Metamorphic Response in the Collisional Damara Orogen

• Tectonic evolution of the Damara Belt [1]
• Metamorphic evolution of the lower-plate [2]
• Metamorphic evolution of the upper-plate [3]
• Collapse of the Damara Belt [bottom of page]

[10] Metamorphic response and crustal architecture in a classic collisional orogen: The Damara Belt, Namibia

Ben Goscombe^1,2*, David A. Foster², David Gray³, Ben Wade⁴.

¹Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.

²Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.

³School of Earth Sciences, University of Tasmania, Hobart Tasmania. Australia.

⁴Adelaide Microscopy, University of Adelaide, Adelaide, 5005, SA, Australia.

Focus Review: Gondwana Research 2017, 52, 80-124.

Damara Belt is well-exposed mid-crustal section through a collisional orogen of Cambrian age that closed the Khomas Ocean basin between passive margins on the Congo and Kalahari Cratons. Collision resulted in a bi-vergent orogen with distinct paired metamorphic pattern of foreland-vergent high-P/low-T orogenic margins and a broad high-grade, low-P/high-T orogen core. Spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using; a large dataset (n~240) of internally consistent quantitative PT determinations, evolution of mineral parageneses and semi-quantitative P-T paths, metamorphic mapping and quantitative metamorphic field gradients. Integration with deformation history, structural profiles, metamorphic chronology, magmatic history and stratigraphy, constrains a dynamic model of crustal architecture during peak metamorphic events. The pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities (MD) with steep pressure gradients, inferring crustal-scale structures that accommodated lateral exhumation of crustal wedges. Discontinuities are confirmed by deformation features in the field, and metamorphic mineral growth indicate that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures. Crustal wedges along the orogenic margins experienced steep clockwise P-T paths with peak-PT conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial. Metamorphic chronology and over-printing metamorphic fabrics constrain a sequence of foreland propagating out-wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins. Peak metamorphism at a geothermal gradient of 20-25 ºC/km and 8-9 kb in the Southern Zone (Wedge I) was attained between ~517-530 Ma, followed by south-directed out-wedging at the Uis-Pass Suture, accommodated by relative extension (MD1) at high structural levels near the boundary with the high-grade orogen core. Out-wedging of the Southern Zone, further buried the Southern Margin Zone (Wedge II) in the footwall below the Uis-Pass Suture. Peak metamorphism at 17 ºC/km and 9.5-11.5 kb in the Southern Margin Zone was attained at 517±4 Ma and followed by out-wedging on basal thrusts, accommodated by vertical flattening and extensional reactivation of the Uis-Pass Suture (MD2). Peak metamorphism at 17 ºC/km and 10.5 kb in a high-P/low-T crustal wedge in the northern margin (Wedge III) occurred at 510±4 Ma. Peak metamorphism in this wedge was terminated by isothermal decompression during north-directed thrusting, accommodated by extension at higher structural levels near the high-grade orogen core (MD3). Granulite facies metamorphism in the high-grade orogen core, which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures (4.3-6.0 kb), higher T/depth ratios (≥38-47 ºC/km) and low DP/DT clockwise P-T paths indicating only moderate burial and protracted high-grade conditions. Metamorphic chronology confirms high-grade conditions (540-505 Ma) persisted beyond isostatic adjustment of the high-P/low-T orogenic margins. High-heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision. In contrast to the high-P/low-T margins that experienced lateral exhumation in a convergent system, the high-grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen. Granulite grade conditions in the central part of the orogen core record higher pressures than marginal lower-grade zones to both the north and south. The contrasting post-peak vertical exhumation after ~505 Ma of the central core was accommodated by brittle extensional detachments at high stratigraphic levels with km-scale omission of crustal section.

[11] Deformation correlations, stress field switches and evolution of an orogenic intersection: the Pan-African Kaoko-Damara orogenic junction, Namibia

Ben Goscombe^1,2*, David A. Foster², David Gray³, Ben Wade⁴, Antonios Marsellos⁵ & Jason Titus²

¹Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.

²Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.

³School of Earth Sciences, University of Tasmania, Hobart Tasmania. Australia.

⁴Adelaide Microscopy, University of Adelaide, Adelaide, 5005, SA, Australia.

⁵Hofstra University, Hempstead, New York, USA

Focus Paper: Geoscience Frontiers 2017, 8, 1187-1232.

 

[12] Evolution of the Damara Orogenic System: Record of West Gondwana assembly and crustal response

Ben Goscombe^1,2*, David A. Foster², David Gray³, Ben Wade⁴.

¹Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.

²Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.

³School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia.

⁴Adelaide Microscopy, University of Adelaide, Adelaide, 5005, SA, Australia.

Chapter 12, SW Gondwana 2017: Springer and Verlag

Damara Orogenic System is a well-exposed orogenic junction that preserves a rich record of West Gondwana assembly and crustal processes in classic examples of transpression (Kaoko Belt) and bi-vergent collisional orogenesis (Damara Belt). Both belts show typical orogenic cycles in common with orogenic belts universally; from rifted passive margin sequences, subduction at continental margin arcs with back arc basins, collision, crustal over-thickening, out-wedging of orogenic margins, detachment of subducted lithosphere, upper-plate lithospheric thinning and eventual collapse. There is no controversy here, and like all orogens, kinematic and metamorphic response is dynamic and strongly zonal, the patterns of which are strong evidence for crustal architectures and tectonic history. Large relational datasets are required to characterize these patterns of orogenic response; both to test robustness, accuracy and internal consistency, and build crustal and tectonic models. For this summary of the Damara Orogenic System, two large-scale internally consistent relational datasets have been integrated. [1] Age calibrated deformation histories have been correlated across the whole system on the basis of stress fields and absolute age constraints in common. [2] Deformation structures have been correlated with mineral growth and patterns of metamorphic response characterized by P-T evolutions, metamorphic maps and field gradients, quantified using a large dataset of PT determinations. Collision of the Rio De La Plata Craton at ~590 Ma resulted in W over E obduction of the Coastal Terrane arc over Congo Craton passive margin. Kaoko Belt subsequently evolved through ~45º clockwise rotation of the stress field showing progressive transpressional orogenesis, steepening and strain partitioning, resulting in a strike-slip shear system between ~550-530 Ma. Collision in the Damara Belt at ~555-550 Ma involved subduction of the Kalahari Craton margin, with highly attenuated Congo Craton passive margin in an upper-plate setting. Kaoko and Damara orogenic fronts were both operating between 550-530 Ma, and with the same NW-SE shortening direction. At ~530-525 Ma a stress switch to NNW-SSE shortening resulted in transtensional reactivation of Kaoko Belt shear zones, rapid exhumation and cooling, terminating orogenesis in this belt. At this time, main phase orogenesis, burial and metamorphism peaked in the Damara Belt, and subsequent contraction in this belt dominated the stress field, which evolved through ~70º clockwise rotation to NE-SW shortening by ~512-508 Ma. Barrovian metamorphism in the southern orogenic margin was diachronous, 530-522 Ma in the north to 517-514 Ma in the south, and accompanied ongoing contraction. Deep burial to 9.5-11.5 kb followed by rapid isostatic readjustment, gave successive foreland propagating exhumation events at <522 Ma and <517 Ma; by southward transport of crustal wedges along basal thrusts. Out-wedging was accommodated by top down to the north transport of hanging walls at higher structural levels, indicated by major metamorphic discontinuities and extensional structures, resulting in extensional telescoping of the southern margin. In contrast, medium-P/high-T granulite facies metamorphic conditions persisted in the orogen core from 540 to 505 Ma, following low P/T clockwise P-T paths indicating moderate burial and stable high-heat flow conditions, best explained by detachment of subducted lithospheric during collision. Rapid stress switch to E-W directed shortening along the orogen at ~508 Ma, generated cross-folding in the orogen core and northern margin. This stress field is inconsistent with any plausible trajectory between Rio De La Plata, Congo and Kalahari Cratons and interpreted as a far-field effect from the orogenic margin of Gondwana at that time; arc collisions in the Ross Orogen. This established a N-S extension direction exploited by 508-504 Ma decompression melts in many parts of the system, and at ~505-500 Ma triggered gravitational collapse and extension of the thermally weakened orogen core, resulting in a broad bivergent core complex, rapid exhumation and cooling from 700 ºC to 400 ºC between 500-470 Ma.

 

[13] Tectonic Evolution of Damara Belt

 

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2018.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2018/XX (in press).

 

Continental breakup, rifting of cratonic margins and formation of passive margins with rhyolitic volcanism, was well under way by ∼750 Ma on all cratonic margins. The rift related felsic volcanics range in age 746-764 Ma on the Congo Craton margin and 729-742 Ma, 780 Ma and 820 Ma on the Kalahari Craton margin. Development of the Adamastor and Khomas Oceans most likely continued until 580 Ma when passive margin sedimentation was terminated. The width of the Khomas Ocean basin have been strongly disputed and was until recently interpreted to be a small ocean basin formed after intracontinental rifting in once contiguous Congo-Kalahari Cratons. However, there is no evidence that the Congo and Kalahari Cratons were once contiguous and that the Khomas Ocean followed from an intracontinental rift between them. The Nosib rhyolites on the Congo margin and the Rosh Pinah rhyolites on the Kalahari margin do not share the same age and cannot be correlated. Detrital zircon provenance studies indicate that sediments deposited on the two passive margins had separate sources and no provenance source in common. Palaeomagnetic data suggests the positions of the Congo, Kalahari and Rio de la Plata Cratons were quite disparate at about 750 Ma, though the position of the Kalahari Craton remains controversial.

Basement-cored domal culminations in the Central Zone and Okahandja Zone and basement-type isotopic signatures in some granite, requires the presence of attenuated Congo continental crust in the northern part of the Khomas Ocean basin. Similarly, the Southern Margin Zone sequences were deposited on an attenuated Kalahari Craton margin. The domain floored by oceanic crust is contained entirely within the now extremely foreshortened Southern Zone, devoid of basement inliers and containing MORB-type Matchless Amphibolite, oceanic chert-Cu-Zn mineralization and a thick monotonous sequence of deep-sea turbidites. The Southern Zone consists of transposed layering, pronounced schistosity and top to the south transport and is interpreted to be an accretionary prism above the down-going plate of an oceanic subduction system. Barrovian-style metamorphism in the Southern Zone is interpreted to result from wedge thickening and higher temperatures during collision, obliterating any earlier subduction-related high-P metamorphic parageneses if any were incorporated within the prism. The geochemistry of more primitive diorites and syenites in early magmas at the attenuated leading edge of the Congo Craton in the Central Zone, supports northward subduction of the Khomas Ocean lithosphere. The Khomas Ocean is interpreted to have extended across the African continent and was closed along the length of the Lufilian Arc, Zambezi Belt and Lurio Belt to the east. Like the Damara Orogen, there is no shared stratigraphy with provenance in common, on both sides of the Lufilian-Zambezi Belt. Furthermore, the Zambezi Belt and Lufilian Arc contains MORB-type eclogite and whiteschists buried to 90 km depth, suggesting an ocean basin with subduction underway at 595-575 Ma.

Seafloor spreading was still underway at 630 Ma in the southern Adamastor Ocean and is recorded by seafloor metamorphism in the Marmora Terrane of the Gariep Belt. An ensimatic, subduction-related origin has been accepted for the Gariep Belt, largely due to the Chameis Complex melange of the Marmora Terrane with its mafic and ultramafic blocks and putative sub-blueschist metamorphism. Detrital zircon populations suggest west-directed subduction in the southern Adamastor Ocean, beneath the Rio de la Plata Craton. This establishes continuity of an arcuate and linked west- to north-directed subduction system that closed the southern Adamastor Ocean and the Khomas Ocean at approximately the same time. Linkage of the Gariep Belt with the Damara Belt is supported by 555-540 Ma collision in the Damara Orogen and 550-540 Ma collision by oblique obduction of the Marmora Terrane over the imbricated passive margin in the Gariep Belt. Together, the arcuate Gariep-Damara Belt constitutes the western-most sector of the greater Kuunga Orogen.

Closure of the former Khomas Ocean basin involved high-angle convergence between the Congo and Kalahari Cratons with over-thrusting at both margins to give a doubly vergent orogen. Closure of the Khomas Ocean occurred at some stage between 580 Ma cessation of sedimentation and 555 Ma oldest deformation of the Naukluft Nappe. The Uis-Pass Line, between the Southern Zone and Southern Margin Zone, marks the true collisional suture between northern limits of all Kalahari basements and is the boundary between distinctly different Neoproterozoic sedimentary sequences on the respective passive margins. Furthermore, sequences north of this thrust have Congo Craton detrital zircon provenance and eNd signatures compared to those south of the Uis-Pass Line, which only show Kalahari Craton signatures. In common with more typical sutures, the Uis-Pass Line contains dismembered lenses of serpentinite after ultramafic protoliths. Deposition of the Fish River Subgroup molasse in the Nama foreland basin occurred at 520-550 Ma indicating main phase convergent orogenesis in the Damara Orogen was well under way by this time.

Closure of the Khomas Ocean was complete by 555 Ma and peak metamorphism during subsequent main phase convergent orogenesis occurred between 535-510 Ma. Convergence between the Congo and Kalahari Cratons resulted in deep burial of the north and south margins, Barrovian metamorphism and later out-wedging accompanying ongoing convergence. Thrusting of the passive margin sequences back over the cratonic nuclei occurred at the Naukluft Nappes and was initiated as early as 555 Ma. The central core of the orogen experienced moderate burial, high heat flow, crustal melting and long-lived high-T/low-P peak metamorphism between 538-505 Ma. Convergence in the Damara Orogen was coincident with low strain warping in the Kaoko Belt and younger thermal and magmatic event accompanying transtensional reactivation of the western shear zones between 530-505 Ma. Main phase convergent orogenesis and peak metamorphism occurred in the same period throughout the length of the Kuunga Orogen, stitching north and south Gondwana in the final phase of assembly. Peak metamorphism took place in the Lufilian Arc and Zambezi Belt at 530-520 Ma. Subduction under the margins of Gondwana was well established by 530 Ma, producing the major accretionary Ross-Delamerian Orogen over the period 520-510 Ma. At the scale of Gondwana the late-stage metamorphic and deformational events in the Damara Orogen and Kaoko Belt, therefore took place in an intraplate setting.

Crustal thickening attendant with final ocean closure and assembly of Gondwana led to post-tectonic magmatism and localized extension. Subsequent to main phase Kuunga and Ross-Delamerian collision and convergence, extension became widespread along the margins of Gondwana as well as within the older major mobile belts. Inboard transmission of stress from the outboard, Gondwana margin (Ross-Delamerian) subduction system caused continued thrusting of the Naukluft Nappes at 500-495 Ma and syn-tectonic sedimentation in the Nama foreland basin. It also led to transtensional reactivation of shear zones, rapid exhumation and decompression induced pegmatites in the Kaoko Belt at 530-520 Ma and 510-505 Ma and shear zone reactivation in the Gariep Belt at 506-495 Ma. Extensional telescoping and out-wedging of the Damara orogenic margins, accompanying ongoing convergence, occurred at 510-490 Ma in the north and 517-490 Ma in the south. Emplacement of post-tectonic A-type granites occurred in the Central and Okahandja Zones at 510-500 Ma and was accompanied by youngest metamorphic parageneses in the Central Zone of 500-490 Ma age. Post-tectonic magmatism was followed by stabilization of the Damara Orogen and diachronous cooling and exhumation occurred through to 480 Ma.

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[14] Metamorphic Evolution of Damara Belt: Part 1: The Lower-Plate Barrovian Margins

 

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2018.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2018/XX (in press).

 

Metamorphic field gradients across the orogen show that the Damara Orogen is a bi-vergent paired metamorphic system. Broadly the Damara Orogen consists of two types of metamorphic terranes of contrasting metamorphic style, peak metamorphic conditions, crustal depth, T/depth ratios and P-T evolutions. These are Barrovian series nappe-fold-thrust belts at the margins of the orogen at low structural levels in the downward advecting lower-plate settings, and the high-grade granite-rich internal part of the orogen at high structural levels in the upper-plate setting. A newly established, large, robust and internally consistent quantitative metamorphic dataset has allowed for characterization of the Damara Orogen by detailed metamorphic field gradients and P-T-t evolutions in different zones. Metamorphic field gradients across the Damara Orogen are considered robust, on the basis of all PT determinations by the same internally consistent manner. Furthermore, unlike many metamorphic belts, the Damara Orogen has not been reworked in later events and all peak metamorphic parageneses were formed in a single metamorphic cycle between 535 and 510 Ma. The metamorphic field gradients constructed for the Damara Orogen are possibly the most detailed complete available for any whole orogenic system on Earth, and presents a strong first-order dataset to constrain the tectonic model for this classic collisional orogenic system. These metamorphic field gradients are further augmented by detailed structural profiles and deformation and kinematic field gradients across the orogen and new metamorphic age data.

Both the eastern part of the northern margin and whole southern margin experienced Barrovian series metamorphism at lower to upper amphibolite facies grades, high pressures between 8.0-11.0 kb and low T/depth ratios between 17-25 ºC/km. All parts of these Barrovian margins experienced steep tight to open clockwise P-T paths with deep burial to near peak metamorphic conditions, followed by near isothermal decompression immediately after or through the peak of metamorphism. These Barrovian margins are in nappe-fold-thrust belts and have inverted metamorphic gradients, with higher temperatures at higher structural levels towards the internal parts of the orogen. The overall metamorphic gradient across these Barrovian margins from foreland to internal parts of the orogen is of systematically increasing temperature and metamorphic grade.

The gradients in pressure (or crustal depth) and T/depth ratio are more complex than temperature and shows significant steps across crustal-scale structures, indicating incision of the metamorphic pattern and extensional telescoping of the orogenic margin. The southern margin consists of two crustal wedges corresponding to the Southern Margin Zone (Outwedge I) and at higher structural levels the Southern Zone (Outwedge II). There is a sharp metamorphic discontinuity (MD) between these two crustal wedges at the Uis-Pass Line (MD1) and another more diffuse discontinuity between Outwedge II and the internal parts of the orogen (MD2), corresponding approximately to the southern edge of the Okahandja Zone. MD1 coincides with the Uis-Pass Line shear zone, an extensionally reactivated major thrust zone that is the true suture between Kalahari and Congo stratigraphy and basement elements, and contains dismembered serpentinite bodies along its length. The southern boundary of Outwedge I is not a sharp discontinuity, but is transitional into the greenschist facies Southern Foreland. Outwedge I shows a steep systematic increase in P from the Southern Foreland to the top of the crustal wedge. This P gradient is steeper than the T gradient and results in a concomitant decrease in T/depth ratio across Outwedge I. At the top of Outwedge I and base of Outwedge II, centred on the Uis-Pass Line, there is a very steep drop in P from 11.0 kb to 8.0 kb, resulting in a drop in the T/depth ratio from 17 to 25 ºC/km. The metamorphic gradient across Outwedge II is flat, with minor concomitant increase in T and P, resulting in near flat T/depth ratios. At the top of Outwedge II at the base of the internal parts of the orogen, there is a very steep drop in P from 9.0 kb to 5.5 kb, resulting in an increase in the T/depth ratio from 25 to 35 ºC/km.

The large pressure drops across the two metamorphic discontinuities indicate excision of metamorphic zonation and post-peak extensional telescoping of the southern margin. Extensional telescoping of the margin was accommodated by top down to the north transport being partitioned into the discontinuity at the top of each crustal wedge. The process of "out-wedging", or lateral exhumation, of each crustal wedge is accommodated by opposing, but coeval, shear kinematics in the lower and upper margins of the crustal wedge. Upthrusting along the basal margin gave a component of vertical exhumation that was accommodated by relative extensional shear at the upper margin; dropping low-P rocks at high structural level, back down to the north against high-P rocks immediately below the discontinuity. Consequently, the metamorphic field gradients across the Damara Orogen constrain both the first-order architecture and crustal evolution of this collisional orogen. Deep burial followed by rapid exhumation of the crustal wedges; is expressed internally by the steep and tight clockwise P-T paths, with near isothermal burial to maximum pressures near peak metamorphic conditions, followed by isothermal decompression. These tectonic constraints from the metamorphic dataset have been combined with the structural and new metamorphic geochronology to outline a fully integrated tectonic model for the Damara Orogen.

Detailed structural profiles across the southern margin confirm the extensional kinematics at the metamorphic discontinuities. The Uis-Pass Line at MD1 is a crustal-scale reverse shear zone with main phase parageneses forming near the peak of metamorphism. This shear zone is strongly retrogressed and reactivated by top-down to the north extensional shear at lower metamorphic grades subsequent to the peak of metamorphism. The region immediately north of the Uis-Pass Line is also retrogressed and over-printed by numerous top-down to the north shearbands and laterally extensive brittle faults. These extensional structures are preceded by an episode of north-vergent folds with shallow north-inclined axial surfaces indicating significant vertical flattening. These ductile to brittle late-stage structures indicate a switch to vertical s1 after the peak of metamorphism consistent with burial loading and subsequent exhumation. The sequence from flattening folds to retrograde extensional shearbands and finally brittle normal faults, documents material trajectories through the crustal column to progressively shallower and more brittle conditions. The MD2 at the top of Outwedge II is broader and more diffuse but nevertheless develops the same sequence of retrograde extensional features, though laterally extensive normal faults have not been identified.

Sequential foreland propagation of the timing of exhumation of the two crustal wedges is constrained by a combination of metamorphic, structural and chronologic arguments. The Uis-Pass Line has an early reverse history accompanying the peak of metamorphism followed by post-peak extensional reactivation. This simple structural history indicates that Outwedge I was initially overthrust (buried) by Outwedge II, followed by exhumation (out-wedging) during later extensional reactivation of the shear zone. The greater crustal depth attained by Outwedge I, also indicate that Outwedge II must have been thrust over Outwedge I first to produce a crustal thrust-stack. The progressive southward propagation, of southward transport of these crustal wedges onto the foreland margin, indicates that out-wedging at the margin occurred during ongoing N-S convergence across the Damara Orogen. Forward propagating of out-wedging is supported by younging of peak metamorphic ages and exhumation ages, from the Southern Zone (Outwedge II) to the Southern Margin Zone (Outwedge I). Peak metamorphism in Southern Zone was between 517-530 Ma and exhumation occurred between 517 and 490 Ma. Peak metamorphism in Southern Margin Zone was later, at between 510-517 Ma and exhumation occurred between 510 and 490 Ma.

The foreland propagating sequence of out-wedging may be the result of thrust stacking giving rise to a southward migration of the gravitational; instability generated by crustal over-thickening. The southward transport of Outwedge II, further loaded and deeply buried the rocks in Outwedge I. This over-loading resulted in relatively fast isostatic rebound of Outwedge I, which in an overall convergent system was facilitated by upthrusting along thrusts in the Southern Foreland and accommodated by top-down to the north extensional reactivation of the Uis-Pass Line (MD1).

The eastern northern margin has a high-P Barrovian crustal wedge (Outwedge III), similar to the metamorphic architecture of the southern margin. Peak metamorphic conditions in Outwedge III were 630 ºC, 10.5 kb and very low T/depth ratio of 17 ºC/km. Consequently; there was a steep increase in T and P from the sub-greenschist facies Northern Foreland. Towards the south was a shallow increase in T and very steep decrease in P into the high-grade internal part of the orogen in the Central Zone. Like the southern margin, these metamorphic variations indicate that the northeastern margin contains a deeply buried crustal wedge that was exhumed by over-thrusting onto the Northern Foreland. This over-thrusting must have been accommodated by extensional telescoping at the boundary with the internal part of the orogen, though there is insufficient outcrop to document these putative extensional structures. Exhumation of Outwedge III was slightly younger than the 517-490 Ma extensional telescoping of the southern margin. Peak metamorphism of Outwedge III is documented by a U-Pb monazite age of 510±4 Ma (this Record). Consequently, exhumation is constrained to between the peak at 510 Ma and cooling through 350 ºC at 490 Ma.

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[15] Metamorphic Evolution of Damara Belt: Part 2: The Upper-Plate High-Grade Orogen Core

 

Goscombe, B., Gray, D., Foster, D. and Wade, B., 2018.

Metamorphic evolution of Gondwana 2. The Damara Orogenic System: amalgamation of central Gondwana and evolution of orogen architecture. Geoscience Australia Record 2018/XX (in press).

 

The upper-plate setting of the Damara Orogen is the broad internal part of the orogen consisting of the Central and Okahandja Zones. All parts of the upper-plate experienced high-T / low-P Buchan type metamorphism and in contrast to the Barrovian margins is associated with large volumes of crustal granitic melts. The upper-plate experienced a large range in M3 metamorphic grade from middle amphibolite to granulite facies, but at uniformly low pressures between 4.0-6.0 kb and high T/depth ratios between 32-50 ºC/km. The Central Zone is granulite facies throughout and peak metamorphic conditions range 632-772 ºC, 4.3-5.1 kb and 38-46 ºC/km. Metamorphic evolutions were also uniform throughout, with shallow tight clockwise P-T paths that experienced only minor burial, with extreme conductive heating resulting in shallow decompressive heating paths from peak pressures and near isobaric cooling after the peak of metamorphism. High thermal regimes and tight P-T paths with isobaric cooling are indicative of metamorphism with long residence times where conduction out-competes advection.

M3 metamorphism occurred over a very protracted period of time from 545 to 505 Ma. Robust U-Pb metamorphic age determinations indicate a continuum of high-grade metamorphism, with clustering around at least three metamorphic events: M3a metamorphism at 534-542 Ma, M3b metamorphism at 516-530 Ma and M3c metamorphism at 505-515 Ma. Detrital metamorphic zircons from molasse in the Fish River Subgroup range in age 530-540 Ma, indicating that the M3a period is the main phase of convergent orogenesis and metamorphism in the Damara Orogen. M3c metamorphism in the Central Zone post-dates main phase N-S convergence and coincides with post-kinematic granites and overlaps extensional telescoping of the north and south margins of the Damara Orogen. Metamorphic mineral parageneses cannot be assigned to these three metamorphic events and the petrology of most samples show no evidence for multiple thermal peaks. It is interpreted that the Central Zone remained at high temperatures through out this period and the thermal maxima and/or crystallization events of zircons and monazites, occurred at different times in different rock samples depending on crossed metamorphic reactions, and in different parts of the Central Zone.

The metamorphic field gradients show systematic variation in metamorphic parameters across the upper-plate. Broadly, there is a decrease in temperature across the Okahandja Zone to a low-T trough at the margin with the Central Zone. To the north, temperature increases across the Central Zone, with maximum-T in the Northern Central Zone, and decreases across the South Ugab Zone into the Ugab Zone of the western northern margin. Pressure also shows a similar field gradient with steep decrease from the Southern Zone, across to a low-P trough within the Okahandja Zone and slight increase northward to the margin with the Central Zone. Pressures only increased slightly across the Central Zone to a subdued maximum-P coincident with the maximum-T in the Northern Central Zone. There was a stepped decrease in pressure across the Autseib Fault into the South Ugab Zone and a further stepped decrease into the Ugab Zone. Because of the steep decline in pressure from Southern Zone and across the southern Okahandja Zone, the T/depth ratio increases steeply across the Okahandja Zone, except for the low-T trough at the margin with the Central Zone. The T/depth ratio increases steeply at the southern margin of the Central Zone and remains high across the whole zone, with a slight maximum-T/depth ratio in the Northern Central Zone. The T/depth ratio only decreases slightly into the South Ugab Zone and remains high to the north within the M3 contact aureoles of the Ugab Zone, which formed at this time.

Superimposed on these simple metamorphic field gradients across the internal part of the orogen are complex interplays with the thermal anomaly associated with the post-kinematic A-type Donkerhoek leucogranite of 505 Ma age, in the central to north Okahandja Zone. The western extent of the Donkerhoek granite coincides with the low-T trough in the northern Okahandja Zone. Consequently, the contact aureole of the granite is of higher grade than the regional metamorphic parageneses and the contact metamorphic isograds can be mapped. Further east in the central Okahandja Zone is a small domain that preserves older (535 Ma) high-grade parageneses adjacent to the Donkerhoek Granite. These rocks are of upper-amphibolite facies grade and of too higher grade to be contact metamorphic parageneses associated with this hydrous granite. The high-grade rocks experienced metamorphic conditions more similar to the Central Zone and may represent the leading edge of the attenuated passive margin, metamorphosed ~20-40 Ma prior to emplacement of the Donkerhoek granite.

The long-lived high-grade residence times in the Central Zone imply little advection of material through the crust and a long-lived thermal anomaly requiring a relatively quiescent tectonic setting. The absence of advection indicates that crustal over-thickening and isostatic induced rapid exhumation did not occur. The Central Zone is characterized by large volume of granite and migmatite (30-60% of the current erosion surface) that are either the cause or consequence of the high T/depth thermal regime. Granite chemistry indicates they were derived from melting of the Damara Sequence, suggesting they are a consequence of a high thermal gradient. There is no exposed evidence for volumous mafic magmatism or a significant volume of highly radiogenic heat producing rocks, leaving the ultimate source of the high temperature conditions necessarily speculative and most probably the result of crustal architecture.

It is proposed that high heat flow in the Central Zone is the result of a combination of lithospheric processes happening in an inter-connected sequence. Initially the north-dipping subduction of Khomas Ocean crust and after collision, partial down advection of Kalahari crust, would have driven upwelling of the asthenosphere above the subducting crust and immediately below the Central Zone. Consequently, the previously thinned passive margin crust of the Central Zone is in a settling akin to a continental "back-arc". These initial conditions generated a high-heat flow that was the ultimate cause of significant partial melting of the crust resulting in the large volume of granites emplaced into the upper-crust level in the Central Zone. Crustal differentiation by partial melting resulted in the formation of a dense residual lower-crust. Extensional telescoping of the north and south orogenic margins during out-wedging exhumation of the deeply buried margins may have resulted in a component of extensional stress in the continental "back arc" setting of the Central Zone. This putative extension may have induced decoupling from the dense lower-crust, resulting in delamination bringing the asthenosphere closer to the surface, ensuring that the already high thermal gradient persisted for a long period of time.

The wider Kuunga Orogen that amalgamated north and south Gondwana in the latest episode of assembly of Gondwana (550-500 Ma) contains a number of low-P/high-T (high T/depth ratio) and UHT domains. The Central Zone of the Damara Orogen is a large contiguous example with the orogenic architecture still intact and thus a potential source for insights into the formation of these "hot orogens" (see above). The Kuunga Orogen contains a disproportional number of "hot orogen" domains compared to other large-scale collisional orogens and orogenic periods in Earths history. Indeed typical collisional orogens such as the Himalayan Orogen are almost absent of high T/depth metamorphic domains.

West of the Damara Orogen there are no high-T/low-P or UHT anomalies of Late Pan-African age (500-550 Ma) recorded in the Lufilian Arc, Zambezi Belt, Malawi Mosaic and Lurio Belt. However, further west is a large number in the Madagascar, Sri Lanka, Southern India and Lutzow-Holm region. Some of the UHT domains in this region formed at high-P and did not experience anomalous high T/depth conditions and are not true "hot orogens" (i.e. T/depth >35 ºC/km). Further west the only other high T/depth domain is the much smaller Prydz Bay region. Consequently, the continent-scale Late Pan-African thermal structure of the Kuunga Orogen consists of three widely separated high T/depth anomalous regions. The majority of the Kuunga Orogen at this time experienced more typical Barrovian series metamorphism, indicating that each anomalous region had independent causes unique to the tectonic settings and histories of each region. However, the tight cluster of anomalies in the central Kuunga Orogen, suggests that the many domains here may have a common cause. Furthermore, the location of this mega-anomaly region in both the centre of the Kuunga Orogen and the Gondwana supercontinent may be a crucial factor in the cause of the high heat flow.

Modelling of mantle thermal structure below different aerial extents of continental crustal lithosphere predicts profound increases in sub-crustal temperatures as supercontinents grew, due to the thermal blanketing effect of continental crust (Gurnis, 1988; Coltice et al., 2007). It follows that latter stages of supercontinent assembly will induce marked changes in thermal structure of underlying mantle and thus profoundly influence the thermal structure of the overlying continental crust. Consequently, it is probable that orogens formed late in the assembly of Gondwana and particularly those sited centrally within the supercontinent, will have higher crustal temperatures. This continent-scale process presents a first-order cause for high thermal gradients in parts of the Kuunga Orogen, in addition to any superimposed second-order processes specific to parts of the orogen.

 

[16] Episodic intra-continental reactivation during collapse of a collisional orogen: the Damara Belt, Namibia

Ben Goscombe^1,2, David A. Foster², Dave Kelsey³, Ben Wade⁴, David Gray⁵, Laura Mulrooney², Peng Jiang², Murray Haseler⁶, Antonios Marsellos⁷

¹Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.

²Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.

³Geological Survey of Western Australia, Perth, WA, Australia.

⁴Adelaide Microscopy, University of Adelaide, Adelaide, 5005, SA, Australia.

⁵School of Earth Sciences, University of Tasmania, Hobart Tasmania. Australia.

⁶2 Mulung Street, Point Lookout, Stradbroke, 4183, QLD, Australia.

⁷School of Earth Sciences, Hofstra University, Hempstead, NY, USA.

Focus Review: Gondwana Research 109,  285-375 (2022).

Collision, high-angle contraction, crustal thickening and heating at 555–516 Ma, primed the Damara Belt ready for crustal collapse, which was triggered by a transition to ENE–WSW contraction along the length of the belt in response to orogenic events in east Gondwana at 516–505 Ma. Along-orogen shortening reworked and thickened the high-grade core of the belt, increasing gravitational instability, and establishing an NW–SE extension direction across the belt that was conducive to reactivation of pre-existing structures and eventual collapse. This extension direction persisted, and the switch to vertical σ1 and collapse was signalled by decompression melting at ~502 Ma and subsequent rapid cooling. Collapse was focused on the high-grade core of the belt that was exhumed as a ~170 km wide, semi-coherent massif-type metamorphic core complex with steep extensional shear zones and faults in the marginal flanks. During exhumation the core complex was reactivated by oblique-slip extensional shear zones that responded to external transient stress fields. Reactivation by middle- to lower-amphibolite facies dextral-normal shear zones at ~500–495 Ma and ~495–490 Ma, involved E–W to ENE–WSW shortening consistent with accretionary events in the west Gondwana margin during the Pampean Orogeny. Reactivation by greenschist facies sinistral-normal shear zones at ~485 Ma, involved N–S shortening consistent with accretionary events in the south Gondwana margin during the Famatinian Orogen. Early stages of exhumation involved decompression melting, flattening folds and ductile ultramylonite zones within carbonate that formed by NW–SE extension. Late stage exhumation in the brittle field from ~480 Ma onwards, involved a stress-switch to radial extension directions dominated by NE–SW. This stage involved flat-lying breccia, inclined faults, vertical fractures, and oxidizing fluids partitioned into the top of the lower-levels of the massif. Ongoing exhumation of the core complex drove localized NW–SE shortening within the flanking margins and hanging-wall, and produced low-strain reverse structures that straddle the ductile to brittle transition. The pressure difference between exhumed massif (4.8–5.5 kbar) and hanging-wall margins (3.9–4.2 kbar), indicate that ~3.2–4.6 km of crust was stripped from above the core complex.

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