Phanerozoic Cover Research Articles

Phanerozoic burial and unroofing history of the western Slave craton and Wopmay orogen from apatite (U–Th)/He thermochronometry

Low temperature thermochronometry of cratonic regions can illuminate relationships among burial and unroofing patterns, surface subsidence and uplift, and lithosphere–asthenosphere interactions. The Slave craton, initially stabilized by the development of a thick lithospheric mantle root in late Archean time, is an excellent location in which to examine these connections. Although the Slave craton currently lacks Phanerozoic cover, Phanerozoic sedimentary xenoliths entrained in ca. 610 to 45 Ma kimberlites indicate that the region underwent a more dynamic history of burial and unroofing than widely recognized. We report new apatite (U–Th)/He thermochronometry data along a southeast to northwest transect from the interior of the Slave craton into the adjacent Paleoproterozoic Wopmay orogen to resolve the region's depositional and denudational history. Six samples from the western Slave craton and three samples from Wopmay orogen yield mean dates from 296 ± 41 Ma to 212 ± 39 Ma. Individual apatite dates are broadly uniform over a wide span of apatite [eU], and this pattern can be used to more tightly restrict the spectrum of viable temperature–time paths that can explain the dataset. When coupled with geologic and stratigraphic information, temperature–time simulations of the thermochronometry results suggest complete He loss from the apatites at minimum peak temperatures of ~ 88 °C in Devonian–Pennsylvanian time, cooling to near-surface conditions by the Early Cretaceous, followed by reheating to ≤ 72 °C during Cretaceous–Early Tertiary time. Consideration of modern and ancient geotherm constraints implies ≥ 3.3 km of burial during the first Phanerozoic heating phase, with an ancillary phase of reburial in late Mesozoic–Cenozoic time. The uniformity of the apatite (U–Th)/He dates indicates that the rocks encompassed by our > 250 km-long sample transect experienced similar Phanerozoic thermal histories. Despite the distinctly different lithospheric architecture on either side of the Paleoproterozoic suture between the Slave craton and Wopmay orogen, the region behaved as a single, broadly coherent cratonic unit since at least ca. 250 Ma. The Phanerozoic burial and unroofing patterns across the craton may be a response to far-field convergent activity at the northern and western margins of North America and processes associated with episodic kimberlite emplacement.

The continuation of the Kazerun fault system across the Sanandaj–Sirjan zone (Iran)

The Kazerun (or Kazerun-Qatar) fault system is a north-trending dextral strike-slip fault zone in the Zagros mountain belt of Iran. It probably originated as a structure in the Panafrican basement. This fault system played an important role in the sedimentation and deformation of the Phanerozoic cover sequence and is still seismically active. No previous studies have reported the continuation of this important and ancient fault system northward across the Sanandaj–Sirjan zone. The Isfahan fault system is a north-trending dextral strike-slip fault across the Sanandaj–Sirjan zone that passes west of Isfahan city and is here recognized for the first time. This important fault system is about 220 km long and is seismically active in the basement as well as the sedimentary cover sequence. This fault system terminates to the south near the Main Zagros Thrust and to the north at the southern boundary of the Urumieh–Dokhtar zone. The Isfahan fault system is the boundary between the northern and southern parts of Sanandaj–Sirjan zone, which have fundamentally different stratigraphy, petrology, geomorphology, and geodynamic histories. Similarities in the orientations, kinematics, and geologic histories of the Isfahan and Kazerun faults and the way they affect the magnetic basement suggest that they are related. In fact, the Isfahan fault is a continuation of the Kazerun fault across the Sanandaj–Sirjan zone that has been offset by about 50 km of dextral strike-slip displacement along the Main Zagros Thrust.

Terrane Map of Europe

The terrane geology of Europe has been hotly debated since the terrane concept became popular with the groundbreaking paper by Coney et al. (1980). There still is no concept for Europe on which a majority of geologists would agree, and even the notion of a terrane is differently understood by many. Seventeen years of data compilation resulted in the Terrane Map of Europe (Oczlon 2006), which is a continent-wide attempt to apply a clear-cut terrane definition to firsthand geological data, collected in over 2700 references for every region of the continent. The applied terrane definition is from Keppie (1989), with modifications. There are large differences in the quality of the data, as well as the data density. Wide parts even remain without any data coverage, in particular the parts of the East European Craton, which are under Phanerozoic cover. The map provides a view on the growth of Europe throughout its billions of years of evolution, applying a self-explanatory colour scheme. It is designed to allow also readers without much background on the geology of Europe to access the basic concepts on the assembly and accretionary history of the continent. Following Late Paleozoic accretion of Gondwana-derived terranes, almost all of the crust that currently makes Europe was already assembled by ca. 280 million years. Therefore, terranedescription focuses mainly on their Paleozoic and older pre-/syn-accretionary evolution. The younger sutures and ophiolite belts of southern Europe are the result of Meso-Cainozoic opening and closure of small oceanic basins within the previously accreted crust. These sutures are shown with a line-framework that outlines the boundaries of the major Alpine tectonic units. For a better understanding of terrane-provenance, paleogeographic reconstructions of Earth at 543, 444, 375, 299, and 195 million years are shown on the map. Colors of the various paleo-continents and terranes are the same as on the Terrane Map, allowing easy recognition of the past position of continental fragments that now form part of Europe. Every terrane or group of terranes with the same provenance and accretion history is described in the map legend with its key attributes. This description is necessarily incomplete and lacks many arguments that lie behind certain assignments. More information can be found in Oczlon et al. (2007), and a simplified version of the map is available on http://www.geobib.unihd.de/terranemap/index.html.

Late Paleoproterozoic to mid-Neoproterozoic history of northern Laurentia: An overview of central Rodinia

At the assumed core of reconstructed Rodinia, northern Laurentia, comprising the Canadian Shield (including Greenland) and its buried western and northern extensions beneath Phanerozoic cover, is considered in two parts. On its southeast side, the Grenville Province is part of a major late Mesoproterozoic orogen inferred to be related to the assembly of the Rodinia supercontinent. The remainder of northern Laurentia experienced a long period of crustal stability beginning at ca. 1.8 Ga, recorded for the most part by intracontinental and passive-margin sedimentation along with mafic igneous activity, mainly in the mid-Mesoproterozoic, related to limited extension. To the south, this stable part of northern Laurentia is overprinted by late Paleoproterozoic orogeny (Yavapai, Mazatzal, and Central Plains orogens) and early Mesoproterozoic magmatism (granite-rhyolite provinces). Deformed and metamorphosed rocks related to these form a major part of the interior of the Grenville orogen, where mid- to late Mesoproterozoic orogenic events led to accretion of terranes to Laurentia and subsequent terminal collision. The first manifestation of Rodinia breakup began in the west at ca. 0.8 Ga, some 0.2 Ga earlier than along the southeast side; the age of initial rifting along the northern margin is not known.

Seismic evidence for Late Proterozoic orogenic structures below the Phanerozoic sedimentary cover in the Kattegat area, SW Scandinavia

Commercial seismic reflection profiles from SW Scandinavia image Sveconorwegian orogenic structures in the late Precambrian basement beneath the thin Phanerozoic cover. We interpret (1) pronounced west to WNW dipping reflections as Sveconorwegian localized ductile shear which overprint; (2) weaker east dipping reflections, interpreted as Gothian or early Sveconorwegian compressional structures. Folded and laminar seismic fabrics may represent F and C type crystalline thrust sheets emplaced during the Gothian and Sveconorwegian events. Some of the dipping reflections are observed to sole into (3) an interval of subhorizontal reflections at 2.5–4.5 s two‐way travel time, which we interpret as a “fossil” ductile to brittle transition that was unroofed during the Gothian Orogeny. (4) NW‐SE striking thrusts cutting the Sveconorwegian structures may indicate a shift in the late Sveconorwegian stress field. They may represent an early stage in the subsequent Phanerozoic basin evolution in the Danish area.

The Neoproterozoic Timanide Orogen of eastern Baltica: introduction

This volume was conceived during EUROPROBE’s investigations into the dynamic evolution of the Palaeozoic Uralide Orogen and relationships northwards into the Eurasian high Arctic. During these European Science Foundation studies, the preservation of Neoproterozoic deformation over large regions of northern Europe became increasingly apparent. This mainly Vendian tectonic event is referred to as the Timanian Orogeny and became the focus of many recent and on-going investigations. Much progress has been made in understanding Timanian Orogeny and a Memoir synthesizing our current knowledge is not only timely, but also relevant to Neoproterozoic global tectonic reconstructions. The type area for the Timanide Orogen is located in the Timan Range of northwestern Russia, which separates the East European Craton from the Pechora Basin and Polar Urals. The orogen extends over a distance of at least 3000 km, from the southern Ural Mountains of Kazakhstan to the Varanger Peninsula of northernmost Norway, flanking the eastern margin of the older craton (Fig. 1). From the Timan Range, it reaches northeastwards below the thick Phanerozoic successions of the Pechora Basin and Barents Shelf (O’Leary et al. 2004), and reappears in the Polar Ural Mountains and northwards through Pai Khoi to Novaya Zemlya. Timanian orogeny thus influenced a vast region of northwestern Russia. The Phanerozoic cover, Arctic shelf areas and, further east, Uralian deformation, obscure the importance of this orogenic event for the geodynamic evolution of Europe. The Timanide Orogen has been referred to by various other names, most frequently as the ‘Baikalides’. The term ‘Baikalian Orogeny’

Detection of subtle basement faults with gravity and magnetic data in the Alberta Basin, Canada: A data-use tutorial

Editor's note: This paper is based on a presentation at the 2003 Canadian Society of Petroleum Geologists/Canadian Society of Exploration Geophysicists Joint Convention in Calgary. Exploration for a wide range of mineral deposits is critically dependent on knowledge of the location and age of fractures and faults. Oil and gas fields in many sedimentary basins are distributed along fault-controlled linear trends, and fault identification is often used effectively for target-area selection in hydrocarbon exploration. Similarly, mineral deposits in various geologic settings are commonly associated with fluid-conducting faults. In the platformal, Phanerozoic Alberta sedimentary basin in western Canada (Figure 1), two fundamentally different types of crystalline-basement structure, formed in different tectonic conditions, are recognized (Lyatsky et al., 1999): Figure 1. Location of the northern Alberta study area in relation to main regional geologic features. 1. Archean and Early Proterozoic (Hudsonian and older) ductile orogenic structures, and 2. Middle Proterozoic to Recent cratonic ones. The influence of ancient ductile structures on the Alberta Basin seems largely confined to the control on Early Paleozoic depositional and drape patterns by the Precambrian erosional basement relief, which may to some extent be related to the basement lithology distribution controlled by ductile ancient structures. These structures were mostly healed and innactive during the Phanerozoic development and evolution of the Alberta Basin, and thus of limited significance to fluid migration. Their geophysical expression typically consists of large magnetic and gravity anomalies, which often obscure the potential-field signatures of the desirable brittle faults. The steep, brittle basement faults in the western Canadian platforms and Cordilleran foreland, although much more subtle than their spectacular counterparts in western United States, exerted considerable syn- and postdepositional influence on the Phanerozoic sedimentary cover. Occasionally, brittle faults and fractures follow the older orogenic basement structures, but commonly cut across them. Even when subresolution seismically, many …

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Mesoproterozoic lithologies of the Namaqua–Natal Belt are exposed in the west and east of South Africa but the central section of the belt is obscured by younger sedimentary cover. This belt is one of several Grenvillian-aged belts world-wide and is of importance in reconstructions of the Meso- to Neo-Proterozoic supercontinent of Rodinia . Zircons from granite and biotite gneiss in two deep boreholes which penetrated the Phanerozoic cover provide SHRIMP U–Pb dates of 1053 +29 / −25 Ma and 1134 +15 / −15 Ma. Sm–Nd model dates for whole-rock samples from these and two other boreholes range from ∼2400 to 1200 Ma, unlike model dates from the eastern (Natal) sector of the Namaqua–Natal Belt which are all younger than ∼1400 Ma. Model dates are similar to those noted in the Bushmanland and Richtersveld Sub-provinces of the western (Namaqua) sector of the Belt. The results are interpreted to indicate that this Proterozoic crust may be an easterly extension of the Bushmanland Sub-province or some equivalent terrane and that there must be a terrane boundary between the eastern-most borehole studied and the surface outcrops further east.

Question of oil-gas prospects in northern regions of the Barents-Kara shelf (Franz-Viktor Trough)

The significant hydrocarbon potential of the shelf areas of the Barents and Kara Seas has been demonstrated by the discovery of very large gas and gas condensate fields in their southern areas. Against this background the increase in oil reserves has not been so significant. Nevertheless there is a geologic basis for predicting large zones of oil accumulation in the north of the Barents-Kara basin, which is an area of border highs. First of all, the depth to source beds is not great, and this provides for the generation of mostly liquid hydrocarbons. The neotectonic environment was also more favourable for formation and preservation of oil pools. Areas of this type are characterised by large zones of oil and bitumen accumulation related to lateral migration of hydrocarbon fluids from more subsided parts of oil-gas basins. All this is substantiated by much geochemical data derived from study of bitumen shows on Franz Josef Land and bottom samples from the surrounding offshore areas. The bitumen shows are largely viscous and solid natural bitumens. Their content in host rock is comparatively insignificant at fractions of a percent. All this indicates migration of predominantly liquid hydrocarbons from deeper horizons of the sedimentary cover as well as absence of significant surface precipitation accompanying destruction of oil pools. In this light the region may be regarded as favourable for oil. It is therefore important that geochemical anomalies be recognised in areas of local positive structures in the north of the Barents-Kara shelf. Information has been gathered in recent years in the Franz-Viktor Trough that indicates a large swell-like structure of north-south trend in its southern part (see Figures 1 & 2). It is designated as the Ladin High in connection with recovery of sediments of the Ladin Stage at station 52 (Figure 2). Reflection and common depth point seismic profiling show the Ladin High as clearly expressed in the relief of the Precambrian (Vendian) basement. Its dimensions are 110 km x 20-40 km. Its base is outlined by the 3 km structure contour, and the crest is at a depth of 1.5-1.7 km. Ladin High retains its inherited form in the Phanerozoic cover, although more gentle, particularly in its upper part. Small cupolas that measure 10-20 km in cross section and with amplitudes of 0.4-0.6 km have been mapped on the reflection profiles. These coincide in direction with the trend of the Ladin High (See Figure 3).

THE CONFIGURATION OF THE BASEMENT BENEATH THE ZAGROS BASIN

It has long been recognised that the deposition and deformation of the Phanerozoic cover in the Zagros Basin (mountains plus foreland) was strongly influenced by the reactivation of old tectonic fabrics in its basement. Facies boundaries and structures trending north‐south and NW‐SE can be attributed to the reactivation of Pan‐African sutures and Najd faults which are exposed in the Nubian‐Arabian Shield. However, to the east of a projection of the Oman line SWwards into the Rhub Al Khali Basin, cover structures have a NE‐SW trend which is not seen in Arabia. This boundary may overlie a Pan‐African suture between Arabia and India (Somalia or Pakistan).Data including magnetic intensities, geothermal gradients and isopach maps are used here to distinguish old faults which were reactivated in the basement from more recent faults formed in the cover by Zagros shortening. Old faults trending NW‐SE are interpreted as having reactivated episodically since the Permo‐Triassic opening of Neo‐Tethys; perhaps more significantly, the basement faults that reactivated in the East Arabian Block since then trend north‐south. The basement configuration is clarified by extending a modified East Arabian Block across the Zagros to an “East Arabian‐Zagros block” in which the NW trend of the Zagros lies between two syntaxes. This suggests a new tectonic framework for the region. The repeated reactivation of basement faults throughout the East Arabian‐Zagros Block controlled source rocks, traps and seals for the supergiant and giant oil and gas reserves which are present at various stratigraphic levels in different areas.

OLD GEOPHYSICAL DATA APPLIED TO MODERN GEOLOGICAL MAPPING PROBLEMS: A CASE-STUDY IN THE SERIDÓ BELT, NE BRAZIL

The Seridó belt, northeast Brazil, is a Brasiliano-Pan-African collisional orogen formed between the West African, São Francisco and Amazon cratons during the west Gondwana assemblage. This area is an excellent example for illustrating the usefulness of gamma-ray spectrometric data to support modern geological mapping in semi-arid terranes. Analyses of K, Th and U channels images, displayed as composite themes, correlate well with the main regional framework of the Seridó Belt. Several distinct mapped units are easily distinguishable in the gamma-ray spectrometric data, as the Paleoproterozoic basement rocks and the supracrustal metasedimentary sequences of the Neoproterozoic Seridó Group. Also, the granitic plutons and the Phanerozoic cover show different geophysical signatures. Definition of contacts among units, faults, folds and shapes of granitic intrusions are considerably enhanced by the airborne gammaray spectrometry data. The results here presented show that there is a strong relationship among geochemical, isotopic and geophysical signatures of different crustal blocks recognized in the region. This kind of approach seems fundamental for investigating regional crustal evolution in overly complex deformed Precambrian areas. It also can be of some help to better define the boundaries of the different lithostratigraphic units that could appear in similar orogenic belts within the Brazilian Shield.

Old geophysical data applied to modern geological mapping problems: a case-study in the seridó belt, ne Brazil

The Serido belt, northeast Brazil, is a Brasiliano-Pan-African collisional orogen formed between the West African, Sao Francisco and Amazon cratons during the west Gondwana assemblage. This area is an excellent example for illustrating the usefulness of gamma-ray spectrometric data to support modern geological mapping in semi-arid terranes. Analyses of K, Th and U channels images, displayed as composite themes, correlate well with the main regional framework of the Serido Belt. Several distinct mapped units are easily distinguishable in the gamma-ray spectrometric data, as the Paleoproterozoic basement rocks and the supracrustal metasedimentary sequences of the Neoproterozoic Serido Group. Also, the granitic plutons and the Phanerozoic cover show different geophysical signatures. Definition of contacts among units, faults, folds and shapes of granitic intrusions are considerably enhanced by the airborne gammaray spectrometry data. The results here presented show that there is a strong relationship among geochemical, isotopic and geophysical signatures of different crustal blocks recognized in the region. This kind of approach seems fundamental for investigating regional crustal evolution in overly complex deformed Precambrian areas. It also can be of some help to better define the boundaries of the different lithostratigraphic units that could appear in similar orogenic belts within the Brazilian Shield.

Unroofing of the southern Front Range, Colorado: A view from the Denver Basin

Fission-track (FT) thermochronology of detrital apatite and zircon from synorogenic sedimentary rocks in the Kiowa Cored Well and the Castle Pines drillhole in the Denver Basin can be used to document a generally predictable unroofing sequence for the southern part of the Colorado Front Range. First, Phanerozoic sedimentary rocks that covered the Front Range in late Mesozoic time contributed recycled sedimentary and far-traveled volcanic zircon and apatite to the oldest sediments as Laramide deformation began. Next, two significant pulses of volcanic grains are recorded in both the detrital zircon and apatite FT age populations in the Kiowa Cored Well as volcanoes along the Colorado Mineral Belt dominated the landscape. The volcanic contribution is not as significant in the Castle Pines well, which is more proximal to the mountain front compared to the Kiowa Cored Well. Higher in the stratigraphic sequence, a mix of volcanic grains, recycled grains from the Phanerozoic cover, and Proterozoic basement grains is present, with the percentage of basement grains in the mixture increasing up section. Finally, in the youngest part of the sequence, metamict zircon grains from the basement and apatite grains, derived from the Proterozoic basement with 50 to 70 Ma apatite fission-track cooling ages derived from below the base of the apatite partial annealing zone in the Front Range, dominate the age populations. Complexities in the form of pulses of volcanic or basement components are superimposed on the simple pattern. This study demonstrates the power of analyzing both apatite and zircon when examining detrital grains derived from a basement-cored uplift where basement resided at shallow levels of the crust (

Paleoproterozoic crust within the Great Falls tectonic zone: Implications for the assembly of southern Laurentia

The Great Falls tectonic zone and the Vulcan structure both have been proposed as the site of a Paleoproterozoic suture between the Archean Hearne and Wyoming provinces. Both hypotheses remain viable because all Precambrian rocks composing the Vulcan structure and much of the Great Falls tectonic zone are buried beneath Phanerozoic cover. The primary exceptions to this are the mafic to felsic igneous and metamorphic rocks of the Little Belt Mountains (Montana), previously considered the northernmost exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite (207Pb/206Pb age: 1864 ± 5 Ma) and a gneissic unit intruded by the Pinto (207Pb/206Pb age: 1867 ± 6 Ma), however, confirm their Paleoproterozoic age. These rocks exhibit an overall calc-alkaline affinity and the depletion in high field strength elements typical of convergent margin environments. Whole-rock Sm-Nd data (initial epsilon of −1 to +4) and a lack of premagmatic zircons indicate that the magmas were principally derived from a depleted mantle source, not from older crust. These data suggest that at least some rocks within the Great Falls tectonic zone originated at a convergent margin that developed during the closure of an ocean basin along the northwestern margin of the Wyoming craton ca. 1.9 Ga.

Structural evidence for the allochthonous nature of the Bulbul terrane in southern Ethiopia: A west-verging thrust nappe

The Neoproterozoic basement of southern Ethiopia links the low-grade Pan-African province of the Arabian–Nubian Shield to the high-grade Mozambique Belt to the south. In this intervening area, a northward terminating low-grade metavolcano–sedimentary and mafic–ultramafic sequence of the Bulbul terrane gently overlies moderately to steeply dipping granitic migmatites of the Alghe gneissic terrane. The contact between the two terranes is a right lateral thrust. In the Bulbul terrane, rocks are part of an overturned sequence with a gently east-dipping composite D1/D2 foliation containing downdip and NE-plunging stretching lineations and westerly verging intrafolial folds. These structures are interpreted to have been developed during westward thrusting. The Bulbul sequence was therefore detached and tectonically transported to the west as a thrust nappe of which the lower inverted limb is still preserved. Structural evidence suggest yet another smaller nappe sequence (terrane) is present in southern Ethiopia and further suggest that Neoproterozoic obducted crust of the Arabian–Nubian Shield in eastern Ethiopia may be located beneath Phanerozoic cover.

The Northwest-Southeast Trending Granite-Gneiss-Migmatite Belt of the Precambrian Basement of Madagascar: Probable Signature of the Collisional Event in the Central-East Part of the East Africa Orogen

Gold occurs in various quantities mainly in the central and southern parts of Sri Lanka. Although gold is recovered mainly from river sediments at present, it has been mined in the past also from a concordant quartz reef in central Sri Lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern Sri Lanka. The sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins, minor pegmatites and sulphide mineralizations mainly of pyrite, pyrrhotite and chalcopyrite. The mineralization of gold is exclusively in the Highland Complex rocks of Sri Lanka which represent Archaean-Late Proterozoic metamorphic complexes.In Madagascar gold is present in the Precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks, concordant quartz reefs and more recent discordant quartz veins. The concordant quartz veins and diffused gold are related to both Archaean and Proterozoic rock associations and the discordant veins which have been identified as Permo-Triassic are found mainly in the north within the contact zone of Proterozoic rocks and the Phanerozoic cover. Gold is also recovered form ancient and recent alluvial accumulations and lateritic earth. The mineralization of vein deposits is mainly gold-quartz and gold-sulphide associations, the sulphides being galena, chalcopyrite and pyrite.In south India, gold occurs in Archaean metamorphic units comprising of granite-greenstone complexes in the Kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic Wynad fields. In both cases gold occurs in gold-quartz veins which occupy fissures and shears within the Archaean metavolcanics. Gold also occurs in the Attapadi area of Kerala both in sediments and in primary epigenetic gold deposits which are hosted by Precambrian amphibolites and granitic gneisses. The Attapadi lode veins are predominantly structurally controlled and are closely associated with the Proterozoic Bhavani shear zone. Gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins, gold quartz carbonate veins and gold quartz sulphide veins. The sulphide minerals are mainly galena, pyrite, chalcopyrite and arsenopyrite.The rocks in the southern Madagascar, southern India and Sri Lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the Pan-African. The emplacement of gold mineralization mainly in Madagascar and south India can be correlated in the Archaean. The gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in Sri Lanka, as a Pan-African mineralizing event. It is also evident that CO2 rich fluids from a deep seated source has continued from Archaean and were still active during the Proterozoic being responsible for the Pan-African event of gold deposition particularly in the younger structures in these countries.

Gold Occurrences in Madagascar, South India and Sri Lanka: Significance of a Possible Pan-African Event

Gold occurs in various quantities mainly in the central and southern parts of Sri Lanka. Although gold is recovered mainly from river sediments at present, it has been mined in the past also from a concordant quartz reef in central Sri Lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern Sri Lanka. The sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins, minor pegmatites and sulphide mineralizations mainly of pyrite, pyrrhotite and chalcopyrite. The mineralization of gold is exclusively in the Highland Complex rocks of Sri Lanka which represent Archaean-Late Proterozoic metamorphic complexes. In Madagascar gold is present in the Precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks, concordant quartz reefs and more recent discordant quartz veins. The concordant quartz veins and diffused gold are related to both Archaean and Proterozoic rock associations and the discordant veins which have been identified as Permo-Triassic are found mainly in the north within the contact zone of Proterozoic rocks and the Phanerozoic cover. Gold is also recovered form ancient and recent alluvial accumulations and lateritic earth. The mineralization of vein deposits is mainly gold-quartz and gold-sulphide associations, the sulphides being galena, chalcopyrite and pyrite. In south India, gold occurs in Archaean metamorphic units comprising of granite-greenstone complexes in the Kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic Wynad fields. In both cases gold occurs in gold-quartz veins which occupy fissures and shears within the Archaean metavolcanics. Gold also occurs in the Attapadi area of Kerala both in sediments and in primary epigenetic gold deposits which are hosted by Precambrian amphibolites and granitic gneisses. The Attapadi lode veins are predominantly structurally controlled and are closely associated with the Proterozoic Bhavani shear zone. Gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins, gold quartz carbonate veins and gold quartz sulphide veins. The sulphide minerals are mainly galena, pyrite, chalcopyrite and arsenopyrite. The rocks in the southern Madagascar, southern India and Sri Lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the Pan-African. The emplacement of gold mineralization mainly in Madagascar and south India can be correlated in the Archaean. The gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in Sri Lanka, as a Pan-African mineralizing event. It is also evident that CO 2 rich fluids from a deep seated source has continued from Archaean and were still active during the Proterozoic being responsible for the Pan-African event of gold deposition particularly in the younger structures in these countries.

The Magondi belt in northeast Botswana: regional relations and new geochronological data from the Sua Pan area

Available data bearing on the extent of the 2.0-1.8 Ga Magondi Orogenic Belt beneath Phanerozoic cover in Botswana are reviewed. Also, new U-Pb and Sm-Nd geochronological data from isolated outcrops of granite on Kubu Island in Sua Pan, ca 60 km west of the main exposures of Precambrian basement in northeast Botswana, are presented. The Kubu Island Granite previously has been considered as a continuation of the Archæan Zimbabwe Craton exposed to the east. U-Pb isotopic analyses of single zircons from the granite yield a crystallisation age of 2039.2 ± 1.4 Ma, indicating that the granite was intruded during Palæoproterozoic orogenesis. One zircon grain interpreted as a xenocryst has a 207Pb/ 206Pb data of 2673 Ma. Sm-Nd isotopic data for the granite yield a T dm model age of 2682 Ma. These data indicate the presence of Archæan crustal components in the lithospheric column sampled by the granite magma, consistent with intrusion of the granite near the edge of the Zimbabwe Craton. Together with regional geophysical evidence, the U-Pb zircon age for the Kubu Island Granite constrains the eastern margin of the Magondi Orogenic Belt to lie between Kubu Island and exposed parts of the Zimbabwe Craton farther east.

THE SIGNIFICANCE OF STRIKE‐SLIP FAULTING IN THE BASEMENT OF THE ZAGROS FOLD AND THRUST BELT

Lateral offsets in the pattern of seismicity along the Zagros fold and thrust belt indicate that transverse faults segmenting the Arabian basement are active deep‐seated strike‐slip faults. The dominant NW‐SE trending features of the belt have undergone repeated horizontal displacements along these transverse faults. These reactivated basement faults, which are inherited from the Pan‐African construction phase, controlled both deposition of the Phanerozoic cover before Tertiary‐Recent deformation of the Zagros and probably the entrapment of hydrocarbons on the NE margin of Arabia and in the Zagros area. We have used observations of faulting recognized on Landsat satellite images, in conjunction with the spatial distribution of earthquakes and their focal mechanism solutions, to infer a tectonic model for the Zagros basement.Deformation in the NW Zagros appears to be concentrated on basement thrusts and a few widely‐spaced north‐south trending strike‐slip faults which separate major structural segments. In the SE Zagros, two main structural domains can be distinguished. A domain of NNW‐trending right‐lateral faults in the northern part of the SE Zagros implies that fault‐bounded blocks are likely to have rotated anticlockwise about vertical axes relative to both Arabia and Central Iran. In contrast, the predominance of NNE‐trending left‐lateral faults in the southern part of the SE Zagros implies that fault‐bounded blocks may have rotated clockwise about vertical axes. We propose a tectonic model in which crustal blocks bounded by strike‐slip faults in a zone of simple shear rotate about vertical axes relative to both Arabia and Central Iran. The presence of domains of strike‐slip and thrust faulting in the Zagros basement suggest that some of the convergence between Arabia and Central Iran is accommodated by rotation and possible lateral movement of crust along the belt by strike‐slip faults, as well as by obvious crustal shortening and thickening along thrust faults.

Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly

Existing measurements from South Australia define a broad (>250 km wide) zone of anomalously high surface heat flow (92±10 mW m −2). This zone is centred on the western margin of the Adelaide Fold Belt (Neoproterozoic to early Phanerozoic cover floored by Palaeoproterozoic to Mesoproterozoic basement), where it borders the eastern Gawler Craton and Stuart Shelf (Palaeoproterozoic–Mesoproterozoic). To the west, in the western Gawler Craton (Archaean to Palaeoproterozoic), heat flow averages ∼54 mW m −2 while to the east in the Willyama Inliers (Palaeoproterozoic) heat flow averages ∼75 mW m −2. We use a regional geochemical dataset comprising >2500 analyses to show that the anomalous heat flow zone correlates with exceptional surface heat production values, mainly hosted in Palaeoproterozoic to Mesoproterozoic granites. The median heat production of Precambrian ‘basement’ rocks increases from <3 μW m −3 west of the anomalous zone to ∼6 μW m −3 within the anomalous zone. In the highest known part of the heat flow anomaly, Mesoproterozoic gneisses and granites of the Mount Painter Province in the northern Adelaide Fold Belt yield an area-integrated mean heat production of 9.9 μW m −3. These data suggest that the anomalous heat flow reflects an unusual enrichment in U and Th in this part of the Proterozoic crust, with the total complement of these elements some 2–3 times greater than would be expected for Proterozoic crust on the basis of the global heat flow database. This extraordinary enrichment has played an important role in modulating the thermal regime of the crust in this region, and particularly its response to tectonic activity.