2016-12-23T18:52:10

[1] Plate tectonics constitutes our primary framework for understanding how the Earth works over geological timescales. High-resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals, mid-ocean ridges, transform faults, and seafloor spreading, cementing the plate tectonic paradigm. However, so-called ''absolute plate motions,'' describing how the fragments of the outer shell of the Earth have moved relative to a reference system such as the Earth's mantle, are still poorly understood. Accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent-ocean distributions change with time. A fundamental problem with deciphering absolute plate motions is that the Earth's rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system. Absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 Ma and ideally would require knowledge about the motions within the convecting mantle. In contrast, models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of Earth's history but cannot provide paleolongitudes because of the axial symmetry of the Earth's magnetic dipole field. We analyze four different reference frames (paleomagnetic, African fixed hot spot, African moving hot spot, and global moving hot spot), discuss their uncertainties, and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a ''hybrid'' model for the time period from the assembly of Pangea ($320 Ma) to the present. For the last 100 Ma we use a moving hot spot reference frame that takes mantle convection into account, and we connect this to a pre -100 Ma global paleomagnetic frame adjusted 5°in longitude to smooth the reference frame transition. Using plate driving force arguments and the mapping of reconstructed large igneous provinces to core-mantle boundary topography, we argue that continental paleolongitudes can be constrained with reasonable confidence.

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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 dam 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.

The Cenozoic intramontane GongheGuide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strikeslip framework of the Kunlun and Altyn Tagh-South Qilian faults in the northeastern Tibetan Plateau. The basin is fi lled 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 fi ve 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 refl ect 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 refl ects 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 fl exure 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.

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This book explains the use and techniques of paleomagnetism to map the movement of major portions of the Earth's surface through time. Written for a geological audience, the initial chapters provide the basic essentials for understanding the value and significance of paleomagnetic results. The later chapters are unique in bringing together the vast amounts of available paleomagnetic data and analyses. This information is integrated with the paleogeography and tectonic movements of the blocks and placed in context with current tectonic hypotheses. A considerable proportion of the present continents are considered – that is the land areas that are now found around the Atlantic and Indian Oceans and in Asia. Also presented is an extensive catalogue of paleomagnetic results for all major continents and the displaced elements that they contain.

Earth's residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core-mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth's surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core-mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth's axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth's mantle may have operated throughout the entire Phanerozoic.plate reconstructions | thermochemical piles T wo equatorial, antipodal, large low shear-wave velocity provinces ( Fig. 1) in the lowermost mantle (1) beneath Africa (termed Tuzo) (2) and the Pacific Ocean (Jason) are prominent in all shear-wave tomographic models (3-7) and have been argued to be related to a dominant degree-2 pattern of mantle convection that has been stable for long times (3). Most reconstructed large igneous provinces and kimberlites over the past 300 My have erupted directly above the margins of Tuzo and Jason, which we term the plume generation zones (1, 2, 5). This remarkable correlation suggests that the two deep mantle structures have been stable for at least 300 My. Stability before Pangea (before 320 Ma) is difficult to test with plate reconstructions because the paleogeography, the longitudinal positions of continents, and the estimates of true polar wander are uncertain (8). It is similarly challenging to reproduce such long-term stability in numerical models (9). However, if the correlation between the eruption sites of large igneous provinces, kimberlites, and the plume generation zones observed for the past 300 Ma has been maintained over the entire Phanerozoic (0-540 Ma), it can provide a crucial constraint for defining the longitudinal positions of continental blocks during Paleozoic time (250-540 Ma).Here we show that a geologically reasonable kinematic model that reconstructs continents in longitude in such a way that large igneous provinces and kimberlites are positioned above the plume generation zones at the times of their formation ( Fig. 2A and SI Appendix, Fig. S2) can be successfully defined f...