Paleogeographic maps for the Mesozoic and Cenozoic can be computed by the fitting together of continental margins or oceanic lineations of the same age on either side of an ocean ridge (Chapters 3, 4). The location of the paleopoles can be determined from paleomagnetic measurements (Section 3.6) and so the only unknown in these reconstructions is the zero meridian of longitude. These combined techniques cannot be used for reconstructions prior to the Mesozoic because in situ oceanic crust is lacking.
Methods of quantifying plate motions in pre-Mesozoic times involve the use of paleomagnetic data coupled with high-precision geochronology. Ancient plate edges, although somewhat distorted, are marked by orogenic belts and ophiolite assemblages (Sections 2.5, 11.4.3), which indicate ancient sutures between welded continents and accreted terranes. Evidence provided from the past distributions of flora and fauna and indicators of paleoclimate also aid these plate reconstructions (Sections 3.4, 3.5). For a particular time the paleomagnetic pole for each ancient plate is rotated to an arbitrary single magnetic pole and the continents on the plate are re-projected using the same Eulerian rotation. The continents are then successively shifted along fixed latitudes, that is, rotated about the magnetic pole, until the overlap of continental margins is minimized. Although the paleomagnetic data do not provide a unique sequence of reconstructions, they clearly indicate the gross trends of plate movements during ancient times. More detailed inferences on the evolution of particular regions are then made from their geology viewed in terms of plate tectonic mechanisms.
The application of paleomagnetic methods for the Precambrian is less straightforward than for Phanero-zoic times for three main reasons (Dunlop, 1981). First, the error limits of isotopic ages typically are larger. Second, isotopic and magnetic records may be partially reset during metamorphism to different degrees, and the distinction between pre- and post-orogenic isotopic and magnetic overprints can be difficult. Third, overprints occur during post-orogenic cooling and uplift, and the temperatures at which isotopic systems close and magnetizations stabilize are different, so that the dates may be younger or older than the magnetizations by intervals of tens of millions of years. However, even given these uncertainties and the gaps in the paleomagnetic record arising from the lack of suitable samples of certain ages, the data allow investigators to test the validity of paleogeographic reconstructions for pre-Mesozoic times based on the geologic record on the continents.
Was this article helpful?
Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.