A synopsis of events related to the assembly of eastern Gondwana

Abstract

The assembly of the eastern part of Gondwana (eastern Africa, Arabian–Nubian shield (ANS), Seychelles, India, Madagascar, Sri Lanka, East Antarctica and Australia) resulted from a complex series of orogenic events spanning the interval from ∼750 to ∼530 Ma. Although the assembly of Gondwana is generally discussed in terms of the suturing of east and west Gondwana, such a view oversimplifies the true nature of this spectacular event. A detailed examination of the geochronologic database from key cratonic elements in eastern Gondwana suggests a multiphase assembly. The model outlined in this paper precludes the notion of a united east Gondwana and strongly suggests that its assembly paralleled the final assembly of greater Gondwana. It is possible to identify at least two main periods of orogenesis within eastern Gondwana. The older orogen resulted from the amalgamation of arc terranes in the Arabian–Nubian shield region and oblique continent–continent collision between eastern Africa (Kenya–Tanzania and points northward) with an, as of yet, ill-defined collage of continental blocks including parts of Madagascar, Sri Lanka, Seychelles, India and East Antarctica during the interval from ∼750 to 620 Ma. This is referred to as the East Africa Orogen (EAO) in keeping with both the terminology and the focus of the paper by Stern [Annu. Rev. Earth Planet. Sci. 22 (1994) 319]. The second major episode of orogenesis took place between 570 and 530 Ma and resulted from the oblique collision between Australia plus an unknown portion of East Antarctica with the elements previously assembled during the East African Orogen. This episode is referred to as the Kuunga Orogeny following the suggestion of Meert et al. [Precambrian Res. 74 (1995) 225]. Paleomagnetic data are currently too few to provide a rigorous test of this proposal, but the extant data do not conflict with the notion of a polyphase assembly of eastern Gondwana. The major conclusion of this paper is that east Gondwana did not exist until its Cambrian assembly.

Introduction

The Mozambique belt lies along the eastern margin of the African continent and is generally thought to represent a zone of continent–continent collision on the scale of the modern Alpine–Himalayan orogen although others have favored a largely ensialic origin Holmes, 1951, Dewey and Burke, 1973, Burke et al., 1977, Stern, 1994, Piper, 2000. For the most part, the formation of the Mozambique belt is discussed in terms of a collision between east and west Gondwana, but such a description oversimplifies both the geometry and timing of Gondwana formation. For example, most analyses view the assembly of the western Gondwana elements (e.g., South American and African blocks) as a series of collisions marked by near final assembly at around 600 Ma (Trompette, 1997). Therefore, if the collisions along the Mozambique belt occurred before 600 Ma, then only parts of west Gondwana were involved in the collision. Stern (1994) and, more recently, Blasband et al. (2000) document evidence of a series of arc-terrane accretions in the Arabian–Nubian shield (ANS) region that spanned at least 100 million years beginning at roughly 750 Ma, indicating a protracted assembly of juvenile terranes and older continental fragments along the northern segment of the Mozambique belt, and referred to this as the East Africa Orogen (EAO).

The term Pan-African (Kennedy, 1964) referred to a sequence of events of tectonothermal events at 500±100 Ma within Africa and adjacent Gondwana elements. The term was broadened by Kröner (1984) to include orogenic events of the same time range (950–450 Ma) on a more global scale. In the intervening years, the ‘Pan-African’ orogenic cycle has been more narrowly defined both spatially and temporally such that it is possible to recognize individual orogenic events within Gondwana Trompette, 1997, Stern, 1994, Meert et al., 1995. The term Pan-African is likely to remain popular, but it no longer provides a level of specificity commensurate with our knowledge of the tectonic history of Gondwana assembly. This paper outlines the orogenic events associated with the assembly of the eastern part of Gondwana.

Although the timing of Gondwana assembly will continue to be debated for some time, the history of eastern Gondwana amalgamation must begin with a discussion of where the cratonic elements originated prior to their late Neoproterozoic to early Cambrian fusion. The notion of a Meso–Neoproterozoic supercontinent is entrenched in the geologic literature Piper, 1976, Bond et al., 1984, McMenamin and McMenamin, 1990, Dalziel, 1991, Hoffman, 1991, Karlstrom et al., 1999 although the configurations differ widely. A number of names have been proposed for this supercontinent including Ur-Gondwana Hartnady, 1986, Hartnady, 1991, Paleopangea (Piper, 2000) and Rodinia (McMenamin and McMenamin, 1990). The name ‘Rodinia,’ which takes its name from the Russian prefix ‘to beget,’ is adopted here. According to Dalziel (1997), the Rodinia supercontinent formed during a series of late Meso to early Neoproterozoic collisions lumped under the term ‘Grenvillian.’ Its breakup is represented by the presence of Neoproterozoic IV-aged rift and passive margin-related sequences Dewey and Burke, 1973, Bond et al., 1984, Dalziel, 1991, Moores, 1991, Hoffman, 1991, Knoll, 2001. Although there are debates regarding the exact position of various elements surrounding Rodinia (e.g., Piper, 2000, Sears and Price, 2000, Dalziel, 1997), there is clear evidence that Laurentia occupied the center of a major landmass. As noted by Dalziel (1997), whatever the final configuration of the supercontinent, the length of rifted margins surrounding Laurentia must be accounted for by a similar length of rifted margins in the formerly contiguous blocks. For simplicity, I adopt a variation of the Rodinia supercontinent shown in Fig. 1 (ca. 800 Ma; Dalziel, 1997, Weil et al., 1998, Torsvik et al., 1996). There is one caveat in adopting this model for the starting point of this paper. The popular model assumes that east Gondwana (Madagascar, Sri Lanka, Australia, India and Antarctica) was united by 1000 Ma. One of the conclusions of this paper is that east Gondwana never existed as a coherent block until all its constituent cratons were assembled in Neoproterozoic to early Cambrian time. Nevertheless, the geometry shown in Fig. 1 allows for a starting point in Gondwana assembly as the elements of Gondwana are more or less dispersed about the Laurentian continent. There are debates surrounding the exact timing of various rift events along the western margin of Laurentia; however, as discussed below, the available paleomagnetic evidence suggests that the rift-to-drift transition was prior to 750 Ma.

Owing to its position in east Gondwana (Fig. 2), the East Antarctic craton is viewed as the keystone continent in east Gondwana (see Yoshida, 1995, Rogers, 1996). Links between the Albany–Fraser belt (Australia) and the Wilkes Province (Antarctica, Fig. 2) were used to argue in support of a Mesoproterozoic (1300–1200 Ma) link between the two continents Sheraton et al., 1995, Nelson et al., 1995, Post et al., 1997. Dalziel (1992) considered the collisional events in the Wilkes Province (Antarctica), the eastern Ghats region of India and the Prince Charles Mountains (Antarctica) as broadly coeval provinces formed during final assembly of Rodinia; however, recent work in the eastern Ghats region and the northern Prince Charles Mountains (nPCMs) of East Antarctica Mezger and Cosca, 1999, Boger et al., 2000 indicate that these blocks were incorporated into east Gondwana during a much younger 900–1000 Ma orogenesis (Fig. 2).

Additional hints that East Antarctica might not comprise a single block came about through the recognition of the similar-aged tectonic histories of the Maud Province and the Kaapvaal craton in the interval 1100–1000 Ma Thomas et al., 1994, Cornell et al., 1996, Jacobs et al., 1996, Jacobs et al., 1998. Gose et al. (1997) argued, on paleomagnetic grounds, that the CMG terrane (Coats Land, Maudheim, Grunehogna) was juxtaposed against the Kalahari craton in early Neoproterozoic times (Fig. 2). Despite the recognition that the CMG terrane was likely a part of Africa, the notion of an undivided east Gondwana during the Neoproterozoic remained largely unchallenged (see Kröner, 1991, Meert et al., 1995, Kröner et al., 2000a, Kröner et al., 2000b for alternative suggestions). Recently, Fitzsimons, 2000a, Fitzsimons, 2000b has noted the possible existence of Cambrian-aged suture zones in East Antarctica that separate the nPCMs/Ghats regions from the Wilkes province. In addition, the possible southern extension of the East Africa Orogen into the Lützow–Holm region would juxtapose the CMG terrane with the remainder of east Gondwana during the Cambrian as argued by Fitzsimons (2000b) and Grunow et al. (1996). Other authors have also argued for a multiphase late Neoproterozoic–early Cambrian assembly of eastern Gondwana Meert and Van der Voo, 1997, Hensen and Zhou, 1997. Madagascar and Sri Lanka were usually considered ‘minor’ elements of east Gondwana, but the recent recognition that Madagascar may itself contain several sutures has sparked renewed interest in the geochronological and tectonic setting of this continental block (Paquette and Nédélec, 1998, Cox et al., 1998; Handke and Tucker, 1999; Kröner et al., 2000a, Kröner et al., 2000b, De Wit et al., 2001).

One of the major advances in understanding the complexity of Gondwana assembly is the ability to date metamorphic and igneous events with high-precision U–Pb geochronology using a variety of methods (e.g., SHRIMP, isotope dilution, Pb–Pb evaporation and electron microprobe). The U–Pb system is particularly robust because a single zircon may contain information regarding both crystallization and metamorphic events. Each of the cratons involved in the assembly of eastern Gondwana now has reliable U–Pb age data that can be tied to a tectonic framework. Other isotopic dating methods, such as ⁴⁰Ar/³⁹Ar and ¹⁴⁷Sm/¹⁴⁴Nd, along with knowledge of closure temperatures in those systems have facilitated the development of detailed cooling histories of orogenic belts. The events in some regions are better constrained than in others (e.g., Arabian–Nubian Shield vs. Kenya–India); however, collectively, they yield important information regarding the assembly of eastern Gondwana. The geochronologic data for each of the elements is reviewed below and placed in a regional tectonic framework.

Paleomagnetic studies from these blocks have the potential to discriminate amongst the various tectonic models Meert, 2001, Torsvik et al., 2001b, Meert, 1999, but the relatively poor quality of the extant database has hindered progress (Meert and Powell, 2001). Nevertheless, recent work (described below) hints that a polyphase assembly of eastern Gondwana is possible. This paper describes the geologic, geochronologic and paleomagnetic evidence for a polyphase assembly of the eastern Gondwana region.