Fossils In Fold Belts

One bad fossil is worth a good working hypothesis.

Rudolf Trumpy, eminent Alpine geologist

Fossils from the deformed zones of mountain belts are rare but important. Relatively few paleontologists study these fossils because they are usually poorly preserved, and are metamorphosed and tectonized; fossils in oro-genic or mountain-building zones are also rare and difficult to collect from often hazardous terrains. Nevertheless, fossils are of fundamental importance in the formulation of tectonic models, providing age and geographic constraints, although the fossils themselves are rarely of great morphological significance. The identification of fossiliferous sequences in thrust belts helped identify large-scale horizontal movements of the Earth's crust in the Swiss Alps, the Northwest Highlands of Scotland and in the Scandinavian Caledonides over a century ago (Box 2.8). In many mountain belts fossil data have provided the only reliable dates for rock successions; unlike radiometric clocks, fossils cannot be reset by later thermal and tectonic events.

The Appalachian-Caledonian mountain belt, developed during the Early Paleozoic, contains large pieces of both North America and Europe, but understanding of its complex history and structure is fairly recent. Parts of the belt have been dissected and investigated by paleontological data. For example, Charles Lapworth's studies on the complex structure and stratigraphy of the Southern Uplands of Scotland in the 1870s were based on recognition of the sequence of graptolite faunas.

_j_j_ boundary of Tethyan and Boreal provinces in Jurassic

—T— T— boundary of high and middle I paleolatitude faunas in P P Triassic and Permian

} paleolatitude faunas in .—P P 1 Triassic and Permian

Figure 2.17 Displaced faunas in terranes within the North American Cordillera together with changing provincial boundaries on the craton. Postulated latitudinal boundaries on the craton during the Permian, Triassic and Jurassic are indicated and confirm the northern movement of these displaced terranes. A dataset of Jurassic ammonoid distributions across the cordilleran terranes is available at http://www.blackwellpublishing.com/paleobiology/. These data may be analyzed and manipulated using a range of multivariate techniques including cluster analysis (see also Hammer & Harper 2005). (From Hallam, A. 1986. J. Geol. Soc. 143.)

Craton

_j_j_ boundary of Tethyan and Boreal provinces in Jurassic

—T— T— boundary of high and middle I paleolatitude faunas in P P Triassic and Permian

} paleolatitude faunas in .—P P 1 Triassic and Permian

Zone of displaced terranes

Tethyan faunas

Permian Triassic

Jurassic (Pliensbachian)

Boreal faunas

Jurassic (Pliensbachian)

Figure 2.17 Displaced faunas in terranes within the North American Cordillera together with changing provincial boundaries on the craton. Postulated latitudinal boundaries on the craton during the Permian, Triassic and Jurassic are indicated and confirm the northern movement of these displaced terranes. A dataset of Jurassic ammonoid distributions across the cordilleran terranes is available at http://www.blackwellpublishing.com/paleobiology/. These data may be analyzed and manipulated using a range of multivariate techniques including cluster analysis (see also Hammer & Harper 2005). (From Hallam, A. 1986. J. Geol. Soc. 143.)

Much more recently in central Scotland, reliable early Ordovician dates from the Highland Border Complex (based on brachiopods, trilobites and a range of microfossils), previously included as part of the mainly Neopro-terozoic Dalradian Supergroup on the continent of Laurentia, suggests that these rocks were deposited in one of a series of basins along the margin of Laurentia. The oceanic terranes, such as volcanic islands and microcontinents, that evolved seaward of the ancient continents are often termed "suspect". In many cases it is not clear to which if any of the continents they were originally attached. The Highland Border Complex was considered a truly suspect terrane. Moreover, the two areas could not have developed together since, firstly, during the Early Ordovician, the Dalradian was deforming and uplifting, and secondly there was a lack of Dalradian clasts in the Highland Border Basin. Some scientists have even suggested the Dalradian was derived

Box 2.7 Latitudinal variation in diversity through time

Today the tropics are teeming with diverse life built around a number of so-called hotspots, small areas that have especially high numbers of species. But is this a modern phenomenon? Recent research suggests that latitudinal gradients have intensified dramatically during the past 65 myr and that biotic radiations in the tropics are based on relatively few species-rich groups in both marine and terrestrial environments (Crame 2001). Part of this may have been driven by evolutionary escalation, part by changing climates. In evolution, sometimes predators and prey evolve rapidly in concert - the predators may adopt ever-more deadly means of attacking their prey, but the prey evolves ever-better means of defense. This kind of escalation, or arms race, has happened in many circumstances (see p. 102), and may have happened in tropical oceans through the past 15 myr. Further, global climate change during this same period probably helped to partition the tropics into a series of diversity hotspots, such as the Indo West Pacific (IWP) center. It is hard to be sure that such hotspots in the geological past will be preserved. How we perceive past diversity may be very much dependent on whether we have or have not properly sampled these hotspots through time.

Other latitudinal diversity gradients tend to confirm current trends. For example, in a study covering the past 100 myr, Markwick (1998) found that crocodilians used to have a wider latitudinal spread than they do today. Modern crocodilians are known primarily from a narrow tropical belt covering the southern United States down to central Brazil, Africa, India and Australasia. Abundant crocodilian fossils from the Cretaceous and Tertiary are known from northern parts of North America and Europe, but the richest finds lie around the paleoequator. So, the tropical, warm-weather part of the world used to be twice as wide as it is today and, in general, global climates have cooled through the last 100 myr. Nevertheless crocodilians are, and were, most abundant round the equator, and their diversity declines the farther one goes away from the tropics.

from Gondwana and has nothing to do with the geological history of North America until later in the Ordovician. This is, however, only one school of thought. New structural data suggest the Highland Border Complex was part of the Dalradian and, indeed, was always intimately linked to the Laurentian craton (Tanner & Sutherland 2007). Elsewhere in the Caledonides, Harper and Parkes (1989) described a series of terranes across Ireland based on paleontological data. While some terranes developed marginal to North America and Avalonia (see above), some smaller ter-ranes in central Ireland almost certainly evolved within the Iapetus Ocean itself, with their own distinctive faunas.

We can thus reassemble ancient mountain belts and trace the origins of their jumbled structure using paleontological data, but can fossils help us understand the rates of these tectonic processes, such as plate movements and the transit of individual thrust sheets within orogenic belts? The Banda Arcs are part of a much younger mountain belt, developed during the Neogene and Quaternary along the continental margin of northern Australia (Harper 1998). A precise stratigraphy based on foraminiferans has allowed the movement of far-traveled thrust complexes to be tracked; thrust sheets were emplaced at rates between 62.5 and 125 mm yr-1 whereas the belt as a whole was uplifted at rates of about 15 mm yr-1.

Fossils, surprisingly, can be of great value to structural geologists, not only in understanding the rates and timing of tectonic events. Structural geologists study rocks that have been folded and faulted, and they want to identify how exactly the rocks have been deformed. If they find a fossil that was originally symmetric, but has since been squeezed, or stretched, in particular directions, they have precise evidence of the magnitude of the tectonic forces that have acted. A famous

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