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1. Paleomagnetism
As the tool that led to the confirmation of the theory of continental drift,paleomagnetic studies have formed the core of paleocontinental reconstructions. The basic principle of this approach utilizes the paleomagnetic determination of the location of the rotational axis of the Earth with respect to a continent at the time at which the rocks acquired their magnetic fabric, the Geocentric Axial Dipole hypothesis ( GAD ) . Such information is acquired by collecting oriented rock samples and then measuring the orientation of aligned remanent magnetic fields of mineral particles within the samples. It is accepted as a working hypothesis that the orientation of the remanent magnetic fields within the sample reflects the GAD at the time at which the rock formed,for example,the time at which an igneous or metamorphic rock cooled or the time at which a sediment became lithified to form a rock. The determination of the significance of a magnetization with respect to the crystallization and / or depositional age of a rock is one of the sources of potential uncertainty in the paleomagnetic technique and paleomagneticists have established a host of criteria and tests that are used to evaluate the viability of a particular set of measurements and the significance of the magnetization. These tests are particularly important as overprints and alteration of the original magnetizations can result in spurious and misleading results,particularly in regions that have been affected by superposed tectonic events such as convergent plate margins. An important component of the magnetic fabric is the tilt or inclination of the magnetization relative to a horizontal datum. This is used to infer the paleolatitudinal position of the rock sample at the time of acquisition of magnetization. This too is subject to some uncertainty as a consequence of uncertainties in assignment of proper hemisphere ( northern or southern) and in correcting for paleohorizontal,which generally is not an issue in sedimentary or layered igneous rocks such as lava flows,but becomes problematic in paleomagnetic studies of rocks from plutonic complexes. Recent approaches have attempted to use barometry of the metamorphic aureoles developed around plutons as a means of determining tilt. When,well- defined,chronologically constrained magnetizations can be measured,paleomagnetic data are used to construct the paleopole positions for continents through time and define apparent polar wander paths which allow for comparison of the positions of different continents through time with respect to a common rotational axis. The latter technique places continental motions in a reference frame that allows for quick comparison of relative motions and determination of drift histories. For example,continents that were separate and then collided should have different APWs ( apparent polar wander paths) prior to collision and similarly,continents that were formerly together and have rifted apart should have similar pre-rift APWs. Such a technique allows for the determination of the timing of paleocontinental drift through time and the generation of paleogeographic maps that are well-advanced for most of the Phanerozoic. Such maps are becoming better constrained as the sensitivity of laboratory techniques has increased such that even very weak magnetizations can be measured,making a much wider range of rock types available for magnetic analysis and study. This has lead to a much expanded and improved database from which to test plate reconstructions. The paleomagnetic determination of paleolatitude through time also offers an important framework constraint on climate and climate change. Paleomagnetic studies offer control on recognizing climactic forcing as a consequence of latitudinal drift as distinct from global cooling climate events that may be relatively independent of latitude.
There are drawbacks in the paleomagnetic determination of continental configurations that result both from inherent limitations in the resolving power of the data as well as complications introduced by secondary magnetizations. For example, paleomagnetic data are incapable of determining longitude so that an iterative process that evaluates continental reconstructions from the perspective of continental overlap and the assumption of minimum movement are used to resolve the most likely plate configurations. Ambiguity of primary magnetic inclination ( paleolatitude) can arise when the inclination of the original magnetization is changed by tectonic tilting. Consequently,such ambiguities in the paleomagnetic analysis of rocks have led to some of the most enduring ( and evolving) controversies in the geosciences. An additional complication in paleomagnetic analysis arises from overprinting, whereby a rock acquires a secondary magnetization related to a younger magnetization event. This can be a result of metamorphism ( regional or burial) ,proximity to an igneous intrusion,or the passage of fluids at elevated temperatures. The latter process is now widely recognized as an important agent of remagnetization,particularly in sedimentary strata within or adjacent to orogenic belts. While such remagnetizations tend to obliterate or complicate the recognition of primary magnetizations, they are an important means of mapping the distribution and timing of regional fluid events. Determining reliable primary paleopoles and magnetizations in older rocks,particularly those that are Precambrian in age,is difficult due to the longer time available for secondary magnetization events and deformation, as well as the limited chronologic resolution in nonfossiliferous sedimentary rocks. Resolving the absolute age of a magnetic imprint in a rock is a problem that is not limited to the Precambrian. It also becomes an issue in regions in which the rocks may have been remagnetized due to processes that affected them after their formation.
2. Biogeography and faunal links
One of the data sets that first suggested former proximity of now widely separated continental terranes resulted from recognition of faunal similarities between fossil assemblages on disparate continental blocks. Such studies are classic examples that were noted well before paleomagnetic techniques had been developed to confirm these ideas. Geographic restriction of reproductive communication leads to evolutionary isolation within a population of organisms and generates a biogeographic province. The isolation of distinct populations may be by geographic or other physical barriers to faunal dispersal and communication as well as by oceanic circulation,which controls sea water temperature and nutrient levels,and by climate. Although there is a fair bit of imprecision in the biogeographic approach,it is an essential element of determining and testing paleogeographic linkages between continental blocks. Statistical approaches that evaluate faunal similarities / dissimilarities offer one means of a more quantitative evaluation of the degree of faunal endemism. Biogeographic associations can provide control not only on potentially contiguous or adjacent continental masses,but has also been used to constrain the affinities of tracts of accreted crust in mountain belts,which can be characterized by faunal assemblages that are quite distinct relative to the adjacent continent. Changes in the degree of faunal isolation ( endemism) as a function of time are produced by changing continental configurations that bring formerly separated populations within the range of reproductive communication and colonization leading to the change from endemic to more cosmopolitan assemblages. One example is the faunal provinces of eastern Laurentia that maintained a high degree of endemism in the Early Paleozoic when Laurentia was an isolated landmass,but began to develop more cosmopolitan faunas as Iapetus collapsed and barriers between European and Laurentian faunas were removed by tectonic reconfiguration. Improved understanding of the controls on the limits of dispersion and development of unique faunas ( endemism) as well as longitudinal control,may help resolve some of the ambiguity and uncertainty in biogeographic studies. Geochemical analysis,for example,the isotope chemistry of planktonic species such as conodonts and foraminifera,of biogeographic realms can be used as a proxy for mapping distinct water masses and reconstructing ancient circulation patterns and water mass characteristics.
3. Abiotic links
The distribution of climatically and tectonically sensitive rocks of the same or similar age offer a first-order framework for constraining paleomagnetic data and commonly are an excellent approximation of paleogeographic patterns. Examples of climatically sensitive sedimentary strata that are latitudinally restricted in their occurrence include deposits of saline evaporites which can, in association with terrestrial red bed deposits,be used to reconstruct arid continental climate zones. Other deposit types include glaciogenic facies,phosphatic facies ( sensitive indicators of zones of climatic upwelling) ,reef ( coral) bearing carbonates and aeolian sandstones. Recent advances in understanding the details of depositional environments have led to refinement of the paleolatitudinal significance of carbonate rocks with the development of the concept of“cool-water carbonates”.
Increasingly,new approaches have been employed in attempts to establish continental configurations,particularly in time intervals where the paleomagnetic data have been strongly affected by younger overprints or where the sensitivity of faunal assemblages is lacking,for example, in the unfossiliferous Precambrian. This is the case for the latest Proterozoic supercontinent Rodinia,which,like its younger analogue Pangea,broke up to form many of the Paleozoic continental landmasses and ocean basins of the Phanerozoic. Paleomagnetic data from this time period ( ca. 800—540 Ma) have been controversial and resulted in a range of different plate reconstructions. Thus reconstruction of this supercontinent has had to rely on alternative techniques,some of which are described below.
Orogenic belts commonly have a distinct polarity,restricted range of ages of plutonic and metamorphic activity and linear strike length measured in the hundreds to thousands of kilometers. Orogenic belts,particularly those that formed during continental collisions,can be used as intracontinental piercing points. In the case of Rodinia,a series of contemporaneous papers used the ca. 1. 1-1. 0 Ga Grenville orogenic belt of eastern and southern North America as a piercing point to stitch together many of the constituent Neoproterozoic continental fragments of Rodinia. Like any technique,this one is not without certain caveats and impediments such as poor geochronologic control and the problem that the edges of post-Rodinia continents ( and hence their basement) are obscured beneath the orogenic cover of younger tectonic processes and plate margins.
An additional approach is to examine the rock record on different continents for evidence of contemporaneous processes and products that would have occurred at a scale beyond that of the individual continental blocks and could therefore be used to reassemble the now dispersed continental fragments. One example is magmatism associated with continental rifting and break- up. Magmatic rocks such as flood basalts and related mafic igneous rocks form above large asthenospheric plumes,with plume head diameters in excess of 2000 km,are chronologically restricted and really widespread. A classic example from the Phanerozoic is the reconstruction of the pre break-up configuration of Gondwanaland on the basis of the distribution of Jurassic mafic rocks in southern Africa,South America,Antarctica and Australia. In recent years,the improved ability to tightly constrain the age of emplacement or eruption of such igneous rocks using U—Pb geochronology has opened possibilities for Precambrian continental reconstructions. The magmatic rocks that formed during the break up of the western Laurentian component of Rodinia are one possible example. At about 723 Ma and 779 Ma dykes,sills and associated flood basalts of northwestern North America appear to have counterparts in Australia,the conjugate margin to western North America. A recent reconstruction using ca. 2. 45 Ga dykes and mafic magmatic provinces,suggest that this approach will help unravel the older Precambrian continental configurations.
A different type of continental overlap offered by magmatic rocks is the presence and isotopic signature of widespread and chronologically restricted ash beds formed by the eruption of silica- rich magmatic centers. A spectacular example is the Ordovician bentonites of northwestern Europe and eastern North America which appear to link these continents in a general way prior to the closure of Iapetus and may also tie western Argentina in at this time interval. Such linkages are smeared somewhat by uncertainties in ash dispersal patterns and the well-known linear continuity of magmatic arcs that could conceivably produce nearly contemporaneous eruptions from a broad line source rather than point source region.
Sedimentary provenance can be used in much the same way as faunal assemblages to link now separate continental blocks. A recent example that is germane to the reconstruction of Rodinia comes for the sedimentary strata of the Mesoproterozoic Belt basin of western North America. This tremendous thickness of clastic sedimentary rocks was derived from a source area that presently lies to the west of the plate edge of North America. The identity of that source can be constrained by geochronologic analysis of detrital minerals contained in the sediment fill which is essentially an inventory of the crystallization