Kaoko Belt Projects

The Kaoko Belt is a classic example of an orogen-scale transpressional orogen, exposed at mid- to lower crustal levels. The Kaoko Belt project started in 1997 with a 2 year mapping contract with the Namibian Geological Survey. Research continued in collaboration with Prof. David Gray (Melbourne University) and Martin Hand (Adelaide University) with ARC funding awarded to Prof. David Gray [1]. Kaoko Belt research has also continued since 2005 in collaboration with Prof. David Foster [2] (University of Florida) with NSF funding. Current research activities by David Foster and students are concerned with exhumation and retrograde history, Ar-Ar thermochronology, zircon provenance and U-Pb insitu dating of metamorphic titanite.

Below are links to conference etc summaries of different aspects of the transpressional Kaoko Belt:

• Structural evolution and architecture [3]

• Mid-crustal extrusion [4]

• Event geochronology [5]

• Transtensional exhumation [6]

• Structure and strain variation [7]

• Coastal Terrane [8]

• Metamorphic architecture [9]

• Palaeoproterozoic Kunene Igneous Complex [10]

• Structural evolution of the Kaoko Belt [11]

• Metamorphic evolution of the Kaoko Belt [12]

(1) Crustal architecture, thermo-mechanical evolution and tectonic setting of the transpressional Kaoko Belt, Namibia.

Ben Goscombe^a, David Gray^b
^aContinental Evolution Research Group, University of Adelaide, 5005, S.A. Australia.
^bSchool of Earth Sciences, University of Melbourne, Victoria, 3010, Australia.

Also see Journal of Structural Geology 25, 1049-1081 (2003).

The obliquely convergent Kaoko Belt in Namibia has crustal-scale shear zones that separate panels with distinct tectono-metamorphic histories. Oblique collision due to docking of an outboard arc terrane between 645 Ma and 580 Ma has caused crustal-scale, oblique-overriding of the arc terrane over the passive margin of the Congo Craton. Ongoing oblique convergence resulted in crustal-scale sinistral shear partitioned into two major ductile shear zones of contrasting kinematics, that accommodated lateral extrusion of high-T metamorphics in the shear zone-bounded orogen core. The orogen margin experienced craton-ward verging basement-cored fold nappes. The major shear zones have inferred W-dipping listric form, as they have steep dips at the surface in the north but become inclined to the west with progressively lower dips southwards. The Kaoko Belt is a thermally softened collisional margin in transpression. The onset of transpressional orogenesis coincided with peak metamorphism and granitoid emplacement simultaneous with pervasive wrench-related shearing, but progressed to more local partitioning of strain into the major shear zones to eventual strain hardening and intrusion of post-tectonic pegmatites. Orogen restoration, through closure of the present day Atlantic Ocean, produces a classic doubly-vergent orogen with shear zone kinematics requiring southward extrusion of the magmatic arc relative to both the bounding Congo and Rio de La Plata cratons.

Zircon and monazite U-Pb dates, garnet Sm-Nd dates and hornblende 40Ar/39Ar data confirm three distinct tectono-metamorphic cycles: M1 (655-645 Ma), M2 (580-550 Ma) and M3 (535-505 Ma). The high-grade M1 metamorphic cycle and associated intrusive complexes are evident only within the westernmost Coastal Terrane. The isotopic data record a progressive and protracted history for the M2 metamorphic cycle that is initiated by collision and terrane docking, but with three distinct tectono-thermal periods including (1) peak metamorphic parageneses and voluminous granitoid emplacement at 580-570 Ma, (2) overlapping whole-scale transpressional orogenesis and reworking dominated by crustal-scale shear zones, throughout the period 575-550 Ma, and (3) cessation of transpressional strain before 530-508 Ma, the age of late-kinematic pegmatite dykes that cross cut the major shear zones and low-grade buckling of the Kaoko Belt during the M3 metamorphic cycle accompanying NNE-SSW directed, high-angle convergence between the Congo and Kalahari Cratons.

The Kaoko Belt provides a well-exposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis. Integration of the kinematic and metamorphic datasets, show that lateral extrusion trajectories occurred within the orogen core, and show that stretching lineations approximately reflect the particle paths experienced in this orogen. The gross regional lineation pattern defines an arcuate array from near orogen-parallel in the orogen core to higher-angle obliquity across the orogen margin, reflecting lateral escape. Two high-grade and relatively high-pressure lobes within the orogen core are coincident with oblique-inclined, upward flow trajectories and represent lateral extrusion of lower-crustal material into middle-crustal levels. Tight and shallow conduction dominated P-T paths are experienced in the orogen core, involving heating-dominated prograde paths followed by decompression, consistent with extrusional tectonics. Steeper and more open advection dominated P-T paths are experienced in the crustal over-thrust orogen margin that experience lower thermal gradients.

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(2) Evidence for oblique, up-plunge extrusion in the core of a transpressional orogen.

Ben Goscombe², David Gray³, Martin Hand²
2. Continental Evolution Research Group, Adelaide University.
3. School of Earth and Environmental Sciences, University of Melbourne.

See Journal of Petrology 46, 1203-1241 (2005)

The Cambrian Kaoko Belt is a sinistral transpressional orogen developed between the Congo and Rio De La Plata cratons at 580-550 Ma. The architecture of the belt can be sub-divided into the following components: (1) a Barrovian-style Escape Zone containing large-scale nappes and over-folds that verge towards the foreland margin; (2) the 20-40 km wide Orogen Core containing mostly high-grade rocks, which experienced intense wrench-style deformation; (3) an exotic outboard Coastal Terrane.

The Orogen Core is a steeply divergent flower or half-flower system, bounded to the east by the obliquely reverse Purros Mylonite Zone and on the west by the extensional strike-slip Three Palms Mylonite Zone. These kinematically contrasted shear zones range from vertical to shallow (30º) inclinations at the surface, and have listric geometries that are inclined to the west at depth. The kinematic array (defined by stretching lineations and shear sense indicators) shows domainal variation in both degree and polarity of lineation obliquity with respect to the grain of the belt. In addition, variations in plunge and polarity of vergence along the plunge with respect to the overall sinistral shear sense, may contain information about the vertical component of particle paths and vergence of transport. Together, these parts of the kinematic array define apparent flow trajectories due to the wrench component of deformation. The gross regional lineation pattern defines an arcuate array from near orogen-parallel to acute obliquity of ≤ 30º in the Orogen Core to higher-angle obliquity across the Escape Zone up to 70-80º, reflecting extrusional escape towards the orogen margin. In the Purros Mylonite Zone, most parts of the Orogen Core and all of the Escape Zone have NNW-plunging lineations, indicating oblique, upward and SSE-directed convergent transport. In contrast, the Three Palms Mylonite Zone, Khumib Domain and most of the Coastal Terrane have stretching lineation trajectories that are obliquely divergent with SSW-plunges, indicating oblique, downward and SSW-directed extensional vergence. Consequently, the kinematic array suggests relative downward transport of the Coastal Terrane and Khumib Domain and upward transport of the two high-grade domains within the Orogen Core and Escape Zone.

On a gross scale the Kaoko Belt shows a smooth variation in metamorphic conditions that reflect the kinematic partitioning of the belt. The Escape Zone represents a high-P/moderate-T Barrovian-style, inverted metamorphic sequence with peak metamorphic conditions of 8-9 kbar and 550-690 ºC with average thermal gradients around 20 ºCkm-1. In contrast, the Orogen Core has two distinct thermobarometric domains arranged into lobe-like patterns characterised by peak conditions ranging between ~700-850 ºC at 7-9 kbar and higher average thermal gradients of around 30-40 ºCkm-1. Within the orogen core, the Khumib Terrane between the two high-grade lobes is a marked low-T/low-P “trough” with conditions ranging from 570-620 ºC at 5-5.5 kbar. In the outboard Coastal Terrane, conditions associated with transpressional reworking are 560 ºC and 4.5 kbar, corresponding to an average thermal gradient of 35-38 ºCkm-1. Clockwise P-T loops are recorded in all parts of the Kaoko Belt. Tight and shallow conduction dominated P-T loops are experienced in all Orogen Core domains, involving heating-dominated prograde paths followed by decompression from near coincident P-max and T-max conditions, consistent with extrusional tectonics. Steep and more open advection dominated P-T loops are experienced in the crustal over-thrust Escape Zone.

The two high-grade lobes within the Orogen Core are coincident with an oblique, upward-inclined convergent kinematic array and the intervening low-grade “trough” with oblique, downward-inclined extensional kinematic array. The recorded barometric differential between high-grade lobes and low-grade “trough” (Dz = 9.5-10.5 km), and longitudinal separation (Dy = 80 km) are entirely consistent with extrusion of the high-grade lobes along particle paths that plunge 7-10º NNW, exactly parallel to the developed stretching lineations. Similarly, the outboard Coastal Terrane was reworked during transpression at 4.5 kb, indicating 6-12 km of downward transport relative to the extruding Orogen Core. Relative downward transport is entirely consistent with extensional shear sense along hinterland-directed oblique and shallow plunging (10º) lineations in both the Coastal Terrane and Three Palms Mylonite Zone. These oblique and plunging particle paths are consistent with being driven by sub-horizontal directed, oblique tectonic convergence between the Rio De La Plata and Congo Cratons. Extrusional flow in the Orogen Core was along shallowly plunging and oblique trajectories that verged upwards due up-ramping along the listric crustal-scale sinistral-reverse Purros Mylonite Zone. Orogen Core extrusion was accompanied by a relative downward trajectory of the outboard Coastal Terrane and sinistral-normal oblique shear in the crustal-scale Three Palms Mylonite Zone.

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(3) Event geochronology of the transpressional Kaoko Belt.

Ben Goscombe², David Gray³, Richard Armstrong⁴, David Foster⁵

2. Continental Evolution Research Group, Adelaide University.
3. School of Earth and Environmental Sciences, University of Melbourne.
4. School of Earth Sciences, Atralian Natioanl University.
5. School of Earth and Environmental Sciences, University of Florida.

See Precambrian Research 140, 103.e1-e41 (2005)

The Kaoko Belt is a classic example of a Neoproterozoic transpressional orogen between the De La Plata and Congo Cratons. It is linked by a triple junction with the high-angle convergent Inland Branch of the Damara Orogen between the Congo and Kalahari Cratons. New U-Pb zircon and monazite age determinations from the Kaoko Belt confirm three metamorphic cycles: M1 (655-645 Ma), M2 (580-550 Ma) and M3 (535-505 Ma). The high-grade M1 metamorphic cycle and associated intrusive complexes are evident only within the western-most and exotic Coastal Terrane. The isotopic data record a history for the M2 metamorphic cycle that is progressive and protracted; initiated by collision and terrane docking followed by three distinct tectono-thermal periods. (1) Peak metamorphic parageneses and voluminous granitoid emplacement at 580-570 Ma. (2) Overlapping whole-scale transpressional orogenesis and reworking dominated by crustal-scale shear zones, throughout the period 575-550 Ma. (3) Ultimately cessation of transpressional strain ceased before 530-508 Ma, the age of late-kinematic pegmatite dykes that cross cut the major shear zones. M1 metamorphism of the exotic Coastal Terrane at 650 Ma must have occurred out-board from the Kaoko Belt passive margin, where M1 intrusives and metamorphic mineral parageneses have not been recognised. Furthermore, this indicates that the Three Palms Mylonite Zone between the Coastal Terrane and the Kaoko Belt proper must be a suture and docking must have occurred at some stage between 645 Ma (M1) and 580 Ma, prior to the peak of M2 metamorphism accompanying transpressional orogenesis. Low-grade buckling of the Kaoko Belt and minor post-kinematic granite and pegmatite intrusions occurred during the M3 metamorphic cycle accompanying NNE-SSW directed, high-angle convergence between the Congo and Kalahari Cratons at 535-505 Ma.

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(4) Rapid exhumation of deep crust in an obliquely convergent orogen: The Kaoko Belt of the Damara Orogen

David A. Foster, Ben D. Goscombe and David R. Gray

Published in Tectonics, 28, TC4002, doi:10.1029/2008TC002317.

The exhumation of deep crustal rocks and juxtaposition of structural-metamorphic domains from different depths in a transpressional orogen may occur during the prograde evolution of the orogen by vertical extrusion or during the retrograde evolution of the orogen via extension. Metamorphic petrology, kinematics, and thermochronology of strike-slip shear zones in the Kaoko Belt of the Damara Orogen are used to evaluate extrusion and extension processes in this transpressional orogen. Mineral assemblages and deformation mechanisms record shearing at pressures of 4–6 kbar and temperatures of _550ºC for the Three Palms mylonite zone, 600–650ºC for the Purros mylonite zone, and <630–700ºC for the Village and Khumib mylonite zones. The Three Palms mylonite zone, which separates the accreted Coastal Terrane from the former passive margin of the Congo Craton, exhibits progressive deformation during decreasing temperatures through lower greenschist facies and into the brittle field, with consistent oblique normal shear indicators. Lower-temperature fabrics and brittle features also overprint the Village mylonite zone. The 207Pb-206Pb (titanite) and 40Ar/39Ar (hornblende, muscovite, and biotite) data indicate rapid cooling, at rates of 30–100ºC/Ma, of all high-grade structural domains in the core of the Kaoko Belt between circa 535 and 525 Ma, which is about 20–30 Ma younger then peak metamorphism. The 40Ar/39Ar data from synkinematic muscovite fish in the retrograde shear zone assemblages indicate that the Khumib, Village, and Three Palms mylonite zones were actively deforming at temperatures below 350ºC at circa 530–524 Ma. These data indicate that the high-grade metamorphic rocks of the Kaoko were rapidly exhumed and juxtaposed after the main transpressive deformation by oblique extension. Reactivation of the shear zones and tectonic exhumation of high-grade structural domains from beneath the accreted Coastal Terrane was caused by collision in the Damara Belt and terminal accretion of Gondwana.

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(5) Structure and strain variation at mid-crustal levels in a transpressional orogen: a review of Kaoko Belt structure and the character of West Gondwana amalgamation and dispersal

Ben D. Goscombe¹, David R. Gray²
1: School of Earth and Environmental Sciences, Adelaide University, Adelaide, South Australia, 5005, Australia
2: School of Earth Sciences, The University of Melbourne, Melbourne,
Victoria, 3010, Australia
See Gondwana Research, Focus paper, in press (2007)

The Neoproterozoic-Cambrian Kaoko Belt is an orogen-scale (800x180 km) transpressional system important in the amalgamation of West Gondwana. Mid-crustal transpression at amphibolite to granulite facies conditions is dominated by two major, >400 km exposed, strike-slip shear zones bounding a 20-40 km wide high-grade Orogen Core. To the east, a deeply buried nappe-dominated Escape Zone has inverted metamorphic sequence and verges outwards onto a platformal foreland. To the west, an arc-like Neoproterozoic Coastal Terrane was amalgamated and variably reworked during transpression. The major Purros and Three Palms Mylonite Zones have calculated shear displacements on the order of 120-180 km. These shear zones are moderately to steeply dipping mylonite zones of 1-5 km width, are arcuate and curvilinear in map view and show along-strike variation in slip kinematics. Also highly curved in vertical section, the shear zones define a flower to half-flower geometry for the Orogen Core. An oblique network of mylonitic shear zones, akin to Riedel shears, links the major shear zones and defines regional-scale shear lozenges internally deformed by tight upright folding and shear fabrics. These shear zones create domains in the Orogen Core with varying dominance of pure shear (in shear lozenges) and simple shear (in shear zones). However, absence of dip-slip domains and the smoothly continuous traces of sub-horizontal to shallow and acute, oblique stretching lineations across all parts of the belt, preclude marked kinematic partitioning and the internal part of the belt resembles large-scale triclinic shear. Clast aspect ratios, boudin train extension, sheath fold aspect geometry, degree of rotation of planes producing flanking folds, composite S-C foliations, pressure fringes on pyrite and garnet porphyroclasts provide a semi-quantitative measure of strain intensity. Average strain ratios are X/Z > 40:1 for the major shear zones, X/Z > 12:1 for the Orogen Core, X/Z > 8:1 for the Escape Zone and X/Z > 3:1 for the Coastal Terrane. A more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index. Foliation traces have a sigmoidal pattern in the Orogen Core, swinging from sub-parallel to the boundary shear zones to higher acute angles in the internal parts. Deformation character also varies from upright open folding in amphibolite facies domains in the north, upright tight chevron folding in a low-grade central domain, to a high-grade domain of tight to isoclinal inter-folded basement and cover, with inclination decreasing towards the south.

The Kaoko Belt is a well-exposed sector of an extensive (3000 km long), broad (400 km) arcuate orogenic system “Adamastor Orogen” that amalgamated West Gondwana, bringing the South American (Sao Francisco and Rio de la Plata Cratons) and African (Kalahari and Congo Cratons) components together. Though a complex system, most sectors involved oblique collision and accretion of magmatic arcs of 660-610 Ma age, followed by peak metamorphism and main phase transpressional orogenesis between 585-560 Ma, with shear zones remaining active until ~530 Ma. This E-W amalgamation immediately pre-dates the final N-S amalgamation of Gondwana along the Kuunga Orogen between 535-510 Ma. The large-scale Adamastor Orogen, consisting of Kaoko, Dom Feliciano, Ribeira, Araçuai and West-Congo mobile belts, also shows broadly similar and symmetric structural architecture throughout. The high-grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form. Either side of the internal zone containing amalgamated arcs and high-grade core, are nappe-fold and thrust belts that rework attenuated passive margin basement and Adamastor Ocean sediments and structures verge outward at moderate to high-angles onto both foreland margins. The Kaoko Belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater “Adamastor Orogen”; such as metamorphic zonation and heterogeneous distribution of deformation style, flow regime and highly variable degrees of reworking strain and recrystallization. This highly partitioned and steep structural grain localized lithospheric extension and rifting 415 Ma later during breakup and dispersal of Gondwana.

Keywords: transpression; shear zones; strain; stretching lineations; mid-crust processes; West Gondwana; amalgamation; dispersal.

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(6) The Coastal Terrane of the Kaoko Belt, Namibia: outboard arc-terrane and tectonic significance

Ben Goscombe^a, David R. Gray^b

^aSchool of Earth and Environmental Sciences, Adelaide University, Adelaide, 5005, Australia^bSchool of Earth Sciences, The University of Melbourne, Victoria, 3010, Australia
See Precambrian Research 155, 139-158 (2007)

The Coastal Terrane, or westernmost part of the Kaoko Belt outboard of the Three Palms Mylonite Zone, has distinct lithology including no basement, distinct eNd sediment signatures excluding the Archaean and suggesting Mesoproterozoic-Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ~650-645 Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high-T/low-P granulite to upper-amphibolite facies and includes migmatisation associated with I-type granitic magmatism of arc affinity. Arc growth at 650-640 Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 Ma and 580 Ma. Lack of ophiolite fragments and evidence of intermediate- to high-P metamorphism along the Three Palms Mylonite Zone suggest that it is not a suture and more likely to be part of an arc- backarc wrench-shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 Ma and 580 Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high-T granulite facies metamorphism in an extruded, shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes.

Keywords: arc terrane; Kaoko Belt; Adamastor Ocean; subduction, transpression

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(7) The metamorphic architecture of a transpressional orogen: the Kaoko Belt, Namibia.

BEN GOSCOMBE^a, MARTIN HAND^a, DAVID GRAY^b, JO MAWBY^a
^aCONTINENTAL EVOLUTION RESEARCH GROUP, DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF ADELAIDE, ADELAIDE, SOUTH AUSTRALIA, 5005, AUSTRALIA.
^bSCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, MELBOURNE, VICTORIA, 3010, AUSTRALIA.
See Journal of Petrology 44, 679-711 (2003)

The Kaoko Belt in Namibia represents the deeply eroded core of a classic sinistral transpressional orogen with a half flower structure centred on the crustal-scale Purros Mylonite Zone. The Kaoko Belt consists of three NW-trending structural zones each with distinct kinemetamorphic style. The Eastern Kaoko Zone contains upright-folded, Neoproterozoic Damara Sequence shelf carbonates. The Central Kaoko Zone comprises an inverted Barrovian metamorphic series within large-scale east-vergent nappes, whereas the Western Kaoko Zone is predominantly of high metamorphic grade and intruded by numerous granitoids. The Western Kaoko Zone has orogen-parallel panels of distinctly different metamorphic grade separated by strike-slip ductile shear zones with overall isograd pattern being indicative of extrusional tectonics in the orogen core. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian. (1) An early Thermal Phase (early-M2) was responsible for pervasive partial melting, high-grade parageneses and granite emplacement between 580-570 Ma. (2) The main deformation Transpressional Phase (580-550 Ma) reworked early-M2 parageneses in the pervasive orogenic fabric producing M2 assemblages that formed due to progressive sinistral transpression that evolved from wrench-style to high-angle convergence accompanying foreland vergent thrusts and nappes. (3) The post-transpression Shortening Phase generated upright, open folds during N-S shortening (530-510 Ma). In the Western Kaoko Zone, peak metamorphic conditions were attained during early-M2 at moderate to high average thermal gradients# (29-40 ºC/km) and were intensely reworked by lower-grade pervasive fabrics during M2. In the northern part of the Western Kaoko Zone, immediately adjacent to the Purros Mylonite Zone, the amphibolite-grade Khumib Terrane experienced peak M2 metamorphism at 573 ºC and 5.4 kb. Along strike to the south the granulite-grade Hoarusib Terrane experienced peak early-M2 conditions at 843 ºC and 8.1 kb and M2 reworking at approximately 560-580 ºC and 4.8 kb. In the western margin of the orogen, the Coastal Terrane experienced early-M2 metamorphism at sillimanite-K-feldspar-melt grades and was reworked during M2 at muscovite-biotite grade. In the Central Kaoko Zone, metamorphic grade increases towards the west to higher structural levels. Peak metamorphic matrix assemblages formed during pervasive deformation in the Transpressional Phase (M2) at conditions ranging 530 to 690 ºC and 8.5- 9.0 kb with consistently low average thermal gradients# (17-23 ºC/km). Clockwise P-T paths were experienced in both the Central Kaoko Zone and Western Kaoko Zone. Garnet Sm-Nd geochronology indicates that matrix parageneses, early-M2 in the WKZ and M2 in the CKZ, formed at the same time within uncertainties (576±15 Ma). This indicates that the thermal peak was contemporaneous across the belt, even though deformational phases of equivalent structural style were diachronous across the Kaoko Belt.
KEY WORDS: Pan-African Orogeny; transpression; metamorphism; geochronology; metamorphic field gradients; orogen architecture

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[8] Satellite ultramafic bodies associated with the largest known anorthosite intrusion; the Kunene Igneous Complex in Namibia.

Ben Goscombe & Eckhart Freyer

Conference abstract and Report for Ambase Exploration Namibia (1999).

The Kunene Igneous Complex is the largest anorthosite intrusion known, being exposed over an area of 18,000 km² in northern Namibia and southern Angola. This layered anorthosite and leuco-troctolite intrusion is thought to have been emplaced at approximately 1500 Ma into the high-grade Palaeoproterozoic Epupa Metamorphic Complex. The KIC was emplaced prior to at least two major tectonothermal cycles that are widespread in southern Africa, the Kibaran (Mesoproterozoic) and Pan-African (Neoproterozoic-Palaeozoic), but never-the-less remains essentially undeformed and unmetamorphosed. At least 19 mafic and ultramafic satellite intrusive bodies have been documented in the region immediately south of the KIC, within 15 km of the margin, with none occur within the KIC. Despite the known association of Cu-Ni-PGE-sulphide and Fe-Ti-V-oxide mineralization (Littmann et al. 1998), few satellite bodies have been documented in any detail, largely due to security problems. In the course of this study, 15 satellite bodies were mapped in detail for the first time and mineralization and structural over-printing relationships evaluated.
Nearly all of the satellite bodies are large elongate tabular bodies or moderately large elliptical lenses, with few being small, circular to amoeboid-shaped plugs. The total range in size is 0.06-1.20 km in width and 0.10-8.70 km in length. All have sub-vertical margins and trend E-W to SE-NW (averaging 103º±18º), sub-parallel to the southern margin of the KIC. Map-scale and outcrop-scale compositional layering and magmatic foliations defined by aligned plagioclase laths, typically trend ESE-WNW, sub-parallel to the length of the bodies. Compositional layering dips shallowly (0-30º), mostly to the NNE and with a regional distribution indicating tilting was around an axis parallel to the length of the satellite bodies and southern margin of the KIC. Rare high-temperature, ductile shear fabrics that formed during or immediately after emplacement (of a still hot body) are sub-vertical and trend E-W. Few bodies have either; oblique compositional layering, are concentrically zoned or have a more complex irregular distribution of rock-types. The small plug bodies are either homogeneous or are a composite of irregular amoeboid shaped domains of different rock-types. Some of the larger bodies are composite, that is, they experienced multiple phases of emplacement, typically of smaller plugs within the core of larger tabular bodies, and a typical emplacement sequence of the different intrusive types can be documented;

(Phase #1) Large tabular anorthosite bodies.

(Phase #2) Moderate sized, tabular to elongate elliptical layered bodies of ophitic to sub-ophitic norite and/or gabbro, rarely also with hybrid (K-feldspar-bearing) rock-types such as monzonite and granodiorite.

(Phase #3) Small ophitic to sub-ophitic massive gabbro plugs.

(Phase #4) Rare, amoeboid and plug-shaped bodies (often composite) of more ultramafic rock-types, such as clinopyroxenite, hyperite and wehrlite.

(Phase #5) Late-stage aplite veins and hornblende-bearing micro-granite dykes.

Sulphide mineralization occurs at the margins of younger intrusive phases (Phases #2-4), only where in contact with earlier satellite bodies and not at contacts with host metamorphic gneisses. For example mineralization at the margin of gabbro and norite bodies (Phase #2 and Phase #3) emplaced into earlier tabular anorthosite bodies. Furthermore, sulphides appear in higher concentrations in the later intrusive phases; such as Phase #4 ultramafics and sulphides are common in Phase #5 aplite veins. Composite satellite bodies with multiple intrusive phases are the best exploration targets and in particular the margins of late-stage ultramafic phases emplaced into early tabular anorthosite bodies.

Most satellite bodies are undeformed, unmetamorphosed and little retrogressed. Margins of bodies are unsheared and ductile tectonic fabrics are nowhere recognised within the satellite bodies, indicating emplacement after all high-grade ductile deformation of the Epupa Metamorphic Complex. Few bodies develop, in part, a variety of weak, penetrative greenschist facies retrograde foliations and most bodies are cut by small-scale retrograde shear zones (RSZ). Both foliations and RSZ trend ESE-WNW (average 112º±18º), dip steeply (80º) NNE and are of the same grade and orientation as dextral RSZ in the Basement, with which they are correlated. These RSZ and retrograde foliations are interpreted to constitute the full expression of Pan-African reworking in the region. Late-stage structures include brittle faulting of Mesozoic age and syenite and nepheline-syenite alkali complexes, carbonatites and dolerite dykes of Cretaceous age. Tilting of layering by 0-30º to the NNE in both the satellite bodies and Cretaceous alkali complexes, may have occurred in the Cretaceous during continent break up.

Littmann, S., Cook, N.J., Teigler, B., Druppel, K. 1998. Ultramafic-mafic intrusives hosting Cu-Ni-sulphide mineralization, Otjitambi area, Kunene region, N.W. Namibia: Investigation of drillcore, hole OTD-1. Comms. Nam. Geol. Surv. 1998.

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[9] Structural Evolution of Kaoko Belt

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2014.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2014/XX (in review).

The Neoproterozoic-Cambrian Kaoko Belt is an orogen-scale (800x180 km) transpressional system important in the amalgamation of West Gondwana. Mid-crustal transpression at amphibolite to granulite facies conditions is dominated by two major, >400 km exposed, strike-slip shear zones bounding a 20-40 km wide high-grade Orogen Core. To the east, a deeply buried nappe-dominated External Nappe Zone has inverted metamorphic sequence and verges outwards onto a platformal foreland. To the west, a magmatic arc Neoproterozoic Coastal Terrane was amalgamated and variably reworked during transpression. The major Purros and Three Palms Mylonite Zones have calculated shear displacements on the order of 120-180 km. These shear zones are moderately to steeply dipping mylonite zones of 1-5 km width, are arcuate and curvilinear in map view and show along-strike variation in slip kinematics. Also highly curved in vertical section, the shear zones define a flower to half-flower geometry for the Orogen Core. An oblique network of mylonitic shear zones, akin to Riedel shears, links the major shear zones and defines regional-scale shear lozenges that are internally deformed by tight upright folding and shear fabrics. These linking shear zones create domains in the Orogen Core with varying dominance of pure shear (in shear lozenges) and simple shear (in shear zones). However, absence of dip-slip domains and the smoothly continuous traces of sub-horizontal to shallow and acute, oblique stretching lineations across all parts of the belt, preclude marked kinematic partitioning and the internal part of the belt resembles large-scale triclinic shear. Clast aspect ratios, boudin train extension, sheath fold aspect geometry, degree of rotation of planes producing flanking folds, composite S-C foliations, pressure fringes on pyrite and garnet porphyroclasts provide a semi-quantitative measure of strain intensity. Average strain ratios are X/Z > 40:1 for the major shear zones, X/Z > 12:1 for the Orogen Core, X/Z > 8:1 for the Escape Zone and X/Z > 3:1 for the Coastal Terrane. A more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index. Foliation traces have a sigmoidal pattern in the Orogen Core, swinging from sub-parallel to the boundary shear zones to higher acute angles in the internal parts. Deformation character also varies from upright open folding in high-grade domains in the north, upright tight chevron folding in a low-grade central turbidite domain, to a high-grade domain of tight to isoclinal inter-folded basement and cover, with inclination decreasing towards the south.

The Kaoko Belt is a well-exposed sector of an extensive (3000 km long), broad (400 km) arcuate orogenic system "Adamastor Orogen" that amalgamated West Gondwana, bringing the South American (Sao Francisco and Rio de la Plata Cratons) and African (Kalahari and Congo Cratons) components together. Though a complex system, most sectors involved oblique collision and accretion of magmatic arcs of 660-610 Ma age, followed by peak metamorphism and main phase transpressional orogenesis between 585-560 Ma, with shear zones remaining active until ~530 Ma. This E-W amalgamation immediately pre-dates the final N-S amalgamation of Gondwana along the Kuunga Orogen between 555-510 Ma. The large-scale Adamastor Orogen, consisting of Kaoko, Dom Feliciano, Ribeira, Araçuai and West-Congo mobile belts, also shows broadly similar and symmetric structural architecture throughout. The high-grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form. Either side of the internal zone containing amalgamated arcs and high-grade core, are nappe-fold and thrust belts that rework attenuated passive margin basement and Adamastor Ocean sediments, and the structures verge outward at moderate to high-angles onto both foreland margins. The Kaoko Belt well illustrates the highly partitioned nature of transpressional systems in general, and patterns in common throughout the greater "Adamastor Orogen"; such as metamorphic zonation and heterogeneous distribution of deformation style, flow regime and highly variable degrees of reworking strain and recrystallization. This highly partitioned and steep structural grain localized lithospheric extension and rifting 415 Ma later during breakup and dispersal of Gondwana.

The Coastal Terrane, or westernmost part of the Kaoko Belt, outboard of the Three Palms Mylonite Zone, has distinct lithology including no basement, distinct e_Nd sediment signatures excluding the Archaean and suggesting mostly Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ~650-645 Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high-T/low-P granulite to upper-amphibolite facies and includes migmatization associated with I-type granitic magmatism of arc affinity. Arc growth at 650-640 Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 Ma and 580 Ma. Lack of ophiolite fragments and no evidence for high-P metamorphism along the Three Palms Mylonite Zone, suggest that this structure was not a suture and more likely to be part of an arc- backarc wrench-shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 Ma and 580 Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high-T granulite facies metamorphism in a longitudinally extruded (see below), shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes at the margin of the transpressional orogen.

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[10] Metamorphic Evolution of Kaoko Belt

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2014.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2014/XX (in review).

The Pan-African Kaoko Belt of NW Namibia provides a well-exposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis. The Kaoko Belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time, with peak M2 metamorphism at 576±15 Ma. The Kaoko Belt may be divided into a Barrovian-style External Nappe Zone with inclined nappes and thrusts that verge outward towards the foreland, a steep high-grade Orogen Core that experienced intense wrench-style deformation and in the hinterland, an older (650 Ma) obducted Coastal Terrane that was reworked at moderate-T/low-P conditions during transpressional orogenesis. Across-orogen metamorphic gradients indicate a marked contrast between the External Nappe Zone and Orogen Core. The External Nappe Zone experienced an inverted Barrovian-style high-P/moderate-T metamorphism and low thermal gradients of 20 ºC/km. The Orogen Core experienced moderate-P/high-T metamorphism and high thermal gradients of 30-40 ºC/km. Clockwise P-T paths are experienced in all parts of the Kaoko Belt. Tight and shallow conduction dominated P-T paths are experienced in all Orogen Core domains, involving heating-dominated prograde paths followed by decompression, consistent with extrusional tectonics. Steeper and more open advection dominated clockwise P-T paths are experienced in the crustal over-thrust External Nappe Zone that experience low thermal gradients.

The kinematic-lineation array (defined by stretching lineations and shear sense indicators) shows variation in degree and polarity of lineation obliquity to the grain of the belt, lineation plunge and different polarity of vergence along the plunge, with respect to the overall sinistral shear sense. The gross regional lineation pattern defines an arcuate array from near orogen-parallel in the Orogen Core to higher-angle obliquity across the External Nappe Zone, reflecting lateral escape towards the orogen margin. Two high-grade lobes within the Orogen Core are coincident with oblique-inclined, upward flow trajectories based on the kinematic-lineation array, and represent oblique extrusion of lower-crustal material into middle-crustal levels. An intervening low-grade metamorphic "trough" within the Orogen Core and the upper-plate Coastal Terrane, coincide with acute oblique downward and outboard-directed apparent flow trajectories. These kinematic patterns are consistent with barometric constraints for peak metamorphism, of 5.2 kb and 4.5 kb in the low-grade "trough" and Coastal Terrane and 8.0 kb in the two high-grade lobes. P-T paths in the panels and shear zones of the Orogen Core, show that decompression during the peak of metamorphism at ~575 Ma, brought all domains together at crustal levels of ~4.0-6.0 kb before the end of the peak metamorphic M2 metamorphic cycle. Integration of the kinematic and metamorphic datasets, show that oblique, shallowly inclined extrusion trajectories occurred within the Orogen Core of this classic transpressional orogen, and show that stretching lineations approximately reflect the particle paths experienced.

Most exhumation of the Kaoko Belt occurred during main phase transpressional orogenesis during decompression in the M2 metamorphic cycle, resulting in foreland vergent nappe transport in the External Nappe Zone and longitudinal shallow upward extrusion of parts of the Orogen Core. Transpressional processes accompanying peak metamorphism resulted in exhumation to crustal levels of 4.0-6.0 kb recorded by assemblages in the major shear zones that remained active throughout transpression to at least 539-549 Ma. A second phase, of now rapid exhumation was rejuvenated at ~530-524 Ma in response to a shift in far-field stress resulting in transtensional reactivation. In particular, the Village-Khumib and Three Palms Mylonite Zones were reactivated by oblique-normal shear through ductile to semi-ductile and into the brittle field. Transtensional reactivation of the belt gave rapid tectonic exhumation and cooling of the Orogen Core beneath the accreted upper-plate Coastal Terrane and Khumib Domain turbidites, and coincides with changes in plate motion after the final amalgamation of Gondwana.

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