Continental growth and recycling by accretion of deformed turbidite fans and remnant ocean basins: Examples from Neoproterozoic and Phanerozoic orogens
Published:January 01, 2007
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David R. Gray, David A. Foster, Roland Maas, Catherine V. Spaggiari, Robert T. Gregory, Ben Goscombe, K.H. (Charlie) Hoffmann, 2007. "Continental growth and recycling by accretion of deformed turbidite fans and remnant ocean basins: Examples from Neoproterozoic and Phanerozoic orogens", 4-D Framework of Continental Crust, Robert D. Hatcher, Jr., Marvin P. Carlson, John H. McBride, José R. Martínez Catalán
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Convergent margin tectonic settings involving accretion of large turbidite fans represent important sites of growth and regeneration of continental crust. The newly accreted continental crust consists of an upper crustal layer of recycled crustal detritus (turbidites) underlain by a lower crustal layer of tectonically imbricated oceanic crust, and/or rifted and thinned continental crust, along with underplated magmatic materials; the new lower crust represents additions to continental crustal volume differentiated from the mantle. This two-tiered crust is of average continental crustal thickness and is isostatically balanced near sea level, resulting in remarkable stability. The Paleozoic Tasman orogen of eastern Australia is the archetypal example of this style of orogeny, representing continental growth rates of cubic kilometers per year of material that does not return to the mantle by oceanic plate-tectonic recycling. The Neoproterozoic Pan-African Damara orogen of SW Africa is a similar orogen, whereas the Mesozoic Rangitatan orogen or Rakaia wedge of New Zealand illustrates the transition of the convergent margin from a Lachlan-type to more recognizable “ring of fire”-type orogen. These orogens illustrate continental growth from the shortening of deep marine successions and their oceanic crustal basement involving subduction-accretion. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of monotonous turbidites and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism reflect the crustal thickening phase, whereas the posttectonic granitoids and surficial volcanic deposits give the timing of cratonization. The turbidites represent fertile sources for crustal melting and are the main sources for the S-type granites.
Continental growth from marine successions due to subduction-accretion processes involves accretion of large volumes of continental detritus (submarine sediment fans, e.g., Ingersoll et al., 2003) and fragments of juvenile oceanic lithosphere (upper crust, e.g., Şengör and Natal'in, 1996), as well as new addition to the nascent continental crust due to magmatic underplating (lower crust, e.g., Albarède, 1998). Turbidite fan accretion represents an apparent continental growth mechanism that was active and important throughout Earth history with examples in the Phanerozoic of central Asia (Şengör and Natal'in, 1996), Proterozoic Yavapai-Mazatzal provinces of SW Laurentia (Karlstrom et al., 2000), and possibly even Archean granite-greenstone belts (Percival et al., 2004). It has a crustal addition rate comparable to the accretion rate for magmatic arcs (e.g., 13–18 km3/km/m.y., Smith, 1981, or ∼1–1.5 km3/yr, e.g., Armstrong, 1981) and may dominate in certain parts of Earth history, for example during the Paleozoic along the Pacific margin of East Gondwana (20,000 km long Tasmanide belt, e.g., Foster and Gray, 2000; see also Cawood, 2005).
Turbidite-dominated orogens are a major location for sedimentary recycling (as opposed to recycling of sedimentary rocks into the mantle) of continental crust in the form of detritus, but may also involve continental growth through new crustal addition from the mantle (e.g., Ketilidian terrain of South Greenland; Patchett and Bridgwater, 1984). Turbidite fan systems on oceanic crust or within oceanic backarc basins form the basis for new continental crust consisting of a mixture of basaltic extractions from the asthenospheric mantle along with recycled continental detritus. These orogens tend to develop with a mafic lower crust and felsic upper crust. It is of course likely that some of the detritus is sourced from new crust (e.g., island arcs, large igneous provinces) as well as old continents. Turbidite-dominated orogens throughout the world, and throughout time, are variable mixtures of recycled continental and mantle-derived end members that depend on the nature of the lower crust and source of the turbidite.
Much of our understanding of accretion has come from the study of modern or recent accretionary prisms (e.g., Lundberg and Reed, 1991; Von Huene and Scholl, 1991). In this paper, we investigate some of the parameters of accretion by analyzing ancient orogens, particularly accretionary-style orogens dominated by turbidites. It is based on the premise that the deformed turbidites preserved in the final crustal orogen architecture must reflect the original tectonic settings of the submarine fan and/or wedge, as well as the overall tectonic process. Their metamorphic character also reflects the original tectonic setting and evolutionary history.
Controls on turbidite fan deformation include the plate-tectonic setting, the tectonic position, on either the overriding slab (hanging wall) or downgoing slab (footwall), the degree of coupling between overriding and subducting slabs, the original thickness of the fan, the residence time of the fan on the seafloor, the degree of lithification (diagenesis/metamorphism), the level in the accretionary wedge, and the fluid availability. The deformation style of turbidites includes mélange or broken formation (e.g., parts of the central and eastern Lachlan orogen of eastern Australia, the Pahau terrane of New Zealand), chevron folding (e.g., the western Lachlan orogen, the Ugab domain of the Northern Zone of the Damara orogen), and fold nappes and accompanying schistosity (e.g., Otago Schist belt of the Rangitatan orogen of New Zealand and the Kuiseb Schist of central Namibia). The strain state is variable, with (1) subvertical plane strain or flattening with mostly coaxial increments (e.g., western Lachlan orogen), or (2) dominantly constrictional strain involving nonco-axial deformation (e.g., Otago Schist belt of New Zealand).
This paper investigates the large-scale architecture of crustal accretion with consideration of the position of turbidites relative to tectonic elements in the archetypal Lachlan (eastern Australia), Damara (Namibia), and Rangitatan (New Zealand) orogens (Fig. 1), as well as the spatial and temporal dynamics of accretion, and geodynamic scenarios of crustal evolution. The paper addresses how and why turbidites of large submarine fan systems are accreted, in what environments and by what processes. The paper aims to (1) characterize the nature of turbidite-dominated orogens, (2) use instructive examples from the Lachlan and Damara orogens and Otago Schist belt of New Zealand to outline structural styles and separate evolutionary histories, (3) demonstrate that there is no simple template, and (4) use regional-scale to mesoscale structure to delineate tectonic process.
OROGEN ARCHITECTURE AND POSITIONS OF TURBIDITES
Figure 1 shows examples of turbidite-dominated orogens from the Gondwana supercontinent, including the Neoproterozoic Damara belt of the Pan-African orogenic system, the Paleozoic Lachlan orogen of the composite Tasman orogen of eastern Australia, and the Mesozoic Rangitatan orogen of New Zealand. Profiles A–A′, B–B′, and C–C′ (Fig. 2) represent, respectively, (1) structurally thickened turbidites making up most of the Rangitatan orogen of New Zealand apart from the Median batholith arc and forearc sequences inboard of the Dun Mountain ophiolite (section A–A′, Fig. 2), (2) deformed and metamorphosed turbidites occupying the central part of the Inland Branch of the Damara orogen (section B–B′, Fig. 2), and (3) predominantly chevron-folded turbidites across the Tasman orogen, in particular the western and central parts of the Lachlan orogen (WLO and CLO in section C–C′, Fig. 2).
The Damara orogen of Namibia (section B–B′, Fig. 2) is a classic divergent orogen between cratonic nuclei with thrusting of the platform carbonate succession or passive margin sequences back over the bounding cratons to the north and south, respectively. The south-vergent part, however, dominates, defined by the generally homoclinally dipping Southern (Khomas) Zone schist fabrics and Southern Margin Zone thin-skinned thrust system (section B–B′, Fig. 2). The other dominant element is the granite-dominated, high-T/low-P metamorphic Central Zone that contains domal culminations of basement (e.g., Puhan, 1983). This magmatism, as well as the structural asymmetry, may reflect a subduction interface dipping beneath attenuated Congo craton to form an Andean/Cordilleran-style margin between ca. 560 and 500 Ma (e.g., Barnes and Sawyer, 1980; Miller, 1983a, 1983b; Kasch, 1983a), although Kröner (1982) has argued for ensialic orogeny without a Wilson cycle ocean closure (a common theme for these types of orogens, e.g., Chappell and White, 2001, for the Lachlan).
The Lachlan orogen of the composite Tasman orogen (section A–A′, Fig. 2) includes the distinct western (WLO), central (CLO), and eastern (ELO) parts. It is made up of an attenuated older arc (Ordovician age), a shear zone–bounded high-T/low-P metamorphic complex with similarities to the Chugach metamorphic complex of the Kodiak accretionary system of Alaska (cf. Hudson and Plafker, 1982), structurally thickened quartz-rich submarine fans where thickening is due to thrust shortening and chevron folding during closure of a marginal backarc basin, and inverted Silurian-Devonian extensional basins within the older arc (e.g., Gray and Foster, 2004a, 2004b). Evidence of subduction is provided by serpentinite-matrix mélanges containing Franciscan-like blueschist blocks or knockers along some of the major fault zones (see Spaggiari et al., 2002a, 2002b, 2004a).
The Rangitatan orogen of New Zealand (section C–C′, Fig. 2) is made up of a structurally thickened sediment wedge (Caples terrane) abutting arc-forearc sequences (Median batholith) and a deformed turbidite fan system (Torlesse of the Rakaia wedge) separated by steeply dipping, fault-bounded, suture-like ophiolite belt (Mortimer, 1993, 2004; Gray and Foster, 2004c). Structural vergence in the schist suggests NE thrusting of the trench volcaniclastic sedimentary sequence over the subducted quartz-rich sedimentary fan (Mortimer, 1993) that lay outboard probably on the subducted slab ocean floor (Coombs et al., 1976).
Evidence of subduction occurs in mafic greenschistchert sequences close to the trench–submarine fan boundary (the Caples-Torlesse boundary) where relicts of blueschist metamorphism are provided by cores of riebeckite and crossite mantled by actinolite (Yardley, 1982). Eventual closure of the Gulf of Oman (section D–D′, Fig. 2) would result in a structurally thickened Makran wedge juxtaposed against the Jaz-Murian arc of Iran, a scenario similar to that shown in the section across the Rakaia wedge of New Zealand (section A–A′, Fig. 2).
EASTERN GONDWANIDES: EXAMPLE OF PHANEROZOIC CONTINENTAL ACCRETION
The East Gondwana margin from ca. 550 Ma to ca. 90 Ma is a classic accretionary margin providing a spectacular example of Phanerozoic continental accretion in a convergent margin setting (e.g., Foster and Gray, 2000; Cawood, 2005). It shows eastward younging in accretion of oceanic domains, as well as accreted submarine fans, fragments of ocean crust, and arc and forearc basin elements from the Tasman and Ross orogens through to New Zealand (Fig. 3). The Tasman and Ross orogens represent at least 5% of the volume of Gondwanan continental crust (ruled pattern, Fig. 1) and with the South American Tasmanides constitute ∼5% of the current continental crustal area. Using a 100 m.y. timescale (e.g., Foster and Gray, 2000), this corresponds to ∼3–4 km3/yr of apparent continental growth, with 1.5 km3/yr of new mantle-derived material composing the lower crust.
In the eastern Australian part, or Tasman orogenic system, accretion occurred in a stepwise fashion with an eastward younging from the Cambrian through the Triassic, reflected by peak deformations of Early Ordovician, Early Silurian to Middle Devonian, and Permian-Triassic age in the respective orogenic belts (Foster and Gray, 2000; Gray and Foster, 2004a). This involved deformation of Neoproterozoic platform and rift basinal sequences (Delamerian orogen), a Cambrian-Ordovician composite turbidite submarine fan overlying Cambrian oceanic crust (Lachlan orogen), and Carboniferous-Permian arc, forearc, trench, subduction complex, and foreland basin sequences (New England orogen) (Gray and Foster, 2004a; Cawood, 2005).
Outboard of the Tasman orogen, the continental landmass of New Zealand records continuous sedimentation and accretionary prism accretion (Rakaia and Pahau terranes; Bradshaw, 1989) in the hanging wall to a long-lived subduction system along the margin of the Gondwana supercontinent from Permian to Late Cretaceous times (Fig. 3) (Bradshaw, 1989; Mortimer, 2004).
The Lachlan Orogen of Eastern Australia
The Paleozoic Lachlan orogen part of the Tasman orogenic system (Figs. 1 and 4A) is a composite accretionary orogen that formed along the eastern margin of Gondwana during late Neo-proterozoic through Paleozoic times (Fig. 3) (Gray and Foster, 2004a). It is dominated by voluminous and extensive, mostly Cambrian to Ordovician turbidites of low metamorphic grade (Fig. 4B) that formed a large submarine fan system in the Late Cambrian to Ordovician that was comparable in size to the present-day Bengal fan (Gray et al., 1991; Fergusson and Coney, 1992a). The fan accumulated on Middle to Late Cambrian back-arc/forearc crust, consisting of predominantly low-K, mid-oceanic-ridge basalts (MORBs) to arc-tholeiite basalts, dolerites and gabbros, high-Mg/low-Ti boninites, ultramafics, and calc-alkaline arc rocks (Crawford and Keays, 1978, 1987), in the Cambrian through Late Ordovician from ca. 505 Ma to ca. 460 Ma (Fergusson and Coney, 1992b; Foster et al., 2005). Closure of the backarc basin took place during plate convergence in an oceanic setting along the eastern margin of Gondwana from ca. 450 Ma to 340 Ma, with accretion of structurally thickened submarine fans, accretionary complexes, former volcanic arcs and oceanic crust, as well as the Tasmania microcontinent (Fergusson and Coney, 1992b; Gray, 1997; Foster et al., 1999; Gray and Foster, 2004b).
The Lachlan orogen comprises three thrust belts that constitute the western, central, and eastern parts, respectively. The western Lachlan orogen (WLO) consists largely of an east-vergent thrust system with alternating zones of northwest- and north-trending structures (Fig. 4A), whereas the central Lachlan orogen (CLO) is dominated by northwest-trending structures and consists of a southwest-vergent thrust belt (Howqua accretion-ary complex in Fig. 4A) linked to a fault-bounded, high-T/low-P metamorphic complex (WOMB in Fig. 4A). The eastern Lachlan orogen (ELO) is dominated by a north-south structural grain and east-directed thrust faults. In the south and in the easternmost part, an east-vergent thrust system (Yalmy-Bungonia thrust belt in Fig. 4A) overrides an older, subduction-related accretionary complex (Narooma accretionary complex in Fig. 4A).
An oceanic setting involving oceanic underthrusting (subduction) for the WLO and CLO for parts of their history (Gray and Foster, 2004a) is suggested by the presence of dismembered ophiolite slivers along some major fault zones (Spaggiari et al., 2003a, 2004a), the lower-T, intermediate-P metamorphic conditions preserved in metasandstone-slate sequences of the WLO and external part of the CLO (Offler et al., 1998; Spaggiari et al., 2003b), the presence of broken formation in the CLO and ELO (Fergusson, 1987; Miller and Gray, 1997; Watson and Gray, 2001), and the presence of serpentinite-matrix mélange incorporating blueschist knockers similar to those in the Franciscan Complex of California (Spaggiari et al., 2002a, 2002b, 2003a).
Lachlan Orogen Metamorphism
Turbidite sequences of the Lachlan orogen are generally of low metamorphic grade represented by greenschist (epizonal) and subgreenschist (anchizonal) conditions (Fig. 5A). Most of the turbidites are within the chlorite zone with localized development of biotite in contact aureoles around granites. High-T/low-P metamorphism is reflected by localized migmatites and K-feldspar-cordierite-sillimanite gneisses in the Wagga-Omeo Complex and Cooma, Cambalong, Jerangle, and Kuark belts of the eastern metamorphic belt (Fig. 5A). Peak metamorphic conditions in the Lachlan orogen were 700 °C and 3–4 kbar in the Wagga-Omeo metamorphic belt (Morand, 1990) and eastern metamorphic complexes (e.g., Cooma Complex). These belts are intimately associated with S-type granitic bodies and anatectic migmatites, and represent deeper parts of the orogenic pile exhumed during crustal-scale extension and voluminous granitic magmatism (e.g., Collins and Hobbs, 2001).
Turbidites from the low-grade parts of the Lachlan orogen show intermediate-P metamorphism based on b0 measurements of phengitic micas (see Fig. 11 of Gray and Foster, 2004b). Intermediate-P metamorphism has been inferred from coexisting chlorite-actinolite from metabasites (Offler et al., 1998), as well as low-T/intermediate-P blueschist metamorphism recorded by metabasite blueschist blocks (knockers) in serpentinite/talc-matrix mélanges within some major fault zones (Spaggiari et al., 2002a, 2002b). Amphibole compositions (winchite and glauco-phane) give estimated pressures of ∼5–6.5 kbar (winchite in the WLO) and ∼6–7 kbar (winchite and glaucophane in the CLO) with temperatures <450 °C (Spaggiari et al., 2002a, 2002b). Metabasite blueschists from the Franciscan Complex of California and the Sanbagawa belt of Japan yield similar P-T estimates. This low-T/intermediate-P metamorphism occurred at 450–440 Ma during the regional deformation of the Lachlan orogen (Spaggiari et al., 2002a, 2002b, 2003a).
Lachlan Orogen Magmatism
Silurian-Devonian granites are a major component of present outcrop (∼20% overall, up to 36% in the eastern and central Lachlan orogen), forming either major batholiths with N-S and NNE orientations roughly parallel to the structural grain, or single plutons (reviews by White and Chappell, 1983; Chappell et al., 1988; Chappell and White, 1992). With few exceptions, the granites are contact-aureole type, i.e., plutons formed at ∼2 kbar pressure or less within low-grade upper crustal rocks. Most are posttectonic and unmetamorphosed, although the oldest intrusions (Wagga-Omeo belt, Kosciusko batholith) carry significant foliation and were emplaced syn- to late-kinematically and at mid-crustal depths (e.g., Hine et al., 1978; Keay et al., 1999). Volcanic sequences, mostly daciterhyolite, are also widespread (15% of area in the eastern Lachlan orogen, significant post-tectonic caldera complexes in the western Lachlan orogen) and associated with shallow granitic plutons. This includes basaltic/andesitic remnants of the dismembered Ordovician Macquarie arc in the eastern Lachlan orogen (e.g., Glen et al., 1998).
K-Ar and Rb-Sr mica, Ar-Ar mica/hornblende, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages indicate a total age range of ca. 430 to 370 Ma (excluding the ≥440 Ma Macquarie arc), with two broad west-to-east younging trends in the eastern (430–370 Ma) and western (410–370 Ma) Lachlan orogen, respectively. Carboniferous (ca. 320 Ma) granites form a 100 km wide strip at the eastern edge of the Lachlan orogen near Bathurst; these are probably transitional to the Permian-Triassic granites of the outboard New England Fold Belt (Chappell et al., 1988).
S-type granites make up roughly half of all granite outcrop in the Lachlan orogen and are concentrated in a NNW-trending belt along the center of the orogen. Granodiorite and granite dominate, and coeval/cogenetic S-type volcanics are locally abundant (e.g., Wyborn et al., 1981). Compositions range from Mg-Fe-rich, cordierite-bearing granodiorites (e.g., the classical SE Australian S types of the Bullenbalong Suite; Hine et al., 1978; White and Chappell, 1988) to highly fractionated granite (e.g., Koetong Supersuite; Price et al., 1983). Felsic S-type granites (410–370 Ma) also occur in the western Lachlan orogen; several of the youngest (370 Ma) intrusions here are associated with extensive caldera complexes (Phillips et al., 1981; Clemens and Wall, 1984; Rossiter, 2003).
I-type granites form a broad belt along the eastern seaboard (e.g., the 8620 km2, 419–370 Ma Bega batholith) but are also abundant throughout the rest of the orogen. I-type volcanics are locally preserved in grabens (Wyborn and Chappell, 1986). Medium- to high-K granodiorites and granites dominate, with lesser tonalite and rare gabbro-diorite (<1%). Surprisingly, trace element patterns in average I and S types are similar and have the Sr-depleted, Y-undepleted (low Sr/Y, i.e., magma sources with residual plagioclase but no garnet) signature found in Australian Proterozoic granites (Wyborn et al., 1992), and in post-Archean upper crust in general. On most Pearce-type trace element discrimination plots, the Lachlan granites plot in the volcanic arc field.
A-type granites are rare and are thought to be derived by high-temperature melting of crustal rocks depleted by earlier I-type magma extraction (Collins et al., 1982; King et al., 1997). Sr-Nd isotope ratios (87Sr/86Sr = 0.704 to 0.720, ϵNd = +4 to −11, e.g., McCulloch and Chappell, 1982; Maas and Nicholls, 2002) define a continuous hyperbolic trend, typical of trends observed in many Phanerozoic granite provinces. I-type granites generally have lower 87Sr/86Sr and higher ϵNd than S types, which trend toward the more evolved (87Sr/86Sr = 0.715 to 0.730, ϵNd = −8 to −12; Adams et al., 2005) compositions of the Paleozoic tur-bidites. However, much isotopic overlap exists between the two granite types. Oxygen isotopes in the I-type (7.9‰–10‰) and S-type (9.2‰–12‰) granites of the eastern Lachlan orogen show correlations with Sr and Nd isotope ratios consistent with mixing of high-δ18O crustal and low-δ18O mantle-like components (McCulloch and Chappell, 1982).
Inherited zircon is ubiquitous in S-type granites of the eastern and central Lachlan orogen and has the characteristic pattern (major peaks at ca. 500 and 1000 Ma, with minor peaks to 3.6 Ga; Williams, 1992; Keay et al., 1999; Maas et al., 2001) found in early Paleozoic turbidites of the East Gondwana margin (Veevers, 2000b). By contrast, only weak, ca. 500 Ma zircon inheritance has been reported in S-type granites of the western Lachlan orogen (Elburg, 1996; Bierlein et al., 2001a). The presence of turbidite-derived detrital zircons in many of the S-type granites supports magma sources at least partly within the early Paleozoic metaturbidites, rather than within a hypothetical (but not exposed) Proterozoic basement. Zircon inheritance in I-type granites is typically weak but of the same pattern as in the S-type granites (e.g., Chen and Williams, 1991; Williams, 1992). This indicates a variable sedimentary component in their magma sources.
Debate about the origin of the Lachlan granites pits models that involve melting of distinct crustal protoliths (Chappell and White, 1992, 2001) against models emphasizing large-scale hybridism of mantle and crustal magmas (Gray, 1984, 1990; Collins, 1996, 1998; Keay et al., 1997). The granite compositions are ambiguous in terms of geodynamic setting, probably due to the crustal (sedimentary) influence even on I types. However, mafic dikes, intrusions, and lavas across the orogen carry inferred back-arc to arc mantle signatures (Bierlein et al., 2001b; Collins, 2002). This suggests that at least some of the granites, e.g., those in the margin-parallel batholiths of the eastern and central Lachlan orogen, formed within arc/backarc tectonic settings, albeit with much sedimentary (turbidite) input. In contrast, 370 Ma postorogenic magmatism in the western Lachlan orogen and elsewhere is probably related to collision (e.g., Soesoo et al., 1997; Soesoo and Nicholls, 1999).
Lachlan Orogen Tectonic Evolution
Extensive turbidite fan deposition took place in a marginal basin off the Gondwana margin between ca. 490 Ma and 470 Ma, with large turbidite fans spreading out onto recently formed oceanic crust of a developing marginal basin (Gray and Foster, 2004b; Foster et al., 2005). Turbidite deposition occurred at the same time as postorogenic magmatism, cooling, and erosional exhumation inboard in the Delamerian orogen from ca. 490 to 480 Ma (Fig. 5B) (Turner et al., 1992, 1996). Outboard subduction-related arc volcanism was initiated at ca. 485 Ma in the oceanic plate some thousands of kilometers away (based on retrodeformation of the WLO and CLO; Fergusson and Coney, 1992b), leading to the development of the Macquarie volcanic arc complex (Glen et al., 1998).
At ca. 460 Ma, inboard of the arc associated with the older, long-lived subduction zone, subduction has been inferred along both sides of a marginal backarc basin (Gray et al., 1997; Soesoo et al., 1997; Foster and Gray, 2000). Closure of the Lachlan marginal backarc basin has been suggested to have occurred by Woodlark basin–style, doubly divergent subduction (Gray et al., 1997; Soesoo et al., 1997). In this scenario, multiple oceanic thrust systems operated in both the eastern and western parts of the basin, i.e., the ELO and WLO, respectively (Fig. 6A), causing duplexing of oceanic crust, and imbrication and chevron folding in the overlying turbidite wedge (e.g., Spaggiari et al., 2004a, 2004b). Structural thickening of the turbidites in the western thrust system led to erosional exhumation and sediment output into the prograding marginal basin (Melbourne “trough”) (Fig. 6D) (Foster et al., 1998; Foster and Gray, 2000). At the same time, west-directed collisional shortening took place in northeastern Tasmania, and deformation of the mineralized Dundas belt (Fig. 4B) was occurring in western Tasmania (Bierlein et al., 2005).
Widespread postorogenic magmatism in the western part of the WLO at ca. 400 Ma (Fig. 5B) was followed by final closure of the marginal basin (Melbourne “trough”) at ca. 390 Ma (Fig. 6F). The collision between the WLO accretionary-style thrust belt and the CLO accretionary complex and magmatic arc was accompanied by localized strong to intense north-south folding and regional, meridional crenulation cleavage development in the CLO (Morand and Gray, 1991), and caused reactivation of shear zones in the Delamerian orogen and Precambrian basement massifs (Mount Painter, Broken Hill, and Tyennan of Tasmania) (Gray et al., 2000). This structural thickening and amalgamation of the WLO and CLO led to cratonization of the inner Lachlan (Gray, 1997; Foster and Gray, 2000). Outboard in the ELO, syn-deformational ca. 440–435 Ma magmatism and high-T metamorphic belts (e.g., Cooma Complex of Fig. 5A) formed during intermittent east-directed thrusting (Fergusson and VandenBerg, 1990) and periods of extension-related volcanism, particularly at ca. 420 Ma, as part of a Cordilleran or Andean-type margin (Fig. 6F) (Zen, 1995).
Postorogenic magmatism in the eastern part of the WLO (central Victorian magmatic province) occurred at ca. 370–360 Ma (Fig. 5B), while east-directed thrusting in the ELO caused inversion of former extensional basin faults and was followed by postorogenic magmatism (Fig. 5B). Cratonization of the Lachlan orogen was completed by ca. 330 Ma (Powell, 1984).
The Rakaia Wedge (Rangitatan Orogen), New Zealand
The Rangitatan orogen is a collage of accreted arc and forearc assemblages and a turbidite-dominated sediment wedge (Rakaia wedge) that formed as part of Jurassic through Cretaceous accretion along the Mesozoic margin of Gondwana (Coombs et al., 1976; Bradshaw, 1989; Mortimer, 1993). Structurally the sediment wedge is made up of two distinct parts (Fig. 7A): (1) the deeper-level Otago Schist belt characterized by schistosity and transposed layering at the mesoscale and by shear zones and apparent recumbent isoclinal fold nappes at the regional scale (Mortimer, 1993; Gray and Foster, 2004c), and (2) the chevron-folded, poorly cleaved younger sediments of the Pahau terrane that make up the bulk of the succession in the central and northeastern parts of South Island (Bradshaw, 1989).
Deep crustal seismic profiling (Mortimer et al., 2003) has shown that the fault-bounded Permian Dun Mountain ophiolite and Maitai terrane mélange (Cawood, 1987) define a steeply north-dipping interface between the gently folded, interleaved arc-forearc sequences (Median batholith; Brook Street and Murihiku terranes) to the south and the deformed Torlesse composite-terrane submarine fan sediments to the north (Fig. 7B). The Torlesse composite terrane includes a crustal section composed of structurally thickened (∼20 km thickness) Permian to Triassic-Jurassic sediments of the Rakaia terrane, structurally overlain by an ∼10 km thick wedge of trench sediments (Caples terrane) immediately adjacent to the Livingstone fault (Norris and Craw, 1987). In the immediate hanging wall to the Livingstone fault, intensely deformed monotonous quartzo-feldspathic schist with minor intercalated micaceous schist, greenschist, and metachert (Otago Schist) occupies a domal culmination with a maximum subsurface width of ∼220 km and ∼20 km of structural relief (Mortimer, 1993) (Fig. 7B). Northwards beyond the Waihemo fault, there is a transition into the tectonically imbricated and weakly metamorphosed Permian-Triassic graywacke sequence of the Rakaia (Older Torlesse) terrane.
South of the Livingstone fault the crustal section is composed of an ∼10–15 km thick succession of Murihiku terrane forearc sediments (Triassic to Jurassic volcanogenic sandstone, siltstone, and tuff) overlying an ∼10 km thick arc sequence of Brook Street terrane volcanic rocks (layered ultramafic-gabbro sequences, diorites, and volcaniclastic sediments that are the roots of a Permian intra-oceanic arc), intruded to the south by the Median batholith (see below).
Rakaia Wedge Metamorphism
The metamorphosed part of the Rakaia wedge (Otago Schist) is an ∼150 km wide, elongate, NW-trending metamorphic belt (Fig. 7A) that occupies a domal culmination defined by a broad warp in flatlying schistosity (Fig. 7B). It has lower-metamorphic-grade units (Caples on the south flank and Rakaia on the north flank) structurally overlying the high-grade schist core consisting of garnet-biotite-albite greenschist facies schist (Mortimer, 1993, 2000). Prehnite-pumpellyite facies on the northern and southern flanks (Torlesse and Caples terranes respectively) grade to biotite grade greenschist facies, and then to epidote-amphibolite facies in the broad central portion (Mortimer, 2000).
Mineral parageneses indicate peak P-T conditions of 450 °C and 8–10 kbar (Mortimer, 2000), suggesting burial to depths of ∼20–30 km and placing the Otago Schist in a moderate- to high-P/T metamorphic series (Barrovian-type). Relict blueschist assemblages occur in mafic greenschist-chert sequences close to the Caples-Torlesse boundary (Yardley, 1982). Riebeckite and crossite relicts occur mantled by actinolite, indicating an earlier high-pressure intermediate-type metamorphism that is overprinted by the Barrovian moderate- to high-P/T metamorphism.
Rakaia Wedge Magmatism
The Rakaia wedge shows no evidence of magmatism, but subduction-related magmatism was long-lived in the overriding plate, typified by the Median batholith (Mortimer et al., 1999a). Granitic complexes exposed in South Island and recovered from submarine extensions range in age from Late Devonian to Late Cretaceous (Tulloch, 1983, 1988; Wandres and Bradshaw, 2005). The oldest granites were emplaced into Paleozoic metasediments of the Western province. This includes the voluminous Late Devonian (ca. 375 Ma) mixed S- and I-type Karamea batholith, the 375 Ma mafic Riwaka Complex, and Carboniferous (360–300 Ma) I-, A-, and S-type plutons (Cooper and Tulloch, 1992; Muir et al., 1996a, 1996b, 1998). The ca. 375 Ma granites are related to a magmatic episode that is also recognized in SE Australia and West Antarctica.
Fragments of a long-lived magmatic arc (arc volcanics, volcanogenic sediments, and granitoids) are preserved in the Mesozoic Median Tectonic Zone, between the Paleozoic Gondwana margin of the Western province and the accreted Eastern province (Bradshaw, 1993). The granitoids, representing semicontinuous (ca. 230–125 Ma) calcalkaline, metaluminous, low- to medium-K, I-type magmatism, are exposed in the northwest and south of South Island and have been assigned to the 10,200 km2 Median batholith (Kimbrough et al., 1993, 1994; Muir et al., 1998; Mortimer et al., 1999a). As in other Mesozoic Andean arcs, mafic granitoids (gabbros, diorites) are common; A-type granites are rare. Although there is little evidence of crustal involvement (no S-type granites, almost no zircon inheritance, 87Sr/86Sr ≈ 0.7037, ϵNd ≈ +4; Muir et al., 1998; Mortimer et al., 1999b), the Median batholith arc is thought to have been emplaced at and into the Paleozoic Gondwana margin of the Western province, in response to outboard accretion of the Eastern province (e.g., Muir et al., 1998; Daczko et al., 2001). However, the geometry of the margin and the locus of subduction responsible for the Mesozoic arc are still unclear.
Final arc-margin collision is marked by a chemically distinct suite of stitching granites, the 124–111 Ma Separation Point Suite (in the north of South Island) and their likely lower crustal equivalents, the 126–119 Ma metadiorites and metagab-bros of the Western Fiordland Orthogneiss (e.g., McCulloch et al., 1987; Muir et al., 1995, 1998). These I-type suites, while covering a wide range in SiO2, are distinctly sodic in character and have high to very high Sr/Y ratios, thus resembling Archean TTG (tonalite-trondhjemite-granodiorite) or modern adakite suites (Muir et al., 1995). Tulloch and Kimbrough (2003) drew attention to the spatial and age pattern of early, outboard low-Sr/Y and later, inboard high-Sr/Y granitic magmatism, represented by the Median batholith and Separation Point–Western Fiord-land Orthogneiss, respectively.
This margin-normal arrangement, mirrored to a surprising extent at other Mesozoic active margins (e.g., Petford and Atherton, 1996), is thought to reflect melting within metabasaltic subarc crust and mantle in a progressively thickening arc. The youngest and most inboard granites related to this “low-Sr/Y, high-Sr/Y” association, the ca. 110 Ma Rahu Suite, is represented by mildly “high-Sr/Y”, I-type and transitional I-S-type plutons in the Hohonu and Paparoa batholiths in NW South Island, intruding Western province Paleozoic metasediments. Zircon inheritance (500–1000 Ma), evolved Sr-Nd isotopic signature (87Sr/86Sr = 0.7062 to 0.7085, ϵNd ≈ −5), and elevated δ18O (Tulloch, 1988; Waight et al., 1998a) indicate that Rahu Suite granites may represent blends of Separation Point–type arc crust- and Paleozoic margin-derived magmas (Tulloch and Kimbrough, 2003). Waight et al. (1998a, 1998c) suggest that Rahu Suite magmatism occurred in a continental margin undergoing a switch from convergence to extension. Advanced extension of basement is marked by ca. 82 Ma mafic dikes and A-type granites in the Western province (Hohonu Dike Swarm, Buller Gorge Dikes, French Creek Suite) that are coeval with the opening of the Tasman Sea between SE Australia and New Zealand (Waight et al., 1998b).
In contrast to the Lachlan and Damara orogens, granites do not occur (or are not presently exposed) in the accreted turbidite terranes of New Zealand's Eastern province. All presently known granites coeval with the Rangitatan orogeny occur within the Mesozoic arc (Median Tectonic Zone, or Tutoko Complex; Wandres and Bradshaw, 2005) or immediately inboard within the Paleozoic Gondwana margin of the Western province. Granite compositions overall are more mafic and almost exclusively I-type (although true granites and A types do occur), zircon inheritance is absent or much weaker (compared to Lachlan orogen granites), and crustally dominated Sr-Nd-O isotopic compositions are rare. This reflects the absence of Proterozoic basement and suggests that equivalents of older oceanic arc terranes of the Eastern province (Permian Murihiku and Brook Street terranes), rather than the crustally derived turbidites accreted farther outboard, became involved in Jurassic–Early Cretaceous granite magma production.
Rakaia Wedge Tectonic Evolution
The generally accepted tectonic evolutionary model for New Zealand (e.g., Coombs et al., 1976; Bradshaw, 1989; Roser and Cooper, 1990) requires convergence of the Permian-Jurassic Torlesse fan with arc–forearc–trench slope sequences (Brook Street, Maitai, Murihiku, and Caples terranes) due to subduction of Permian oceanic crust beneath the Gondwana margin (Fig. 8). Coupling related to this subduction caused structural thickening of the Torlesse turbidite fan in the Late Jurassic (ca. 150–140 Ma), with Early Cretaceous (130–120 Ma) structural interleaving of Caples and deformed Torlesse sediment resulting in final accretion of the turbidite fan to the margin (Bradshaw, 1989; Mortimer, 1993; Gray and Foster, 2004c). This coincided with collision between the Median Tectonic Zone (Median batholith) and the Western province and was marked by the sudden onset of Separation Point–type magmatism at 125–118 Ma (Waight et al., 1998c; Kimbrough et al., 1994). Erosional exhumation and core complex development occurred in the late Early Cretaceous (110–90 Ma) as a precursor to Tasman Sea opening (Ireland and Gibson, 1998; Spell et al., 2000). Thermal decay from Tasman Sea spreading commenced at ca. 84 Ma and was well under way at 70 Ma (Johnson and Veevers, 1984).
NEOPROTEROZOIC CONTINENTAL GROWTH (THE DAMARA BELT OF THE DAMARA OROGEN, NAMIBIA): RECYCLED TURBIDITES INVOLVING A WILSON CYCLE?
The Neoproterozoic Damara orogen is part of the Pan-African orogenic system (e.g., Coward, 1981, 1983) and within Namibia consists of the Damara belt, or Inland Branch, and coastal branches including the Kaoko and Gariep belts in the north and south respectively (Fig. 9A) (e.g., Porada et al., 1983; Prave, 1996). Turbidites constitute a major component of the Damara belt or Inland Branch (Martin and Porada, 1977; Hoffmann, 1983; Martin, 1983; Miller, 1983a, 1983b; Kukla et al., 1988) and represent a Neoproterozoic submarine fan system and/or accretionary complex that formed in part of an ocean basin (Khomas trough) between the Congo and Kalahari cratons (Miller et al., 1983a; Kukla et al., 1988; Kukla and Stanistreet, 1991).
Relict oceanic lithosphere is possibly preserved by the thin (200–300 m thick), but laterally extensive (over 350 km in length) (Fig. 9B), shear zone–hosted Matchless amphibolite (Barnes, 1983; Killick, 2000). This unit consists of intensely deformed basalt, pillow basalt, chert, and gabbro, with tholeiitic geochemistry (Schmidt and Wedepohl, 1983), as well as dispersed serpentinite bodies (boudins). The basalt chemistry has been used to argue for an ensialic origin with basalts formed on thin crust (Schmidt and Wedepohl, 1983), although the dispersed ultramafic bodies have Alpine chemical affinities more typical of oceanic lithosphere (Barnes, 1983).
The flanks of the former basin now consist of craton-verging thrust systems within shelf carbonates (e.g., Hakos and Naukluft nappes in the Southern Margin Zone) (Korn and Martin, 1959; Coward, 1983; Miller, 1983b). The Northern Zone underwent metamorphism from ca. 540 to 530 Ma (Goscombe et al., 2004). The basinal Khomas turbidite sequence is metamorphosed to form the Kuiseb Schist now subdivided into two major structural/metamorphic zones, referred to as the Central and Southern Zones (Fig. 9B). The granite-dominated Central Zone (Fig. 10A), metamorphosed under high-T/low-P conditions between 538 and 505 Ma, is floored by an attenuated Congo-like craton, with dome and basin fold interference (e.g., Jacob et al., 1983; Coward, 1983; Miller, 1983b, p. 459; Kröner, 1984; Kisters et al., 2004), and possible extensional structures (e.g., Oliver, 1994). The Central Zone has two phases of metamorphism at 538–516 Ma and 511–505 Ma with granitic magmatism between ca. 560 Ma and ca. 470 Ma (Jacob et al., 2000; Jung and Mezger, 2003a, 2003b).
The thicker basinal sequence has undergone intermediate-T/intermediate-P (Barrovian series) metamorphism and is now part of the ∼100 km wide Southern or Khomas Zone (Kasch, 1983a; Kukla, 1993). This zone consists of homoclinally north-dipping Kuiseb Schist with transposed foliation and schistosity (Hälbich, 1977; Miller, 1983b, p. 461–466; Kukla et al., 1988; Kukla and Stanistreet, 1991), which represents a major shear zone interface transitional into the basement-cored fold-and-thrust nappes of the Southern Margin Zone (Fig. 9B) (Miller, 1983b, p. 463–468; Coward, 1983). Based on U-Pb ages for metamorphic monazite, Kukla (1993) suggests an age of 525–515 Ma for peak metamorphism in the Kuiseb Schist; a lower limit is given by the intrusion of the 505 ± 4 Ma posttectonic Donkerhuk granite (Kukla, 1993).
The western Northern Zone (Ugab domain) is influenced by transition between basinal turbidite facies and slope-to-platform carbonates (Miller et al., 1983b; Swart, 1992; Passchier et al., 2002). It also represents the junction between Kaoko and Inland Branches, and is intruded by granites of the Central Zone (Coward, 1983; Maloof, 2000; Passchier et al., 2002; Goscombe et al., 2004).
Damara Belt Metamorphism
The Damara belt has a distinct metamorphic zonation with belts of intermediate-T/intermediate-P (Barrovian-style) metamorphism bounding the Central Zone (Fig. 10B), which has undergone moderate-P/high-T metamorphism with peak temperatures ∼650 °C and pressures ∼4.3 kbar (Kasch, 1983a). The Central Zone has garnet-cordierite granulites (e.g., Puhan, 1983; Masberg, 2000) and clockwise P-T paths and shows a high thermal gradient (30–50 °C/km) (e.g., Jung, 2000). It contains Atype, posttectonic granites that originate from mid-crustal, low-pressure melting (McDermott et al., 2000).
Intermediate-T/intermediate-P (Barrovian-style) metamorphism occurs within the Southern and Northern Zones. The Southern Zone metamorphic conditions are moderate-P and -T, with peak conditions of ∼600 °C and ∼10 kbar but showing a decrease in pressure northwards to ∼4 kbar toward the Okahandja Lineament (Kasch, 1983a). The Southern Zone has a low thermal gradient (18–22 °C/km), with syntectonic garnet and posttectonic staurolite in metapelites defining a clockwise P-T path. The Northern Zone shows high P and moderate T with a clockwise P-T path; peak conditions are estimated at 635 °C and 8–7 kbar and an average thermal gradient of 21 °C/km (Goscombe et al., 2004).
Damara Belt Magmatism
As in the Lachlan orogen, granitic rocks are a major component of the Damara orogen (Inland Branch), composing ∼74,000 km2, or roughly 25% of outcrop (see review by Miller, 1983a). Most of the granitic magmatism was concentrated in the high-temperature Central Zone, with minor activity in the Northern Zone; the very large posttectonic Donkerhuk granite (5000 km2) represents the only significant granitic complex in the lower-temperature Southern Zone. Granites dominate the compositional spectrum, with an areal ratio of granite to grano-diorite-tonalite to diorite-gabbro of 96:2:2. Characterization of Damara granites is hampered by the structural and lithological complexity of syn- and late- to posttectonic intrusions, the high metamorphic grade of the deeply eroded orogen, and the lack of reliable high-precision geochronology. The pre-1990 granite age framework, based largely on Rb-Sr whole-rock isochrons, suggests a period of 190 m.y., from 650 to 460 Ma, for the Pan-African granitic magmatism. More recent studies (U-Pb monazite, titanite, zircon, allanite; Sm-Nd garnet) indicate an age range for granitic magmatism from 560 to 470 Ma (e.g., Jacob et al., 2000; de Kock et al., 2000; Jung and Mezger, 2003a), similar to the age range of regional metamorphism.
McDermott et al. (1996) used geochemistry to divide the granites into three groups irrespective of timing relative to deformation. Group 1 comprises the widespread, mostly peraluminous (S-type) leucogranites (including, for example, the large posttectonic Donkerhuk batholith) and Salem-type granites (a generic term for a diverse group of porphyritic, biotite-rich granites) and the volumetrically minor but economically important alaskites (e.g., Rössing; Basson and Greenway, 2004). Mildly peraluminous, Nb-Zr-Y (high field strength elements, or HFSE) -rich Atype granites with within-plate signatures make up group 2, while group 3 encompasses the more mafic, metaluminous and partly calc-alkaline quartz diorites and granodiorites.
Group 1 granites, volumetrically by far the most important, show an exceptionally wide range in Sr-Nd isotope ratios (87Sr/86Sr = 0.708 to 0.740, ϵNd ≈ −2 to −19; Haack et al., 1982; McDermott et al., 1996; Jung et al., 2001, 2003), reflecting involvement of negative ϵNd Archean-Mesoproterozoic basement gneisses in some complexes (e.g., Jung et al., 2003). Many of the S-type granites, however, have ϵNd near −5, similar to Neoproterozoic (Swakop Group) turbidites (McDermott and Hawkesworth, 1990; Jung et al., 2001). δ18O values are typically high (11‰–15‰, e.g., Jung et al., 2000, 2003), similar to both Damara metasediments and pre-Damara basement. McDermott et al. (1996) suggest a quasi-secular decrease in ϵNd with decreasing granite age, perhaps due to a retreat of the melting zone into the lower crust after the metamorphic peak (<500 Ma), but this appears to be an oversimplification: The Rb-Sr ages used are unreliable, and large ranges exist even within single complexes (e.g., ∼9 ϵNd units for posttectonic Donkerhuk granite).
Chemical and isotopic data for the S-type granites indicate overwhelmingly crustal sources within the Damara metasediments and/or pre-Damara basement highs, with melt generation by dehydration melting at mid-crustal levels (pressures >6 kb). The distinct chemistry of the alaskites suggests they represent rapidly withdrawn (disequilibrium) melts generated by muscovite dehydration melting of pelites (McDermott et al., 1996).
Group 2 granites, with A-type affinities, show a range of ages (525 Ma syntectonic to ca. 486 Ma posttectonic; Jung et al., 1998a, 2000; McDermott et al., 2000), similar to the S-type granites. Sr-Nd isotope signatures are less evolved (87Sr/86Sr = 0.7034 to 0.709, ϵNd = 0 to −6, as low as −16 for the Sorris-Sorris A type), but δ18O is still high (10‰–13‰). Generation of hot (>800 °C) A-type magmas is thought to involve remelting of a deep-seated (pressures of 8–10 kb) metatonalite source followed by extensive differentiation in some cases. Mixed (mantle + crust) isotopic signatures may be a feature of the metaigneous lower crustal source rocks.
Like group 2, group 3 quartz diorites and granodiorites are quite rare (2%) but have received attention because they may help establish the tectonic setting of the Pan-African granitoids. Group 3 diorites are metaluminous, carry hornblende and relict clinopyroxene and are clearly I-type. However, isotopic signatures vary conspicuously (87Sr/86Sr ≈ 0.704 to 0.713, ϵNd = 0 to −20, δ18O = 7‰ to 13‰; Haack et al., 1982; Hawkesworth et al., 1981; Miller, 1983a; McDermott et al., 1996; Jung et al., 2002; van de Flierdt et al., 2003) and clearly imply a range of magma sources. Titanite and zircon U-Pb ages near 540 Ma suggest the diorites are consistently among the oldest Pan-African intrusives, coeval with the onset of high-grade metamorphism (Jung and Mezger, 2003a). A distinct suite of ca. 540 Ma syenites (with more limited Sr-Nd isotopic signature; Jung et al., 1998b) was also formed at that time. Petrogenetic models for at least some dioritic complexes (Goas, Okongava in the Central Zone, Bandombaai in the southern Kaoko belt, with mild to high Sr/Y ratios and depletion in heavy rare earth elements) involve remelting of high-K basalts in the deep crust. Occurrence of very low ϵNd, high δ18O, and strongly retarded Pb isotope ratios suggest involvement of Archean-Proterozoic granulitic basement, presumably via deep crustal assimilation. Other diorites, with more primitive Sr-Nd-O isotope signatures (Hawkesworth et al., 1981) appear to lack this assimilation component.
The dominance of S-type granites and the absence of clearly mantle-like (positive ϵNd, low δ18O) isotopic signatures even in mafic I-type granitoids indicate the importance of crustal recycling rather than growth during the Damara orogeny (McDermott and Hawkesworth, 1990; Jung et al., 2003). Clearly, tectonic processes, thermal conditions (including the well-known high heat production in Damara metasediments; Haack et al., 1983), and high fertility combined to produce massive mid/lower crustal melting. The issue of the tectonic setting for Damara Pan-African magmatism is far from settled. Magmatic arc models (e.g., Kasch, 1983a) have been questioned because of the lack of margin-parallel linear batholiths and margin-normal geochemical gradients, the preponderance of felsic S-type granites, and the scarcity of mafic granitoids with mantle-like isotopic signatures (e.g., Jung et al., 2003). With respect to the last point, it should be kept in mind that pervasive crustal melting would set up a density filter preventing further ascent of mafic magmas, and would greatly increase the chance of magma hybridization in the crust. The small volume and crustal isotopic signatures of the rare I-type granitoids thus does not rule out greater mantle contributions at depth.
Damara Tectonic Evolution
Rifting between the Congo and Kalahari cratons has been inferred to develop along three northeast-trending intracontinental rifts between ca. 840 Ma and ca. 730 Ma (Miller, 1983b), although dating of Northern Zone rift-related rhyolites has provided ages 746 ± 2 Ma and 747 ± 2 Ma (Hoffman et al., 1996). Synrifting silicic volcanism in the Central Zone took place at ca. 750 Ma (de Kock et al., 2000). Eventual development of an oceanic spreading center and growth of oceanic crust at ca. 700 Ma in the southern part led to development of the Khomas ocean basin (Miller, 1983b). Like the Lachlan orogen, there are conflicting tectonic hypotheses for closure of the Khomas basin and the nature of the substrate to the turbidites (e.g., Porada, 1979, 1983; Martin, 1983).
The Khomas basin has been depicted either as an intracontinental basin between the Congo and Kalahari cratons with turbidites sitting on attenuated continental lithosphere (Porada, 1979, 1983; Martin and Porada, 1977; Hawkesworth et al., 1983), or as an ocean basin floored in part by oceanic lithosphere that closed by subduction beneath the leading edge of the Congo craton (Fig. 11) to form an Andean-type margin with development of voluminous magmatism (e.g., Barnes and Sawyer, 1980; Miller, 1983b; Kasch, 1983b; de Kock, 1992), not unlike the Silurian-Devonian of the eastern Lachlan orogen (e.g., Zen, 1995). Basement-cored domal culminations in the Central Zone (Jacob et al., 1983; Puhan, 1983; Kröner, 1984; Oliver, 1994; Kisters et al., 2004) and basement-type isotopic signatures in some granites require the presence of attenuated Congo continental crust in this part of the Khomas ocean basin, probably not unlike the present-day Japan Sea, which has a complex distribution of attenuated continental crust, rifted crust, and oceanic crust (Tamaki, 1995).
Closure of the former Khomas ocean basin involved high-angle convergence with overthrusting at the margins to give a doubly vergent orogen (Fig. 9C). Apart from the laterally continuous, shear zone–hosted Matchless amphibolite there is no classic suture. The distinct metamorphic zonation of the Inland Branch, with Barrovian metamorphism on the orogen flanks, reflects structural thickening at the craton margins, while Andean- or Cordilleran-style high-T/low-P metamorphic conditions in the Central Zone and accretionary prism-like features of the Southern Zone Kuiseb Schist (Kukla and Stanistreet, 1991) reflect subduction beneath this zone (cf. Maloof, 2000). Arguments against subduction-related closure of the Khomas ocean basin have been based on granite geochemistry (Hawkesworth et al., 1983), as well as the lack of blueschists and eclogites or typical subduction zone low-T/high-P metamorphism (Kröner, 1982); but see Kasch (1983a) for a discussion.
PROCESSES OF ACCRETION IN TURBIDITE-DOMINATED OROGENS
Crustal accretion for the Lachlan, Rangitatan, and Damara orogens as described in this paper involves deformation of tur-bidite-dominated submarine fans and the underlying oceanic basement, resulting in marked (>50%) shortening and crustal thickening. In the deformed state, the turbidites now occur either as belts of thrust-imbricated, upright chevron-folded, low-grade turbidites or as zones of higher-metamorphic-grade, inclined, homoclinally dipping schistosity and transposition layering.
Turbidite deformation resulted from convergent margin tectonism driven by closure of marginal basins (e.g., western Lachlan orogen), accretion of intraoceanic plateau (e.g., Chatham Rise of the Rangitatan orogen), and Wilson cycle closure of a small ocean basin by convergence between cratonic nuclei (e.g., Damara belt of the Damara orogen). The turbidites are the common link, but each orogen has its individual style or crustal architecture as a function of the tectonic setting and the position of the turbidites in this setting. The Lachlan, Rangitatan, and Damara orogens show that parameters such as the thickness, structural style, metamorphic character, and tectonic position of the turbidites, or former submarine fan, as well as the age and nature of the underlying substrate, are all important variables for crustal evolution (Table 1; Fig. 12). Proximity to large continental landmasses with active uplift and relatively rapid erosion is required to produce the large turbidite fans that characterize these orogens and others like them (e.g., Ingersoll et al., 2003).
Positions of Turbidites in Ancient Orogens
The tectonic setting and position of the turbidite fans affect the final architecture and characteristics of the deformed turbidite in each of the three orogens. In most orogens, tectonic reconstructions are based on recognition and interpretations of key geologic or tectonic elements that make up the orogen, as well as the structural architecture and tectonic vergence defined by fault or shear zone dips, the distribution of rock types, and the temporal and spatial distributions of metamorphism and igneous activity (e.g., Gray and Foster, 2004a, 2004b, for the Lachlan orogen; Mortimer, 2004, for the Rakaia wedge of New Zealand). The thickness of the turbidite successions and the degree of shortening (>50%) and thickening (up to 300%) supports an oceanic depositional setting so that the final geometry is not the result of ensialic deformation as inferred from consideration of granite petrogenesis as the determinative factor. The Lachlan orogen has an extensive volume of turbidites, extending over the 750 km width, thrust systems of mixed vergence, and an included high-T/low-P metamorphic complex. How this belt evolved and in what setting has been debated, but there are certain characteristics of the Lachlan orogen that are important for any tectonic reconstruction (see Gray and Foster, 2004b, p. 792–793). These include
three simultaneously operating oceanic thrust systems in different parts of a marginal ocean basin that was behind a long-lived, outboard subduction system along the eastern margin of Gondwana;
blueschist blocks in serpentinite-matrix mélange along major faults in the WLO and CLO;
mélange and broken formation along faults within the frontal fault system of the CLO (the Howqua accretionary complex);
discordant posttectonic granites in the WLO and large elongate composite granitoid bodies in the CLO (northwest-trending) and ELO (north-trending); and
shear zone–bounded high-T/low-P metamorphic complex with regional aureole-style metamorphism related to significant high-level S-type granites.
These features indicate that Lachlan orogen accretion was by thickening and imbrication of extensive and thick submarine fans, including the underlying oceanic lithosphere, in a “Woodlark basin style” doubly divergent subduction system (e.g., Gray et al., 1997; Soesoo et al., 1997).
In this scenario, subduction zones, essentially zones of underthrusting, were initiated on the opposing sides of a former Cambrian-Ordovician backarc basin (Fig. 6). Low-angle under-thrusting in the western Lachlan orogen (WLO, Fig. 4A) resulted in marked shortening (∼65%–70%) by chevron folding, cleavage development, and thrust imbrication of the turbidites (e.g., Gray and Willman, 1991; Gray and Foster, 1998) but without development of a magmatic arc. However, subduction along the eastern side of the marginal basin, represented by the Howqua accretionary complex (Fig. 4A), produced large elongate composite batholiths that define a shear zone–bounded, northwest-trending magmatic arc, as well as the associated high-T/low-P metamorphism (WOMB of Fig. 4A).
Although there is now a general acceptance that the Lachlan turbidite fans developed on backarc basin lithosphere (e.g., Crawford and Keays, 1978, 1987; Foster et al., 2005), the Lachlan substrate is analogous to the Philippine Sea or Japan Sea (see Gray and Foster, 2004b). Each analog has different consequences with possible microcontinental ribbons (e.g., Tasmania and/or the Selwyn block; Cayley et al., 2002) in the former, and relict arcs (e.g., Licola arc; Spaggiari et al., 2003a, 2003b) in the latter providing significant rigidity contrasts during crustal shortening and thickening.
For the Damara orogen, despite the controversy over the tectonic setting of the Central Zone, the accretionary prism-like nature of the Southern Zone schists (Kukla and Stanistreet, 1991) as well as their strong similarities to those of the Otago Schist belt in New Zealand (see photos in Gray and Foster, 2004c) suggest that this orogen is another example of continental growth by subduction-accretion of turbidites (cf. Kukla and Stanistreet, 1991). Damara orogen evolution, however, involves a full Wilson cycle with the opening and closing of a small ocean basin partly floored by attenuated continental crust of the Congo craton and partly underlain by ca. 700 Ma oceanic lithosphere formed during opening of the Khomas ocean basin (e.g., Prave, 1996).
The Rangitatan orogen of New Zealand provides the least complicated setting, representing part of the Mesozoic convergent margin of Gondwana facing a large ocean basin (cf. Busby, 2004), the Rangitatan orogen evolved in a forearc position typical of the modern-day Aleutian arc-subduction complex (“ring of fire”-type arc). In contrast, the Lachlan orogen involves closure of a backarc basin related to possible multiple subduction systems (e.g., Spaggiari et al., 2004a), while the Damara belt of the Damara orogen involves a single subduction system but with final continent-continent collision (e.g., Barnes and Sawyer, 1980).
Parameters Affecting Accretionary Orogen Development: Fan Thickness and Timing
Apart from their tectonic positions, other parameters that affect accretionary orogen development include the thickness of the fan, the depositional age of the fan, the timing of fan shortening and thickening, and the age of the oceanic lithosphere relative to basin closure (Fig. 12). These can be measured or determined for the individual orogens, whereas parameters such as the residence time on the seafloor are derivative of the former (see Table 1).
For the Lachlan orogen, the age of backarc basin crust is 505–495 Ma (e.g., Spaggiari et al., 2003a; Foster et al., 2005), the age of the developing submarine fan system is from 490 to 460 Ma, and the deformation that caused the backarc basin closure was from ca. 450 Ma to 420 Ma (i.e., some 50 m.y. after oceanic lithosphere formation and some 30–20 m.y. after submarine fan deposition). These conditions favor chevron folding, without the significant stratal disruption and mélange or broken formation typical of shallow levels of modern accretionary complexes. Chevron folding requires lithification or precompaction of the sediment fan and therefore time for burial and dewatering on the seafloor before deformation (e.g., closed-system vein formation during shortening; Gray et al., 1991). Metamorphism due to sediment loading is known to occur in the modern-day Bengal fan (Curray et al., 1982).
In this accretionary process, oceanic lithosphere is preserved as fault-bounded slices derived from “peeling” of oceanic lithosphere in the subduction zone (e.g., Kimura and Ludden, 1995). The temperature, and therefore age, of the oceanic lithosphere is important. The western Lachlan orogen shows that thick, lithified or partially lithified turbidite fans (4–5 km thicknesses) sitting on old, cold oceanic lithosphere produce dominantly chevron-folded, thrust-interleaved packages that incorporate fault-bounded, duplexed slivers of the upper parts of the backarc basin oceanic lithosphere (i.e., Cordilleran-style ophiolitic fragments). This process is inferred to occur by low-angle underthrusting with the turbidites predominantly deforming in the overriding plate (see Gray and Foster, 1998; Spaggiari et al., 2004a).
In the case of the Rakaia wedge of New Zealand, the age of the underlying oceanic lithosphere as inferred from the Dun Mountain ophiolite is ca. 280 Ma (Kimbrough and et al., 1992) with Permian to Triassic submarine fan sedimentation (ca. 280 to 200 Ma; Bradshaw, 1989) and eventual wedge thickening at 160–150 Ma (Gray and Foster, 2004c) (i.e., some 120 m.y. after oceanic lithosphere formation and some 40 m.y. after fan deposition).
In this respect, plate convergence for significant parts of the Lachlan and the Rakaia wedge of New Zealand involves old, cold oceanic lithosphere and thick turbidite fans (4–5 km thicknesses). The deformation of the turbidites occurs either in the hanging wall above a shallow-dipping subduction zone or on the down-going slab (e.g., Spaggiari et al., 2004a, for the Lachlan orogen; Coombs et al., 1976, for the Rakaia wedge deformation). We speculate that the deformation and metamorphism of the Kuiseb Schist of the Damara Inland Branch also occurred on the down-going slab (cf. Barnes and Sawyer, 1980).
Both the Rangitatan (Otago Schist) and Damara orogens (Southern Zone Kuiseb Schist) show evidence for coupling between the overriding margin and the subducting slab, with the thick turbidite fan sitting on the subducting plate, producing an intensely deformed, highly thickened wedge that undergoes Barrovian-style metamorphism. The result is intensely deformed schist with transposition layering in the former turbidites. In both the Rakaia wedge and Southern or Khomas Zone of the Damara, the wedge underwent significant structural thickening by shear-related, noncoaxial deformation at the subduction interface (e.g., Mortimer, 1993; Gray and Foster, 2004b) to produce transposition layering or schistosity and a pronounced rodding lineation (i.e., S-L tectonites). These are the dominant fabrics produced in this subduction interface environment that have been previously attributed to underplating in a coaxial flattening strain environment (e.g., Lundberg and Reed, 1991; Von Huene and Scholl, 1991).
In both these cases, arc magmatism occurs in the overriding plate but not in the intensely coupled zones. In New Zealand, magmatism occurs inboard of the ophiolite “suture” with a complete lack of granitoids in the thickened wedge (Fig. 7C), presumably a reflection of underlying thin crust and deformation directly above the subducted slab rather than over areas of the asthenospheric wedge. Both arc regions, inboard of the subduction complex schistose rocks, contain granulites in the lower parts of the magmatic arcs: Fiordland in New Zealand (Claypool et al., 2001) and the Central Zone of the Damara (Puhan, 1983).
In the case of the Damara, the presence of oceanic lithosphere, and with it the role of subduction, have been disputed (e.g., Martin, 1983). The MORB character of the Matchless amphibolite and the association of chert, pillow basalt, and gabbros is suggestive of remnants of the upper parts of oceanic crust (e.g., Killick, 2000); similar lithological packages are present in the Cambrian ophiolite fragments of the Lachlan orogen (e.g., Spaggiari et al., 2004a). The unusual elements of the Matchless amphibolite are its length (∼350 km), its uniform lateral continuity, and style of deformation. The extent and fault-bounded and/or shear zone–bounded nature of the Matchless amphibolite, however, has certain similarities to the Dun Mountain ophiolite of New Zealand that extends for some 600 km, but separates the arc-forearc sequences from the deformed fan turbidites (Fig. 7) 702, rather than occurring within the deformed sediment wedge (Fig. 9C).
The Template of Accretion
Turbidite-dominated orogens tend to (1) have greater width than the classic Alpine orogen typical of continent-continent collision, and (2) be dominated by turbidites instead of batholithic rocks of the Andean orogen typical of modern Pacific-type subduction. The Lachlan, Damara, and Rangitatan orogens have ∼750, ∼450, and ∼400 km widths, respectively. Despite providing a disconnected history of the tectonic process of accretion through time from the Neoproterozoic to Holocene, the Damara, Lachlan, and Rangitatan orogens clearly show that there is no simple template for the accretionary process. This is reflected in the crustal-scale architecture of each orogen, as well as the temporal and spatial variations in the development of deformation fabrics, metamorphism, and magmatism. The Lachlan orogen is made up of three thrust belts with contrasting vergence, whereas the Damara orogen shows divergent thrust systems, and the Rangitatan orogen one thrust system and a steeply dipping backstop behind the sediment wedge.
Within the orogen architecture, the positions of metamorphic rocks and the spatial variations in different types of metamorphism delineate or help reconstruct the tectonic setting. To a first-order approximation, the low-T/high-P metamorphic rocks (blueschists and eclogites) define the subduction channel, the moderate-T, moderate- to high-P (Barrovian-style) metamorphic rocks define the regions of structural thickening above the subduction interface, and the high-T/low-P regional aureole metamorphic rocks intruded by large, elongate composite granitoids define the magmatic arc.
The Rakaia wedge part of the Rangitatan orogen has relict blueschist metamorphism preserved in intercalated metabasites near the interface between the volcaniclastic trench sediment and the deformed quartz-rich fan (Yardley, 1982), whereas the wedge proper shows Barrovian metamorphism with garnet-oligoclase assemblages in the Otago Schist core (Mortimer, 2000).
Similarly, the schistose part of the Damara orogen shows intermediate-T/intermediate-P (Barrovian-style) metamorphism (Kasch, 1983b), but relict blueschist metamorphism has not been recognized. In the Damara, the schistose belt dips toward, but verges away from, the Central Zone belt of large, irregularly shaped, syntectonic granitoids and the high-T metamorphism (Puhan, 1983).
The Lachlan orogen shows background low-grade, intermediate-P metamorphism (Gray and Foster, 2004b), but has two belts of high-T regional aureole-style metamorphism associated with northwest-trending (Wagga-Omeo belt of the central Lachlan orogen) and north-trending (eastern Lachlan orogen), large, elongate, composite granitoids. In the western and central Lachlan orogen, blueschist metamorphism is preserved in knockers within serpentinite-matrix mélange of the major faults zones (Spaggiari et al., 2002a, 2002b).
Turbidite-dominated orogens therefore develop in convergent margin settings involving subduction of oceanic crust containing large submarine turbidite fans, but have no specific template or orogen design. They have special character reflected by the large volumes of structurally thickened and metamorphosed turbidites and relicts of oceanic crust as fault-bounded slivers.
The Role of Magmatic Underplating: Chemical Maturation of Recycled Crust
Granitic magmatism plays an important role in the chemical maturation of these structurally thickened and accreted turbidite fan systems. Turbidite-dominated orogens tend to be characterized by large volumes of syn- to posttectonic granite, such as those in all zones of the Lachlan orogen, and in the Central Zone of the Damara orogen.
Turbidites, in particular the more feldspathic (graywacke) and pelitic lithologies, represent fertile sources for crustal melting (Clemens and Vielzeuf, 1987; Thompson, 1996). There is clear isotopic evidence in both the Lachlan and Damara orogens for derivation of most of the S-type granites from the local metaturbidites. Additional source components are sometimes required to explain the chemical/isotopic constraints, e.g., contributions from underlying Cambrian oceanic crust and synmagmatic mantle-derived melts (Lachlan orogen; Keay et al., 1997; Maas et al., 2001), or from old crystalline basement (Damara granitoids with very low ϵNd; Jung et al., 2003).
The large volumes of granite imply significant crustal melting, with a requirement of mafic magma underplating at the base of the crust. Thermal conditions related to tectonic setting as well as a high fertility and, in the case of the Damara, high HFSE contents of the crust, combine to produce the observed massive mid/lower crustal melting in turbidite-dominated orogens.
Ancient orogens dominated by turbiditic interbedded sandstone and mudstone sequences, such as the Neoproterozoic Damara Inland Branch (Namibia), the Paleozoic Lachlan (eastern Australia), and the Mesozoic Rangitatan (New Zealand) orogens, show that marked shortening and thickening of the former turbidite fan sequence is a requisite for accretion, or incorporation into eventual crust of continental character and thickness. This process occurs along former convergent margins by subduction-related closure of former backarc basins (e.g., Lachlan orogen) and small ocean basins (e.g., Damara orogen), and commonly involves the underlying oceanic lithosphere.
While it is clear that there is no simple template for accretion, the final architecture of turbidite-dominated, accretionary-type orogens is variable and dependent on their previous history and tectonic setting. The marked shortening and thickening of the crust and the involvement of predominantly oceanic basement are key factors in recognizing ancient subduction-accretion processes in such orogens. The presence of the lower crustal layer of imbricated oceanic crust coupled with the overlying thickened sedimentary succession yields a density structure that is remarkably stable and isostatically balanced near sea level. Magmatic underplating and marked mid/lower crustal melting, resulting in significant volumes of granites, are also an important part of the later stages of this continental growth and surficial crustal recycling through the accretion of deformed turbidite fans.
This review and write-up was undertaken as part of an Australian Research Council (ARC) Australian Professorial Fellowship (DP0210178 awarded to Gray). Research on the Tasmanides of eastern Australia has been undertaken over a period of 20 years, supported by ARC grants E8315666, E8315675, A38615754, A38715383, A38930784 (awarded to Gray), and A38615754 (Durney, Gray, Gregory); additional funding is from the Australian Geodynamic Cooperative Research Centre (to both Gray and Foster) and National Science Foundation (NSF) grant EAR-0073638 (to Foster). Research on New Zealand has been supported by ARC grants A39927139 and A39030706 (awarded to Gray) and a Monash University Small Grant (awarded to Gray). Research on Namibia has been funded by ARC Large Grant A00103456 (awarded to Gray) and NSF grant EAR-0440188 (awarded to Foster). We acknowledge discussions with (1) Chris Fergusson, Vince Morand, Nick Woodward, Clive Willman, John Miller, Chris Wilson, Thomas Flöttmann, and Ron Berry on Tasmanides geology; (2) Simon Cox, Richard Norris, Dave Craw, Nick Mortimer, Moses Turnbull, and Alan Cooper on New Zealand geology; and (3) Cees Passchier, Rudolph Trouw, and Thomas Becker on Namibian geology.
Figures & Tables
4-D Framework of Continental Crust
- absolute age
- accreting plate boundary
- alkaline earth metals
- alluvial fans
- clastic rocks
- continental crust
- Damara System
- igneous rocks
- imbricate tectonics
- isotope ratios
- Lachlan fold belt
- New South Wales Australia
- New Zealand
- ocean basins
- oceanic crust
- orogenic belts
- Pan-African Orogeny
- plate tectonics
- plutonic rocks
- S-type granites
- sedimentary rocks
- Southern Africa
- stable isotopes
- stratigraphic units
- tectonic units
- trace elements
- upper Precambrian
- Rangitatan Orogen
- Rakaia Wedge