Terrane provenance and amalgamation: examples from the Caledonides

The Scottish Caledonides have grown by the accretion of terranes generated somewhere along the Laurentian margin. By the time these terranes had been emplaced along the Scottish sector, they were structurally truncated then reassembled to form an incomplete collage of indirectly related tectonic elements of a destructive margin. The basement to some of these tectonic elements and the basement blocks belonging to the previously accreted Precambrian are of uncertain provenance and a source in the Pan-African craton is possible. As terranes migrate along the orogen they generate in the fault zones and on their periphery a reservoir of mature sediment. This mature sediment is produced because of the recycling produced during the generation and destruction of sedimentary basins developing during terrane translation. At each period of recycling the mature sediments are mixed with less mature sediments yielded from local uplifts generated by the new basin formation. If a large part of the orogen suffers orthogonal closure, giant river systems may form and disperse sediment across terranes. This is likely to have happened during the Devonian-Carboniferous of parts of N. Europe.

Synopsis of late Palaeozoic and Mesozoic terrane accretion within the Cordillera of western North America

Establishing the paleogeographic origin of most of the terranes within the Cordillera remains an ellusive goal; despite more than 10 years of multidisciplinary research, the home port of any major terrane has not been identified unequivocally. Even most continental fragments that show affinities to North America cannot be repositioned confidently along the Cordilleran margin, and some continental fragments (e.g. Chulita) probably are not North American in origin. Cordilleran oceanic terranes, including island arcs, seamounts, off-ridge islands, and scraps of ocean basins, are especially difficult to reposition because Panthalassa has been destroyed. Faunal studies with emphasis on palaeobiogeographic affinities are the most useful, particularly when coupled with analyses of faunal diversity and endemism. Such studies suggest that some terranes previously thought to have formed near the Cordillerran margin were situated thousands of kilometres to the west, and were separated from the continent by broad ocean basins, rather than by a narrow marginal sea.

Concluding remarks

During the course of this Discussion Meeting, a very large amount of regional tectonic geology was displayed, and debated critically in a terrane framework, on scales ranging from the whole of the North American Precambrian or the Mesozoic-Cenozoic Tethys down to particular segments of the Caledonides and Alpides. A wide spectrum of opinion was expressed from those who believe that the terrane methodology is a critical and essential objective stage in data handling before any rational palaeogeographic and palaeotectonic synthesis can be attempted in plate boundary zones to those who believe that the terrane philosophy is fundamentally flawed, dangerous, and pernicious, in that it leads to random data collection and the obscuring of fundamental plate tectonic processes. Another view was that terranology has been useful in drawing our attention to the importance of large pre-collisional strike—slip or transform motions in orogenic belts and the juxtaposition of disparate elements and zones. Yet another position was that there is nothing new in terranology that is not implicitly and explicitly inherent in plate boundary processes and that terrane analysis is simply another harmless word for what most careful regional geological synthesizers have been doing since the early 1970s. Naturally, no coherent consensus view emerged from the discussion, but an important result was that a huge amount of excellent regional and global geology and tectonic ideas were discussed in the context of the problems and complexities of plate boundary zone evolution and the mechanisms by which objects from the size of ‘knockers’ to continents, detach, move and weld to form collages at all scales.

Palaeomagnetism, North China and South China collision, and the Tan-Lu fault

Refined Apparent Polar Wander (APW) paths for the North and South China Blocks (ncb and scb) are presented and the collision between the NCB and SCB discussed. We suggest that the amalgamation of the NCB and SCB was completed in the late Triassic-early Jurassic, during the Indosinian Orogeny. This proposed timing is based on an analysis of palaeomagnetic signatures relating to continental collisions, such as the convergence of palaeolatitude, deflections of declination, hairpin-like loops in and superposition of APW paths. Like the Cenozoic India—Eurasia collision, the Mesozoic NCB- SCB collision reactivated ancient faults in eastern China, converting some of them into transcurrent faults, of which the Tan-Lu fault is the most famous.

General discussion

A. Trench (Department of Earth Sciences, University of Oxford, U. K. ). Several participants at the meeting commented upon the impact of the terrane hypothesis in directing geological thinking to consider strike-slip movements within the British Caledonides. It is perhaps pertinent to recount its effect on geophysical research. The terrane hypothesis essentially makes two geophysically testable predictions. These are (i) that crustal fragments of disparate geographical origin are juxtaposed in the final crustal collage, and (ii) that terrane movements bring together blocks of different crustal character. The former of these predictions lends itself to palaeomagnetic study whereas the latter might be addressed using a combination of seismic, electrical and potential field methods.

On terrane analysis

Many recent papers on displaced crustal masses are organized around ‘terranes’ given only geographic names, and are bewildering for other than local experts; self-explanatory descriptive or genetic terms should be incorporated in most designations. Plate-tectonic interpretations of aggregated terranes that incorporate awareness of how modern arc systems vary and evolve contain implicit and testable predictions, but too many interpretations are based instead on invalid assumptions. As actualistic models are applied to analyses of orogenic belts, much more mobilistic interpretations than those generally now visualized will probably emerge. An example is made of the Carpathian region.

Allochthonous terrane processes in Southeast Asia

Southeast Asia comprises a complex agglomeration of allochthonous terranes located at the zone of convergence between the Eurasian, Indo-Australian and Philippine Sea plates. The older continental ‘core’ comprises four principal terranes, South China, Indochina, Sibumasu and East Malaya, derived from Gondwana-Land and assembled between the Carboniferous and the late Triassic. Other terranes (Mount Victoria Land, Sikuleh, Natal, Semitau and S.W. Borneo) were added to this ‘core’ during the Jurassic and Cretaceous to form ‘Sundaland’. Eastern Southeast Asia (N. and E. Borneo, the Philippines and eastern Indonesia) comprises fragments rifted from the Australian and South China margins during the late Mesozoic and Cenozoic which, together with subduction complexes, island arcs and marginal seas, form a complex heterogeneous basement now largely covered by Cenozoic sediments. Strike-slip motions and complex rotations, due to subduction and rifting processes and the collisions of India with Eurasia and Australia with Southeast Asia, have further complicated the spatial distribution of these Southeast Asian terranes. A series of palinspastic maps showing the interpreted rift-drift-amalgamation-accretion history of Southeast Asia are presented.

Geological constraints on the origin of the mantle root beneath the Canadian shield

Cratonic North America is composed of a cluster of Archaean microcontinents centred on the Canadian shield, and juvenile Proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms. The shield is underlain by an anomalous low-temperature mantle root that is absent beneath the platform. As there appears to be no systematic difference in crustal thickness or density between the shield and the platform, the long-lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature. Isotopic and seismic anisotropy data indicate an Archaean age for the mantle root, close to the time of formation of the overlying crust. The preferential development of the mantle root beneath Archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere, assuming that buoyant subduction was more common in the Archaean. Formation of the mantle root was not dependent on collisional orogenesis, as has been suggested, but the Archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening. The mantle root persists beneath Archaean crust that was transected by mafic dyke swarms and subjected to short-lived episodes of post-orogenic crustal melting, but the root is reduced at mantle plume initiation sites. The partitioning of Archaean and Proterozoic crust between the shield and the platform, respectively, causes the shield to misrepresent Precambrian crust as a whole. Studies of the shield falsely conclude that a high percentage of Precambrian crust formed in the Archaean, and that the Proterozoic was characterized by epicontinental volcanism and sedimentation, and crustal ‘reworking’. Furthermore, the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust.

Laboratory measurements of deep-water breaking waves

The results of laboratory experiments on unsteady deep-water breaking waves are reported. The experiments exploit the dispersion of deep-water waves to generate a single breaking wave group. The direct effects of breaking are then confined to a finite region in the wave channel and the influence of breaking on the evolution of the wave field can be examined by measuring fluxes into and out of the breaking region. This technique was used by us in a preliminary series of measurements. The loss of excess momentum flux and energy flux from the wave group was measured and found to range from 10% for single spilling events to as much as 25% for plunging breakers. Mixing due to breaking was studied by photographing the evolution of a dye patch as it was mixed into the water column. It was found that the maximum depth of the dye cloud grew linearly in time for one to two wave periods, and then followed a t 1/4 power law (t is the time from breaking) over a range of breaking intensities and scales. The dyed region reached depths of two to three wave heights and horizontal lengths of approximately one wavelength within five wave periods of breaking. A detailed velocity survey of the breaking region was made and ensemble averages taken of the non-stationary flow. Mean surface currents in the range 0.02-0.03 C (C is the characteristic phase speed) were generated and took as many as 60 wave periods to decay to 0.005 C. A deeper return flow due to momentum lost from the forced long wave was measured. Together these flows gave a rotational region of approximately one wavelength. Turbulent root mean square velocities of approximately 0.02 C were measured near the surface and were still significant at depths of three to four wave heights. More than 90 % of the energy lost from the waves was dissipated within four wave periods. Subsequently measured kinetic energy in the residual flow was found to have a t -1 dependence. Correlation of all the above measurements with the amplitude, bandwidth and phase of the wave group was found to be good, as was scaling of the results with the centre frequency of the group,. Local measures of the breaking wave were not found to correlate well with the dynamical measurements.

A geochemical approach to allochthonous terranes: a Pan-African case study

The recognition of Mesozoic and Cenozoic terranes can best be made from palaeomagnetic, structural and palaeontological studies, but older regions of continental crust require geochemical constraints to evaluate crustal growth through terrane accretion. For Precambrian shields, the pattern of Pb and Nd isotopic provinces may reveal the mechanism of crustal growth. The Afro-Arabian Shield was generated by calc-alkaline magmatism between 900 and 600 Ma ago. This example of Pan-African crustal growth underlies an area of at least 1.2 x 10 6 km 2 , which may extend to 3.5 x 10 6 km 2 beneath Phanerozoic sediments and Tertiary volcanic cover. Field evidence and trace element geochemistry suggest that Pan-African tectonics began as a series of intra-oceanic island arcs that were accreted to form continental lithosphere over a period of 300 Ma. The great majority of Nd and Pb isotope ratios obtained for igneous rocks from the shield are indicative of a mantle magma source. Although many of the dismembered ophiolites cannot be identified with inter-terrane sutures in their present location, the eastern margin of the Nabitah orogenic belt is a major tectonic break that coincides with a critical boundary between Nd and Pb isotopic provinces and is marked by a linear array of ophiolite fragments across the length of the shield. Other terrane boundaries have not been identified conclusively, both because coeval island arcs can not be distinguished readily on isotopic grounds and because many ophiolites are allochthonous. However, the calculated rates of crustal growth (measured as volume of magma, extracted from the mantle per unit time) between 900 and 600 Ma are similar to those calculated for Phanerozoic terranes from the Canadian Cordillera. Such high rates in the Afro-Arabian Shield suggest that island arc terranes have accreted along a continental margin now exposed in NE Africa, together with minor continental fragments. If crustal growth rates during this time were no greater than contemporary rates, ca . 4000 km of arc length are required, which is considerably less than that responsible for crustal growth in the SW Pacific.