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Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic Paleoclimate

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Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic Paleoclimate 3 Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic Paleoclimate Dennis V. Kent and Giovanni Muttoni everal recent analyses of paleomagnetic data the late Paleozoic and the early Mesozoic, but it can- support the concept of Pangea, an assemblage of not explain the presence or absence of continental ice Smost of the world’s continents that was mobile sheets. in terms of large-scale internal deformation and with respect to paleolatitude. The main feature of internal deformation involved the transformation from a Pan- gea B–type configuration in the late Paleozoic, with The supercontinent of Pangea included most of the northwestern South America adjacent to eastern North continents over the late Paleozoic and the early Me- America, to a more traditional Pangea A–type config- sozoic (ϳ300 Ma to ϳ175 Ma). Over its existence, uration in the early Mesozoic, with northwestern Af- Pangea experienced a wide range of global climate con- rica adjacent to eastern North America. Pangea B thus ditions, from extensive Gondwana glaciations in the seems to coincide in time with extensive low-latitude Permo-Carboniferous to generally drier continental coal deposition and high southern-latitude Gondwana climates and the absence of evidence for high-latitude glaciations, whereas Pangea A coincides with generally ice sheets in the Triassic and Jurassic (Frakes 1979). If drier conditions over the continents and no polar ice there was indeed little change in continental positions sheets. Although the configuration of Pangea may have from the late Paleozoic to the early Mesozoic, as is been more stable as an A-type configuration in the often supposed, then the role of paleogeography in ex- Early and Middle Jurassic prior to breakup, the paleo- plaining the large contrast of climates across these in- magnetic evidence suggests that there was appreciable tervals is not very obvious (Hallam 1985). latitudinal change of the assembly. Such changing However, some recent analyses of paleomagnetic tectonic boundary conditions emphasize the practical data support the concept of a more mobile Pangea, in importance of age registry of paleoclimate data in which the relative arrangement of the main continental making valid comparisons with model results. A sim- elements and the paleolatitudinal setting changed ap- ple zonal climate model coupled with the geocentric preciably with time. The concept of a mobile Pangea axial dipole hypothesis for establishing paleolatitudes expands opportunities for testing the sensitivity of tec- in precisely controlled paleogeographic reconstruc- tonic boundary conditions in climate simulations; tions can explain many of the climate patterns in both however, the changing tectonic boundary conditions 12 • Dennis V. Kent and Giovanni Muttoni also emphasize the importance of precise age correla- nonzonal climate, or simply poor or undiagnostic tion of paleoclimate data in making valid paleogeo- data) can be better understood. graphic reconstructions. This chapter highlights some According to the GAD hypothesis, paleomagnetic recent developments in our understanding of the evo- directions of similar age from any locality on a rigid lution of Pangea in the late Paleozoic and early Me- tectonic plate should give the same paleomagnetic pole sozoic and calls attention to their possible paleoclimate and hence the location of that plate with respect to the implications. geographic axis. In practice, there will be some scatter in such determinations, and the resulting uncertainty in the mean pole position is usually expressed by the Paleomagnetic Data radius of the circle of 95% confidence (A95), according to Fisher statistics (Fisher 1953). The uncertainty in Paleomagnetism provides one of the best recorders of paleopole determinations, where A95 is typically a few past locations of the geographic axis and therefore can degrees and higher, is thought to be caused mainly by place critical constraints independent of paleoclima- experimental and observational errors rather than by tology on the latitudinal distribution of landmasses. complexities in the paleofield (for a caveat on Paleo- Descriptions of the methodology and assumptions of zoic and Precambrian data, see Kent and Smethurst the subject are discussed extensively elsewhere (for a 1998). recent general treatment, see Butler 1992, and for a A temporal sequence of paleopoles or an apparent modern and comprehensive analysis of tectonic appli- polar wander (APW) path is a convenient way to rep- cations, see, especially, Van der Voo 1993). Some com- resent the motion of a tectonic plate with respect to ments are offered here to remind the reader of the the rotation axis. Van der Voo (1993) has introduced scope and limitations of paleomagnetic data when a set of seven minimum reliability criteria for evalu- such data are used for paleocontinental reconstruc- ating published paleopoles. A low overall quality factor tions. is grounds for rejection, but relatively few results satisfy A central assumption of paleomagnetism is that the all the threshold criteria for reliability. Van der Voo Earth’s magnetic field, when averaged over some thou- (1993) suggested that only those results that pass three sands of years, will be approximated closely by that of or more reliability criteria should be retained as the a geocentric axial dipole (GAD). Thus an observed most suitable candidates for documenting polar wan- mean inclination, I, can be related to geographic lati- der. Acceptable paleopoles from specific rock units are typically averaged over intervals, usually something 2 ס tude, L, according to the dipole formula (tan I tan L). Paleomagnetic observations for the past few such as a geologic epoch or 10 to 30 million years de- million years, where accumulated plate motions are pending on the availability of data, in order to enhance small or can be taken into account, indicate that the the coherence of the APW path. Paleomagnetic data GAD model is an accurate representation of the pa- are not uniform in quality or abundance, so there is leofield to within a few degrees (e.g., Opdyke and considerable variability in the definition and reliability Henry 1969; Schneider and Kent 1990b). The detailed of APW paths for different plates or time intervals. configuration of the paleofield is known less precisely Paleocontinental reconstructions accordingly are not for earlier eras (e.g., Livermore, Vine, and Smith 1984; uniformly constrained and may change as better data Schneider and Kent 1990a), and evidence for the gen- become available. eral validity of the GAD hypothesis comes to rely Relative motions between tectonic plates or their mainly on the degree of congruence with the inferred continental proxies are detected by systematic differ- latitudinal dependence of various climate indicators ences in their respective APW paths. It is also possible (e.g., Blackett 1961; Briden and Irving 1964). It is that the whole Earth rotated with respect to the rota- therefore useful to maintain a distinction between es- tion axis (e.g., Goldreich and Toomre 1969), a phe- timates of latitude obtained from paleomagnetic data nomenon referred to as true polar wander (TPW). For and those obtained from paleoclimate data so that the the late Mesozoic and Cenozoic, where plate motions nature of any discrepancies (e.g., those possibly arising are constrained by evidence from the seafloor and by from more complicated magnetic fields, regionally a hot-spot reference frame more or less fixed to the Mobility of Pangea • 13 solid Earth, the amount of any TPW has been esti- minimum motion arguments, must be relied on to mated to have been much smaller than plate motions provide some constraints on the relative longitudinal in accounting for polar wander (e.g., Courtillot and distribution of tectonic plates. Besse 1987; Gordon 1987). Some researchers have sug- gested episodes of larger TPW for some earlier inter- vals, such as the Permo-Triassic (Marcano, Van der Pangean Configurations Voo, and Mac Niocaill 1999), but TPW will remain a in the Permian, Triassic, moot issue for paleoclimatology provided that the and Jurassic GAD hypothesis remains valid; that is, the dynamo in the fluid outer core always responds to the geographic Although a variety of data have demonstrated deci- or rotation axis so that the dipole field tends to remain sively the existence of Pangea in the late Paleozoic and aligned along it. early Mesozoic, uncertainty remains on its precise con- There have been occasional suggestions, although figuration. For example, the positions of the various they are not well posed as a testable hypothesis, that tectonic blocks in eastern Asia may not have become the paleomagnetic and geographic axes were signifi- assembled with the rest of Laurasia until sometime in cantly decoupled for long intervals. For example, the Jurassic (Enkin et al. 1992). However, a first-order Donn (1982, 1989) attempted to explain apparently problem considered here is the position of the south- large disagreements between paleomagnetic data and ern continental assembly of Gondwana with respect to other paleolatitudinal indicators for the Triassic to Eo- the northern continental assembly of Laurasia. Two cene with what might be referred to as paleomagnetic general models have been proposed: (1) a supercon- polar wander. Some discrepancies between paleomag- tinent that maintained a more or less static Pangea netic and paleoclimatic estimates of paleolatitude have A–type configuration over the late Paleozoic and early been resolved with better paleomagnetic data—for ex- Mesozoic; and, largely on the basis of paleomagnetic ample, in the Devonian of North America (Heckel and results, (2) a supercontinent that experienced appre- Witzke 1979; Miller and Kent 1988; Stearns, Van der ciable internal deformation and evolved from a Pangea Voo, and Abrahamsen 1989)—or have been taken as B–type to a Pangea A–type configuration over this an indication of a significant climate problem, as in interval. the case of high-latitude warmth in the Cretaceous and The Pangea A–type model is the traditional Wege- Eocene (Barron, Thompson, and Schneider 1981; Bar- nerian configuration, whose salient feature for this dis- ron 1987).
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