outline maps for those who prefer to make their own paleogeographic interpretations. Several projections are used-Mercator, Mollweide, and stereographic polar-to suit the various requirements of paleogeographic work.

Tethyan subducted slabs under India

Abstract Tomographic imaging of the mantle under Tibet, India and the adjacent Indian Ocean reveals several zones of relatively high P-wave velocities at various depths. Under the Hindu Kush region in northeastern Afghanistan and southern Tajikistan, a regional northward-dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the Indian plate. Under northern Pakistan this same slab shows a roll-over structure with the deeper portion overturned and dipping southward, as can also be seen in the distribution of earthquake hypocenters. Farther east-southeast (e.g., in the vicinity of Nepal), a well-resolved anomaly below 450 km depth is connected to the slab under the Hindu Kush, but seems to be separated from the lithosphere above 350 km. These upper-mantle anomalies are interpreted as the remnants of delaminated sub-continental lithosphere that went down when Greater India continued to converge northward with Asia after ∼45 Ma. The deeper high-velocity anomalies under the Indian sub-continent appear clearly separated from the shallower ones as well as from each other, and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the Indian plate. They occur at depths between 1000 and 2300 km and occasionally descend down to the core-mantle boundary. The anomalies form three parallel WNW-ESE striking zones. We interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the Neo-Tethys Ocean closed between India and Tibet in the Cretaceous and earliest Tertiary. The northern deep-mantle zone under northern Afghanistan, the Himalayas and the Lhasa block in southern Tibet may represent the last-subducted remnant of the Paleo-Tethys Ocean, which is thought to have closed before the Hauterivian stage of the Early Cretaceous. The middle zone continues southeastward as a rather straight high-velocity zone towards Sumatra, where it becomes convex southward and parallel to the subduction zone under the Sunda arc. Comparison of this straight middle zone near India with the shallower (upper 600–1000 km) northern zone, which displays a cusp-like shape near the Yunnan (SW China) Syntaxis of the eastern Himalayas, supports the notion that the shallow northern zone represents later subduction than the deeper middle zone. The suggestion of a counterclockwise rotation (>20°) of the Indian plate during Tertiary indentation of Asia is supported by these features. The present-day latitudes of 5°–35°N of the deep slabs under India and adjacent areas correspond to the approximate paleolatitudes of the Cretaceous subduction zones. The slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey, which implies that lateral movements in the deeper mantle were not large.

The reliability of paleomagnetic data

Abstract A set of seven reliability criteria has been applied to a previously published Phanerozoic paleopole database for Europe and North America and a Late Precambrian data set for Africa. A quality factor (0 ⩽ Q ⩽ 7) is assigned to a result, based on the number of criteria satisfied. Three criteria, dealing with age reliability, structural control and laboratory demagnetization analysis are deemed the most important; for the Phanerozoic results these are satisfied by a large majority of the results, whereas for the majority (up to 80%) of the African Late Precambrian results such criteria are not met. Criteria based on tests that constrain the age of the magnetization, such as those dealing with folds, conglomerates, contacts or reversals, enhance the reliability of a result; for the Phanerozoic, they are generally satisfied by about one third of the data, but for the Precambrian only a few results incorporate such tests. The assertion is made in this study that these criteria indeed qualitatively describe the reliability of results in broad terms, so that a data set satisfying on average most of the criteria (Q ⩾ 4) can be described as more robust than a data set with average Q = 2. Statistical evaluations illustrate the difference in robustness of paleopole data sets between the well-studied Phanerozoic Era and the much more uncertain Late Precambrian.

Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: Comparisons of paleomagnetism with paleoclimatology and biogeographical patterns

New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging.During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counter-clockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.

The Proterozoic supercontinent Rodinia: paleomagnetically derived reconstructions for 1100 to 800 Ma

Abstract Well-dated paleomagnetic poles for the interval 1100–800 Ma have been compiled for the Laurentia, Baltica, Sao Francisco, Congo and Kalahari cratons in order to construct apparent polar wander paths (APWPs) for this interval. Laurentias APWP consists of a well-determined Keweenawan track for 1100–1000 Ma and a 1000–800 Ma Grenville loop. We use a counterclockwise APW loop for the Grenville poles based on ages for post-metamorphic cooling through ∼500°C for the Grenville Province between 1000 and 950 Ma, and the temporal and spatial similarities with Proterozoic counterclockwise APWPs for other cratons. Balticas APWP is comprised of seven dated poles that define a similar loop, counterclockwise and hinged at 950 Ma, that can be superimposed on the Laurentian Grenville loop. This loop is also seen in the seven poles of the APWP for the combined Sao Francisco–Congo craton; superposition of these loops leads to a reconstruction in which the Sao Francisco–Congo craton is to the south-southeast of Laurentia in present-day coordinates. A long 1090–985 Ma APWP track for the Kalahari is in reasonable agreement with the roughly coeval Keweenawan track, when the Kalahari craton is rotated ∼40° counterclockwise away from the Congo craton while remaining hinged at the Zambezi belt. The resulting Rodinia reconstruction resembles those previously proposed on geological grounds for Laurentia, East Gondwana, Baltica, Sao Francisco–Congo, and the Kalahari craton.

Mesozoic subducted slabs under Siberia

Recent results from seismic tomography demonstrate that subducted oceanic lithosphere can be observed globally as slabs of relatively high seismic velocity in the upper as well as lower mantle,. The Asian mantle is no exception, with high-velocity slabs being observed downwards from the west Pacific subduction zones under the Kurile Islands, Japan and farther south, as well as under Asias ancient Tethyan margin. Here we present evidence for the presence of slab remnants of Jurassic age that were subducted when the Mongol–Okhotsk and Kular–Nera oceans closed between Siberia, the combined Mongolia–North China blocks and the Omolon block. We identify these proposed slab remnants in the lower mantle west of Lake Baikal down to depths of at least 2,500 km, where they join what has been interpreted as a ‘graveyard’ of subducted lithosphere at the bottom of the mantle. Our interpretation implies that slab remnants in the mantle can still be recognized some 150 million years or more after they have been subducted and that such structures may be useful in associating geodynamic to surface-tectonic processes.

The assembly of Gondwana 800-550 Ma

Abstract The formation of the supercontinent Gondwana heralded the beginning of the Phanerozoic following a complex series of collisional events after the break-up of earlier supercontinental assemblages. Paleomagnetic data are used to help distinguish between these events and it appears that there are three critical periods of mountain building during Gondwana assembly. The first major orogenic event took place between 800 and 650 Ma and has been termed the East Africa Orogeny. This tectonic episode formed the Mozambique Belt and likely resulted from the collision of India, Madagascar and Sri Lanka with East Africa. The second and third orogenic periods during Gondwana assembly partially overlap in time. The Brasiliano orogeny (600–530 Ma) resulted in the amalgamation of the South American nuclei and Africa. The Kuunga Orogeny was proposed, in part, because of the recent collection of geochronologic data indicating a 550 Ma granulite forming event in East Gondwana and the observation that the apparent polar wander path for Gondwana does not form a spatially and temporally coherent pattern until roughly the same time. The Kuunga orogeny may have resulted from the collision between Australia and Antarctica with the rest of Gondwana.

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China

The Cenozoic intramontane Gonghe–Guide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strike-slip framework of the Kunlun and Altyn Tagh–South Qilian faults in the northeastern Tibetan Plateau. The basin is filled with thick Cenozoic clastic sedimentary formations, which provide important evidence of the deformation of this part of the plateau, although they have long lacked good age constraints. Detailed magnetostratigraphic and paleontologic investigations of five sections in the Guide Basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit (the Guide Group) into six formations (where ages are now magnetostratigraphically well established, they are given in parentheses): the Amigang (1.8–2.6 Ma), Ganjia (2.6–3.6 Ma), and Herjia formations (3.6 to ca. 7.0–7.8 Ma), and the older Miocene Ashigong, Garang, and Guidemen formations. These rocks document a generally upward coarsening sequence, characterized by increasing accumulation rates. Increasing gravel content and sizes of its components, changes of bedding dips and source rock types, and marginal growth faults collectively reflect accelerated deformation and uplift of the NE Tibetan Plateau after 8 Ma, punctuated by a sharp increase in sedimentation rate at ca. 3.2 Ma that reflects the boulder conglomerates of the Ganjia formation. Interestingly, much of the vergence of the compressional deformation in the basin is to the south, accommodated by a sequence of six thrusts (F1–F6), which become active one by one progressively later toward the south, undoubtedly contributing to the uplift of this part of the plateau. F1 likely initiated the Guide Basin due to crustal flexure in the Oligocene, F2 was active in the early Miocene, F4 and F5 at ca. 3.6 Ma, and F6 was active in the early Pleistocene. The detailed late Miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and, hence, uplift of northeastern Tibet, which, when compared with ages for events on other parts of the plateau, provides important boundary conditions for the geodynamical evolution of Tibet.

Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China

Abstract This study provides a detailed magnetostratigraphic record of subsidence in the Linxia Basin, documenting a 27 Myr long sedimentary record from the northeastern edge of the Tibetan Plateau. Deposition in the Linxia Basin began at ∼29 Ma and continued nearly uninterruptedly until ∼1.7 Ma. Increasing rates of subsidence between 29 and 6 Ma in the Linxia Basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to Late Oligocene–Late Miocene time. By Late Miocene–Early Pliocene time, a decrease in subsidence rates in the Linxia Basin associated with thrust faulting and a ∼10° clockwise rotation in the basin indicates that the deformation front of the Tibetan plateau had propagated into the currently deforming region northeast of the plateau.

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous–Early Tertiary apparent polar wander (APW) paths (300–40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous–Early Jurassic and Late Cretaceous–Tertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland’s position is problematic in a Bullard et al. fit, because of a Late Triassic–Early Jurassic regime of compression (>300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland ‘locked’ to Europe for the Late Paleozoic–Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous.