Structural Geology of Robben Island: Implications for
the Tectonic Environment of Saldanian Deformation
Christie D. Rowe*, Nils R. Backeberg, Tamsyn van Rensburg, Scott Angus
Maclennan, Carly Faber, Catherine Curtis and Pia A. Viglietti
Department of Geological Sciences, University of Cape Town, Private Bag X3,
Rondebosch 7701, South Africa
*Corresponding author, christierowe@gmail.com, now at: Department of Earth and
Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz,
CA 95064, USA
Abstract
We present a detailed structural and lithologic map of Robben Island, offshore Cape
Town, South Africa. Robben Island is underlain by the Tygerberg Formation, part of the
Neoproterozoic to early Cambrian Malmesbury Group of the Saldania Belt. The
depositional setting and structural history of the Tygerberg Formation are poorly
constrained due to limited outcrop and lack of previous structural studies.
Sedimentary structures are indicative of deposition at relatively high rates in a high
energy environment and we concur with previous workers that deposition occurred on
turbidite fan systems in a tectonically deepening basin. By comparison with active and
ancient examples, we suggest that a forearc or trench slope, supra-subduction zone basin
is a possible match to the setting of the Tygerberg Formation. However, limits on
preservation and insufficient age data prevent comparisons in basin geometry and
deposition rates which could be used to test depositional setting with more certainty.
Northwest-southeast striking subvertical pressure solution cleavage is pervasive
throughout the exposures. Upright folds, with axial planes parallel to the cleavage, plunge
10-15° to the northwest or southeast with approximately 20° variation in trend azimuth.
The folds are limited in along-axis extent and often occur in asymmetric pairs. Subtle
bedding-parallel shear zones divide folds of different plunge directions. This pattern of
folds is consistent with experiments and observations of en echelon folding during
distributed strain associated with oblique transpression. This finding is consistent with
previous studies of parallel, slightly earlier orogenic belts to the north (Gariep and Kaoko
Belts) although our observations do not allow us to distinguish whether transpressional
strain was sinistral or dextral. Sinistral transpression is considered more likely given the
dominantly sinistral strike-slip history on the nearby Colenso Fault and the southward
migration of collision along the western margin of Africa during the late Neoproterozoic
to early Cambrian.
Key words
Robben Island, Malmesbury Group, Saldania Belt, Adamastor Ocean, Pan-African, slate
belt, transpression, oblique strain
Introduction
The Saldania Belt is one of several deformed belts of “Pan-African” age recording the
closure of the Adamastor Ocean with the construction of southwestern Gondwana
(Frimmel and Fölling, 2004), collectively referred to as the Brasiliano Orogeny (e.g.
Cawood and Buchan, 2007, and references therein) or Adamastor Orogeny (Goscombe
and Gray, 2008). In southern Africa, these belts are represented by the Kaoko Belt in
Namibia, the Gariep Belt in Namibia/South Africa, and the Saldania Belt in South Africa.
Dextral transpression along NW-striking, east-ramping thrusts has been suggested for
parts of the Gariep Belt (Hälbich and Alchin, 1995), where sheath folds also suggest a
strong component of shear-parallel extrusion. The Kaoko Belt displays highly oblique
sinistral shortening (Goscombe et al., 2003; Goscombe and Gray, 2008). The Saldania
Belt has not been subject to the same level of structural investigation, due largely to
relatively poor exposure.
As noted by Rozendaal et al. (1999), the deformation of the Saldania Belt is not
consistent with a ”proper collisional orogeny”. The Saldania Belt is represented in the
western Cape by two terranes: the inland, slightly higher grade Swartland Terrane and the
coastal Malmesbury Terrane (Figure 1; Belcher and Kisters, 2003). Belcher and Kisters
(2003) gave a detailed overview of structural fabrics in the Swartland Terrane (northeast
of the Colenso Fault, Figure 1) and showed that transposed fabrics therein were formed
by high shear strains. Belcher (2003) suggested that a subduction trench active during the
complex, multi-phase closure of the Adamastor Ocean could provide an appropriate
model in which deposition of deep-marine sediments, and structures relating to highshear strain, could be roughly co-eval and therefore a good match to his observations of
the Swartland Terrane. The Colenso Fault is a poorly exposed strike-slip fault with a
complex strain history comprising a reversal from sinistral to dextral motion during rapid
regional exhumation at 540Ma (Kisters et al., 2002). The Malmesbury Terrane, mostly
represented by the Tygerberg Formation, has never been the subject of a similarly
detailed and regionally extensive structural study.
With this contribution we aim to document the detailed structures within the Tygerberg
Formation exposed on Robben Island and compare the sedimentary facies and structures
to the proposed environments of deposition and deformation.
Geologic setting of Robben Island
Robben Island is a World Heritage site of tremendous importance to South Africa in
terms of both human and natural history. The coastline of the island (Figure 1) provides a
unique opportunity to observe the structural fabrics of the Neoproterozoic to Cambrian
Saldania Belt.
The tectonic history of the Saldania Belt is poorly understood. The belt is composed of
low-grade (low greenschist and below) metasediments of the Malmesbury Group
(Hartnady et al., 1974). The depositional age is not well constrained but a few detrital
zircon dates suggest that the sediments range from 750-560Ma (Armstrong et al., 1998).
Rozendaal et al. (1999) described the Saldania Belt rocks as marine sediments deposited
in a deepening basin. The Malmesbury Group was divided by Hartnady et al. (1974) into
three tectonic terranes separated by the Colenso Fault and the (inferred) PiketbergWellington Fault. Detailed field mapping by Belcher and Kisters (2003) revised the
stratigraphy by demonstrating that the eastern terranes (Swartland and Boland Terranes
of Hartnady et al. (1974)) are overlain, either structurally or unconformably, by the
western Malmesbury Terrane, represented in coastal outcrops by the Tygerberg
Formation. The Swartland Terrane is deformed by broad regional doubly-plunging
anticlinoria, with higher metamorphic grade (up to biotite-bearing schists) in the center of
the folds. Most of the Swartland Terrane is chlorite-white mica schists and psammites
with strong shear fabrics transposing bedding, overprinted by horizontal shortening. The
Malmesbury Terrane is dominated by fabrics suggesting significant horizontal
shortening. Although the contact is nowhere exposed, Belcher and Kisters (2003) and
Belcher (2003) inferred a depositional unconformity based on the strong shear fabrics of
the Swartland Group which are absent from the Malmesbury Group, the presence of
Swartland-derived vein quartz clasts in the Piketberg formation which forms the base of
the Malmesbury Group, and the horizontal shortening which represents the latest
deformation of both groups.
The Saldania Belt is intruded by the Neoproterozoic to Cambrian Cape Granite Suite
(Figure 1). Complicating structural interpretations, these units were later involved in the
Carboniferous-Permian Cape Orogeny (Frimmel et al., 2001). The Cape Granites are
spatially separated into a seaward belt of locally tectonised S-type granites and a
landward belt of undeformed I-type granites (Scheepers, 1995). Available dates are
suggestive of a southward-younging trend in the S-type granites although the resolution
of the ages is not sufficient to confirm this trend (Figure 1). Post-tectonic A-type granites
and ignimbrites occur in both terranes (Scheepers, 1995). The granites and their intrusive
contacts appear to be undeformed during the Cape Orogeny, and they clearly crosscut
pre-existing folds and foliations in the Saldania Belt meta-sediments (von Veh, 1982).
Therefore, it has been generally assumed that the majority of structural development in
the Saldania rocks can be attributed to pre-intrusion deformation. This assumption is
difficult to test however, as the fold axes and regional shortening fabrics are parallel in
the Saldania metasediments and the overlying Paleozoic Cape Basin, and the maximum
metamorphic grade at the present level of exposure in both cases is lower greenschist or
lower. There is little clear evidence of structural or metamorphic overprinting during the
second orogeny, which may be used to interpret the “pre-Cape” condition of the Saldania
Belt.
Von Veh (1982) studied detailed structural and sedimentological transects across the
intrusive contact between the Cape Granite and the Tygerberg Formation at Sea Point,
Cape Town. He established that the Tygerberg sediments were folded prior to granite
intrusion at Sea Point, and suggested that additional flattening had occurred associated
with the intrusion of the magma. He reported northeast vergence in the meso-scale folds
crosscut by the intrusive contact. The Peninsula Batholith intruded at 540 ± 4 Ma
(Armstrong et al., 1998).
Tygerberg Formation on Robben Island
The Tygerberg Formation sediments can be locally divided into three general lithological
groups: tan to grey sandstones, interbedded greywacke and siltstone, and fine dark grey
slates. These lithologies are similar to those described by von Veh (1982) at Sea Point
(Figure 1). These rocks form the bedrock of Robben Island and outcrop along the rocky
coasts and in quarry excavations. Rare interior outcrops on the island are badly weathered
and were not utilized in this study. A dolerite dyke cuts across the Tygerberg sediments
in the southwest corner of the island. The general distribution of these facies is shown on
Figure 2, which also gives the location of photos in the descriptions which follow. These
units are unconformably overlain by Quaternary beach and dune sands and calcrete which
cover nearly the whole interior of the island (Theron et al., 1992).
Sandstones
The sandstones of the Tygerberg Formation outcrop on the southeastern and westnorthwestern coasts of Robben Island (Figure 2). They are tan to light grey in colour,
corresponding to slight changes in composition, although they tend to discolour into a
darker brown with weathering. In general, the Tygerberg Formation sandstone is medium
to coarse grained. Bedding thickness is typically in the range of 10-30 cm but reaches 50
cm locally in the northwest corner of the island. Orthogonal joints cut the sandstones in
three major orientations: N-S, E-W and NW-SE (Figure 3A). Grey-green reduction stains
may be observed on the surfaces of these sandstones and are concentrated along the joint
surfaces. Spheroidal weathering between joint surfaces is a characteristic feature of the
outcrop appearance (Figure 3A).Where bedding and cleavage are sub-parallel and
cleavage is closely spaced, the sandstones may take on a slaty appearance but bedding
structures are still very well preserved. Parallel lamination and trough cross-bedding are
common. Discrete intervals of convolute bedding (also reported by Nakashole, 2004) are
locally present and sometimes deformed by widely-spaced pressure solution cleavage
(Figure 3B). In general, pressure solution cleavage in the sandstones is weaker and more
widely spaced than in more pelitic units of the Tygerberg Formation. The spacing and
orientation of cleavage planes varies with bedding thickness and with slight, often
gradational variations in grain size and clay content (Figure 3C).Sand-filled burrows were
reported by Nakashole (2004), suggesting active bioturbation in these facies, although
this study did not confirm these observations.
The sandstones are similar to Facies A of von Veh (1982), who interpreted the
mechanism of deposition as fluidization, grain flow or high density turbidity currents.
The frequency of convolute bedding horizons suggests high rates of deposition and
burial.
Interbedded shales and greywacke/siltstones
The siltstones of the Tygerberg Formation are darker in colour than the sandstones and
contain metamorphic chlorite and diagenetic or metamorphic pyrite (Mbangula, 2004;
Nakashole, 2004). The greywacke grain fraction contains sub-rounded to angular lithic,
feldspar and quartz grains in a clay-rich matrix. Typically, the siltstones are interbedded
with greywackes and slight cleavage refraction is observed across lithologic contacts.
These siltstone/greywacke intervals are 1- 20 cm thick, separated by thin shale beds.
Differential erosion of the shale beds is common along the coastline, resulting in
excellent bedding plane exposures in the axial region of 10's of metres-scale folds (Figure
3D). These strata are similar to Facies B defined by von Veh (1982), who interpreted
them as deposited by turbidity currents in a “proximal” setting.
Dark grey slates
Dark grey fine-grained slates outcrop on the southern tip and northwestern corner of the
island and were quarried at Jan van Riebeek quarry in the south and Rangatira Quarry in
the north (Figure 2). Delicate sedimentary and soft-sediment deformation features are
common, including combined ripples and bidirectional cross-bedding (Figure 4A),
sinusoidal ripple lamination, graded bedding, formsets of ripple-drift cross laminations,
regular cross laminations, light grey rip-up clasts (Figure 4B), and delicate soft-sediment
deformation structures (e.g. Figure 4C). The slates have gently anastomosing cleavage,
which refracts slightly across bedding when cutting at a high angle (Figure 4D). The
cleavage-bedding intersections as viewed on the bedding surface (Figure 4E) are also
anastomosing and variable in spacing.
This lithology is consistent with Facies D of von Veh (1982), who also noted very fine
laminations alternating between dark (clay rich) and light (silt rich) as well as the delicate
sedimentary structures characteristic of fluid escape processes.
Dolerite dyke
A single dolerite dyke occurs on Robben Island. The dyke is similar in lithology and
orientation to the local Cretaceous (132 ± 5 Ma) False Bay dyke swarm (Reid et al.,
1991). The dyke is tabular and trends ESE-WNW across the south coast of the island,
crosscutting bedding and cleavage in the Tygerberg Formation. It is preferentially eroded
and the contacts are not exposed. Although the dolerite does not outcrop in situ, it is
abundant in boulders and cobbles within a tabular zone approximately 20m thick (Figure
2).
Summary
The sedimentary units on Robben Island are dominated by turbiditic sequences (similar to
the Sea Point exposures, von Veh (1982)) and all the representative lithologies show
evidence of rhythmic normal grading. Coarse sediments dominate most sections and the
sediments are chemically immature. Sole marks and rip up clasts (Figure 4B) testify to
high rates of sediment flow and deposition. The rate of deposition often exceeded the rate
of static dewatering, as demonstrated by symmetric dewatering structures such as
convolute bedding, load casts and flame structures which are present in all facies. Fine
horizontal laminations are also common in all facies (Figure 4D.) Locally, bidirectional
ripples and cross-bedding suggest multi-directional currents were active during
deposition. Massive muddy olistostromal beds were observed at Sea Point (von Veh,
1982), and ~8 km north of Robben Island at Silwerstroomstrand (C. Rowe et al.,
unpublished data).
Conclusive evidence of bioturbation is rare, although sand-filled burrows were described
by Nakashole (2004). The lack of fossils in the Tygerberg Formation limits the precision
of palaeo-environmental interpretations. In the modern world, shallow marine
environments are characterised by intense bioturbation and biological reworking.
Nakashole (2004) relied on the rarity of bioturbation fabrics to prefer deep water turbidite
origin over a shallower water tempestite or contourite setting for the Tygerberg
Formation. However, an age constraint on the Tygerberg Formation is given by the
intrusion of the Cape Granite Suite. The nearest intrusion to Robben Island is the
Peninsula Pluton cropping out at Sea Point (540± 4 Ma, Armstrong et al. (1998)).
Globally, the widespread appearance of burrowing behaviours which contribute to
bioturbation is known as the “substrate revolution” (Bottjer et al., 2000). This
stratigraphic event is recorded in Cambrian shallow marine sediments worldwide.
Therefore, the paucity of bioturbation in the Tygerberg Formation is not considered an
indication of deep water, and the depth remains unconstrained.
Structural Features
The structural features exposed at Robben Island are dominated by: strong vertical
northwest-southeast pressure solution cleavage, asymmetric meso-scale folds, cryptic
bedding-parallel shear zones (sometimes bearing quartz-cemented breccia), and east-west
vertical joints and quartz veins (Figure 5).
Pressure Solution Cleavage
All outcrops of Tygerberg Formation on Robben Island are crosscut by strong pressure
solution cleavage. The cleavage in the clay-rich lithologies is planar and parallel on a
regional scale (Figure 4F) but in sandier lithologies the cleavage shows distinct local
fanning around fold axes (Figure 5A). As described above, the pressure solution cleavage
is locally anastomosing and refracts gently through graded beds. In general cleavage is
axial planar to the major folds and bedding parallel between fold axes. On the scale of the
whole island, the cleavage is very well aligned (Figure 4F) with the mean cleavage plane
oriented 330/86NE (axial planar to the upright folds.)
Folds
The structure of Robben Island is dominated by NW and SE plunging folds which often
occur in anticline-syncline pairs (Figure 5). The half-wavelength of the folds is typically
40 ± 10m. Axial planes are upright and parallel to the regional pressure solution cleavage.
The shape of fold hinges varies, with concentric folds occurring in sandier sections (e.g.
Figure 3D) and increasingly similar fold morphologies observed in shalier units (e.g.
Figure 5A). Asymmetry of anticline-syncline pairs generally suggests northeast vergence.
Parasitic folding at scales below the meso-scale folds was only observed in one location.
The folds are open to closed with interlimb angles ranging between 35-75°.
The eighteen mapped meso-scale folds on Robben Island are coaxial with the regional
tectonic fabrics and plunge 5-20° in either direction (Figure 5C). The fold axes cluster in
either orientation but do not define a continuous distribution in plunge angle. Folds are
cylindrical and doubly-plunging folds were not observed on the island. This could be
attributed to limited along-trend exposure, however, one would expect that random
exposure of doubly plunging folds would produce an array of fold axes around a central
mean value. Our data clearly form two discrete clusters at 10-15° plunge. The clusters
trend in opposite directions although there is considerable scatter in trend direction. It
was not possible to trace fold axes across Robben Island where they might be covered by
Holocene sediments, suggesting that axial traces may not be continuous on the km-scale.
This distinctive pattern will be further discussed below.
In more than one location, oppositely plunging folds are closely associated, apparently
sharing roughly parallel limbs (e.g. Figure 5B). The transition zone between the two
anticline axes in Figure 5B does not show a gradual rotation of bedding as would be
expected if a syncline separated the anticlinal hinges. Limb orientation is maintained at
relatively constant curvature from both folds, up to a zone approximately 1.5m wide
where bedding is not exposed. These observations suggest that the unexposed zone
represents a cryptic wrench fault along bedding separating the shared limbs. A similar
fault is observed farther south on the west coast of the island (Figure 5D.)
Faults and shear zones, quartz-cemented breccias
The majority of faults are inferred from poorly exposed narrow zones across which
bedding and/or cleavage change orientation sharply (e.g. Figure 5B described above).
Although direct indications of shear sense are rare, the bedding orientation discontinuities
require significant rotation along the faults. A few small faults showed quartz
slickenfibres but in most cases the direction of shear is inferred from drag folding of
bedding. In all cases these indicate oblique slip.
Rarely, quartz-cemented breccias are found in these zones (Figure 6A), most commonly
in sandstone units although they are inferred in other lithologies by the presence of
quartz-cemented breccia boulders observed in float. The breccias have a chaotic fabric
and quartz veins around them are deformed (flattened and boudinaged) along the pressure
solution cleavage, suggesting that these breccias are pre-to syn-shortening. Stretching
direction recorded by the boudinage of these early quartz veins is parallel to the strike of
the pressure solution cleavage.
At the southern end of the island, an intensely deformed breccia zone cemented by quartz
indicates the presence of a brittle shear zone (Figure 6B). A tight anticline is
asymmetrical toward the southwest, suggesting southwest vergence. This anticline
structurally overlies an intensely veined shear zone and is interpreted as a drag fold in the
hanging wall of a southwest-vergent, bedding-parallel thrust fault. This unique exposure
suggests an association between the early quartz-cemented breccias and shortening by
brittle faulting.
Joints and fibrous/blocky extensional quartz veins
Quartz veins were observed throughout the strata at Robben Island and crosscut the
pressure solution cleavage and folds (Figure 6C.) Veins are concentrated in the
sandstone-dominated units and do not have any apparent association with fold hinges or
faults. They generally strike E-W and dip steeply south, although there is quite a bit of
scatter in the orientations (Figure 6D). The veins are generally blocky but where aligned
euhedral fibrous crystals are present, they plunge gently N-NW suggesting gradual
oblique opening for the veins (Figure 6F). The veins are roughly parallel with the tabular
Cretaceous-age False Bay dykes (only one on Robben Island; Figure 2).
These veins are therefore kinematically consistent with, and most likely associated with
Cretaceous extensional rifting coincident with the intrusion of the False Bay dykes (Reid
et al., 1991).
Environment of deposition and deformation
The depositional environment of the Tygerberg Formation is not well constrained by
previous studies, although two general themes exist in the literature: Rozendaal et al.
(1999) suggest a deepening ocean/continental margin setting, and von Veh (1982)
suggests intercalation of proximal and distal distributary fans. The significant volume of
turbidites implies a dynamically deepening depocenter, while the sedimentary structures
imply high energy.
One possible match to these environmental conditions, and the stratigraphic relationship
to the more deformed Swartland terrane, is the setting of a forearc or trench slope basin
overlying an active subduction zone. Depositional basins often develop in the forearc
region of a subduction margin due to high sediment supply from the nearby volcanic arc,
and the topographic basins created by the intricate processes of compression and
extension in accretionary wedges. These may be separated or continuous with
depositional basins on the trench slope, which share many characteristics with forearc
basins but may be distinguishable by their deeper water and steeper slope, expressed in
finer overall grain size and increased relative importance of gravity slides. Both basin
geometries are predicted by Coulomb models for wedge evolution, especially models
incorporating erosion (where appropriate) and thermo-mechanical evolution of the wedge
sediments (e.g. Fuller et al., 2006; Kimura et al., 2007).
Lithofacies of forearc and trench slope basins
Several recent and modern examples of forearc and trench slope basins are presented for
comparison to the Tygerberg Formation. Our examples range from the landward edge of
the forearc basin (the Pliocene Quinault Formation, Cascadia (Campbell et al., 2006)),
broad forearc settings (the modern Kumano Basin, Japan (Park et al., 2002; Moore et al.,
2009) and the Cretaceous Great Valley Sequence, California (Ingersoll, 1979)), outer
forearc basin (the Mio-Pliocene Awa Group, Japan (Tokuhashi, 1989)), trench slope
(Cretaceous Matanuska Formation, Alaska, USA, (Trop, 2008), and trench slope-trench
(the Eocene Sitkalidak Formation, Alaska, USA (Moore and Allwardt, 1980)). All the
examples are dominated by turbidite fans which overlap laterally and through vertical
section. They are all relatively long, narrow basins which unconformably overly higher
grade, more deformed meta greywackes and shales, comprising accretionary wedges.
They share the characteristic paleocurrent patterns with two maxima parallel and
perpendicular to the basin axis, representing sediment flow into the basin and along the
axis. Large-scale slumping and slope collapse deposits are prominent in all the examples,
particularly in the trench slope settings. Lithologically, coarse sands and conglomerates
dominate the landward forearc basin (Campbell et al., 2006), transitioning to finer silts
and shale in the trench slope to trench deposits (e.g. Moore and Allwardt, 1980; Trop,
2008). Some of the basins contain regionally continuous tuff beds: airfall deposits from
the nearby arc (Great Valley, Kumano: Ingersoll, 1979; Moore et al., 2009), while the
others do not but may have a significant volcaniclastic component in the sandstones.
Water depth indicators in the mid-forearc basin, where they are reported, are generally
bathyal (~200-2000m) (Awa Group, Great Valley, Kumano: Tokuhashi, 1989; Ingersoll,
1979; Park et al., 2002). Several examples show evidence of tectonic deepening of the
basin by localized normal faulting either pre- or syn-depositionally (Kumano, Awa,
Matanuska: Park et al., 2002; Tokuhashi, 1989; Trop, 2008).
In terms of the basin geometry, the Kumano Basin sediments onlap the irregular
topography created by the extension of the wedge landward of the outer rise of the
accretionary wedge, and even lap seaward over the outer rise into the trench slope basin
(Moore et al., 2009). Moore and Allwardt (1980) distinguished two structural zones in the
Sitkalidak Formation: the seaward edge of the basin was sheared and then horizontally
shortened, while the landward part of the basin shows upright folds and axial planar
cleavage, with no evidence of shearing or substantial intraformational faulting.
Comparison to the Tygerberg Formation
Several of the diagnostic features indicating relatively high-energy, episodic deposition
for the forearc and trench slope basins described above were also observed on Robben
Island and reported elsewhere in the Tygerberg Formation. The dominance of turbidites
and occurrence of olistostromes and slump features suggest deposition by periodic highvolume events. The combined ripples (Figure 4A) require multi-directional flow, the ripup clasts (Figure 4B) suggest transient high-velocity flow conditions, and the abundant
convolute bedding (Figure 3B) and dewatering structures (Figure 4C) present in all facies
suggest high rates of deposition. Hummocky cross stratification in massive sandstone
beds was not observed on Robben Island, but has been recorded in the Tygerberg
Formation at Sea Point (von Veh, 1982) and at Grotto Bay about 10 km north (C. Rowe
et al., unpublished data). Scoured bedding contacts were not observed in the Tygerberg
Formation on Robben Island although they have been reported elsewhere (von Veh,
1982). Von Veh (1982) also reported varying but generally bimodal current directions for
the Tygerberg Formation turbidites at the Sea Point Contact, consistent with the other
examples.
Based on the multi-directional current indicators, frequent coarse sands, rip-up clasts and
abundant dewatering structures, we concur with the interpretation by previous researchers
that the Tygerberg Formation was deposited in a deepening basin (Rozendaal et al., 1999)
by migrating distributary channels on coalescing fans (von Veh, 1982). Although there is
yet insuficient data to uniquely determine the depositional setting of the Tygerberg
Formation, we note the similarities to forearc and trench slope basins in which large
volumes of chemically immature marine sediment are deposited, dominantly as turbidites
building distributary fans, with tectonic basin deepening acting to generally maintain
consistent water depth. As pointed out by Cawood et al. (2009), forearc basins are
dominated by lithic greywacke-shales as the primary source is an arc, but backarc basins
are more quartzose as they also receive sediment from the landward continent. In
addition, the closure of a back-arc basin is associated with high geotherms while the
closure of a fore-arc basin remains cold (Cawood et al., 2009). The unconformable
relationship to the Swartland terrane described by Belcher and Kisters (2003) and von
Veh (1982) is consistent with the relationship between a flat-lying forearc and the
underthrusted sediments of the accretionary wedge.
Implications of fold orientation distribution
Models of folding within transpressional shear zones predict parallel fold axes which
rotate with material lines toward the shear direction during increasing strain. This
paradigm treats fold axes as features which, once formed, rotate as passive markers
during increasing shear. These folds are typically shown as symmetrical and upright
horizontal or gently doubly-plunging, with parallel trends (e.g. Davis and Reynolds,
1996; Sanderson and Marchini, 1984; Krantz, 1995, and references therein). In this
paradigm, the long axis of the strain ellipse within the shear zone rotates from fold
initiation at approximately 30° to the shear zone boundaries (perpendicular to maximum
compressive stress) to smaller angles as it extends with increasing oblique strain. A
prediction of this model is that the angle between fold axes and the direction of shear
depends on both the angle of obliquity and increasing strain.
A contrasting paradigm is provided by the experiments of Tikoff and Peterson (1998) and
the observations of Titus et al. (2007). Tikoff and Peterson (1998) performed experiments
using sheets of plasticine and silicon to investigate the fold morphologies and orientations
formed during transpression. They varied the angle of the shear zone with respect to the
direction of maximum compression ( = 0° when compression is parallel to the margin;
 = 90° when compression is normal to the margin). Their results indicate that fold axes
initiate with plunges around 5-10° even while compression and shear are horizontal, and
that significant scatter in trend of the fold axes does not decrease with increasing strain.
Most importantly, they showed that fold axes do not follow material lines within the
shear zone – therefore the fold axis orientation is decoupled from both angle of obliquity
and the total strain, but would be expected to steepen with increasing strain if rocks
behaved passively as the system evolved. In essence, these experiments show that fold
axes are not maintained as passive markers, but rather that they can continue to move
relative to bedding surfaces which roll over the fold axes as the folds tighten (Smallshire,
1999)
Our fold axis observations (Figure 5C) show a similar pattern to that produced by Tikoff
and Peterson (1998). Two distinct clusters of fold axes are shown. The northwest
plunging folds trend 310 ± 14° and plunge 13 ± 3°. The southeast plunging folds trend
136 ± 14° and plunge 13 ± 3°. The divergent plunges and consistently variable trends of
fold axes are consistent with transpressional strain.
Titus et al. (2007)'s structural and paleomagnetic study of en echelon folds formed
around a slightly transpressive segment of the San Andreas Fault in central California
confirmed the patterns of fold axis trend produced by Tikoff and Peterson (1998).
Further, their paleomag results require vertical axis rotations of the folded strata in
addition to the horizontal axis rotation imposed by tightening folds. The juxtaposition of
these two rotations suggests complex rotational shearing is necessary along limbs of folds
in order to accommodate the necessary stretching of limbs. Scissor faults such as those
shown in Figures 5B and 5D may have formed during similar intrafolial rotations.
Shortening recorded by the structures on Robben Island
The pervasive NW-SE striking vertical pressure solution cleavage on Robben Island
demonstrates strong NE-SW shortening. Spread in cleavage orientation (Fig. 4F) reflects
anastomosing (Fig. 4E) and fanning (Fig. 5A) of cleavage planes, suggesting the cleavage
formation was early with respect to folding. The fold asymmetries suggest predominantly
northeast-vergence with local southwest-vergent folds and faults (e.g. Figure 6B)
representing antithetic structures. These are preferentially associated with zones of
quartz-cemented breccias, suggesting they represent zones of fluid advection and
hydrofracture. Although strain markers are scarce, where the cleavage is observed
crosscutting sedimentary structures it is possible to comment on the bulk shortening
achieved by pressure solution. In the sandstone units where cleavage crosscuts convolute
bedding (e.g. Figure 3B) we estimate shortening on order 10-20%. In clay rich lithologies
where pressure solution is more efficient, the volume loss, and therefore total shortening,
is much greater. In Jan van Riebeeck Quarry, the rip-up clasts shown in Figure 4B are
moderately well-aligned. The large clasts are extremely thin along an axis normal to the
pressure solution cleavage (for example, the large clast in Figure 4B is 15cm : 5 cm :
0.2cm; sampling limitations in the World Heritage Site prevented quantitative
investigations of a statistically relevant number of clasts). On this basis, we estimate that
pressure-solution shortening in the clay-rich lithologies may be several times greater than
the shortening in the sandstones. As demonstrated by Tikoff and Peterson (1998)'s
experiments, cross-section balancing across transpressional folds is likely to overestimate
the total shortening as it does not account for out-of-plane extrusion. Strain ellipses in
transpression show flattened “pancake” shapes (Fossen and Tikoff, 1993), regardless of
strain partitioning or the local mechanisms of strain accommodation. Analog models have
predicted a transition from strike-slip dominated, sub-vertical structures toward thrustdominated, moderately dipping structures at oblique convergence angles in the range of 
= 15-25° (Casas et al., 2001). Given the sub-vertical orientation of faults and folds
observed on Robben Island, and the wrench and rotational fault motions, we suggest that
the Saldanian margin during late-Neoproterozoic to early Cambrian was oriented at  =
15-25° or less, relative to the direction of plate convergence. Simply stated, the direction
of maximum shortening on Robben Island, as in other examples of transpressional
deformation, does not correlate to the direction of maximum principle palaeostress.
The numerical models of Merle (1997) demonstrate that the orientation of the plane of
principle flattening (represented on Robben Island by the axial planar pressure solution
cleavage, Figure 4F) does not vary in orientation when the angle of oblique convergence
varies. Merle (1997) points out that the unusual symmetry of the finite strain ellipse
produced during transpression renders finite strain analyses relatively useless in
interpreting strain paths during transpression. Similarly, the experiments of Tikoff and
Peterson (1998) and the fold axis orientations observed by Titus et al. (2007) are
symmetrical and do not distinguish between sinistral and dextral transpression.
As such, we are unable to measure the exact shortening represented by the structures on
Robben Island, or determine the sense of transverse motion (whether sinistral or dextral).
To differentiate, we would need specific measurements of the rotation in plunge direction
of the long axis of the finite strain ellipse (Merle, 1997). Unfortunately, no conclusive
strain markers recording this information were observed during our field studies. Citing
Kisters et al. (2002)'s observations of dominantly sinistral motion for the Colenso Fault
(followed by a very brief episode of dextral slip at about 540Ma), and the southward
propagation of collision indicated in the Kaoko and Gariep Belts, sinistral shear sense is
considered more likely.
Regional Tectonic Events
Numerous orogenic belts dividing older crustal terranes formed during the construction
of southwestern Gondwana in the late Neoproterozoic – early Cambrian. The closure of
the Adamastor Ocean, uniting African and South American blocks, was a complex,
multiphase event. Many crustal blocks were assembled (Rapela et al., 2007; da Silva et
al., 2005), probably involving rotations in plate motions including changes in subduction
polarity as seen in similar modern settings (Cawood et al., 2009). The southwardyounging orogenic belts along the western margin of southern Africa are enigmatic
because structural studies suggest highly oblique strain, with the consequence of lateral
displacement of terranes, and the geometry of subduction and collision is often unclear.
The Saldania Belt, at the southern end of this trend, introduces particular questions since
although there is abundant evidence of crustal shortening, there is no direct evidence of
continental collision or an associated volcanic arc that would indicate subduction
polarity. Therefore, the tectonic environment of deformation has not been adequately
explained.
The Kaoko Belt (northern Namibian coast) records oblique accretion of an exotic terrane
(the Coastal Terrane, during east-dipping subduction; Goscombe and Gray, (2007), or
west-dipping subduction at 650-600 Ma, with the Granite Zone of the Dom Feliciano Belt
as the associated arc complex; Oyhantçabal et al. (2009)) and compression beginning at
660 Ma, with peak metamorphism at 585-560Ma and shear zones active until 530Ma
(Goscombe and Gray, 2008).
In the Gariep Belt (southern Namibia-South Africa), basin filling and shortening is dated
at 576-573 Ma and deformation peaked between 545- 530Ma (Frimmel and Frank, 1998;
Frimmel et al., 1996; Frimmel and Basei, 2006). In this case, the basin is interpreted as a
retro-arc extensional setting during west-dipping subduction, again citing the Dom
Filiciano granites as the associated arc (Frimmel and Basei, 2006; Basei et al., 2005).
Meanwhile, de Ameida et al. (2002) interpret the Camaquã Basin (and equivalents) in
southern Brazil to also represent retro-arc spreading during the same west-dipping
subduction. These two possibilities are geometrically incompatible with one another
based on current plate reconstructions.
The relationship between the Gariep Belt and the Saldania Belt is essentially unknown,
although some authors have suggested lateral equivalency based on the older
Neoproterozoic correlations between Gariep rocks and the Cango Inlier (e.g. Frimmel et
al., 1996) or apparent strike-alignment and similar age (e.g. Dunlevey, 1992). More
recent reconstructions align the Saldania Belt with the ∼550 Ma Ross Orogen in
Antarctica (e.g. da Silva et al., 2005), separated along strike but more consistent with the
orientation of the Kalahari craton in some reconstructions (e.g. Gray et al., 2008).
In the Saldania Belt, oblique shortening was complete by the intrusion of the undeformed
units in the Cape Granite Suite (e.g. Peninsula Batholith at 540 ±4 Ma; Armstrong et al.
(1998)), simultaneous with the late-stage reversal in transpression direction at about 540
Ma (Kisters et al., 2002). In contrast to the Kaoko and Gariep Belts, no collisional phase
or metamorphic event followed. As pointed out by Frimmel and Frank (1998), this
southward-younging tectonism is consistent with a zipper-like, south-progressing closure
of the Adamastor Ocean, although other models are possible (e.g. southward extrusion of
the arc complex, as discussed by Miller et al. (2009)).
The youngest detrital zircons reported in the Tygerberg Formation are ∼560 Ma,
(uncertainty not reported; Armstrong et al., 1998), roughly the same age as the oldest
Cape Granites (552 ±6 Ma; Scheepers and Armstrong, 2002). The Cape Granites are the
only known source for zircons of this age. Folds and cleavage in the Tygerberg
Formation are crosscut by the late-tectonic Sea Point granites (540 ± 4 Ma; Armstrong et
al., 1998; von Veh, 1982), which solidified at 3-4 kbar (Villaros et al., 2006)
(approximately 11-15 km depth assuming a typical psammitic crustal density of
2650kg/m3). Volcanics erupted at the same structural level as the granites (following
their exhumation) by 515 Ma (Scheepers and Poujol, 2002). These constraints allow a
minimum estimate of the regional uplift rate during the early Cambrian, between the
intrusion of the Sea Point granite and the eruption of the Postberg Ignimbrite: 11-15 km /
∼25 Ma gives an uplift rate of ∼0.4-0.6 mm/yr. This minimum estimate of uplift rate is
less than or similar to uplift rates in currently active areas of transpression (e.g. Southern
Alps at 6-9 mm/yr; Little et al. 2005, San Andreas fault in the Santa Cruz Mountains at
0.13-0.3 5mm/yr; Valensise and Ward 1991).
This uplift was associated with a brief period of local extension by the formation of
elongate grabens parallel to regional structure, (the intra-orogen pull-apart basins
suggested by Rozendaal et al. (1999)) followed by long term regional subsidence
(Tankard et al., 2009). This fault-parallel, graben-bounded geometry is well preserved
today (Figure 1) and the conglomerates are unconformably overlain by the regionally
extensive Paleozoic Table Mountain Group (Hartnady et al., 1974; Theron et al., 1992;
Tankard et al., 2009). Although some previous authors have suggested that the rifting was
regionally significant and the extent of the conglomerates was originally much greater
than is seen today (Theron et al., 1992), no direct evidence for either voluminous
deposition or significant normal faulting at this time has been reported.
Two tectonic settings are suggested which could explain the deposition and deformation
of the Tygerberg Formation in a supra-subduction setting, link the regional uplift to the
switch in shear sense on the Colenso Fault at 540 Ma, and provide context for the
deposition of the graben-bounded conglomerates. Triple-junction migration (or ridgetrench interaction) is associated with transient localized extension caused by the passage
of a thermal high with or without associated slab window volcanism (Unruh et al., 2007;
Farris and Paterson, 2009; Ayuso et al., 2009; Cole and Steward, 2009), which we
suggest may correlate to the Cape Granite Suite. Alternatively, migration of releasing
bends along a transform boundary can produce chains of grabens along strike-slip faults
without regional extension (Seeber et al., 2010; Wakabayashi et al., 2004). These two
models can be compared and tested by further studies distinguishing: the age and
distribution of the lower Cambrian conglomerates, and the age progression and tectonic
setting of melting which produced the Cape Granite Suite, in particular, the relationships
in age and petrogenesis between the S-type and I-type granites.
Although numerous authors have suggested a subduction setting for the Saldania Belt and
associated granites (Belcher, 2003; Da Silva et al., 2000; Dunlevey, 1992; Scheepers,
1995; Stevens et al., 2007 and others), no associated arc complex has been uniquely
identified. Our data presented here do not allow us to suggest a direction of subduction
vergence, only that the sedimentological and tectonic setting is consistent with a suprasubduction zone basin, as would be expected around the edges of the disappearing
Adamastor Ocean basin. If the closure of the Adamaster Ocean was a complex, multistage event involving multiple terranes as implied by Germs and Gresse (1991) and
Stanistreet et al. (1991), a forearc basin may display complex provenance and source
directions (e.g. the Wrangel Mountains Basin, which laps onto the accreted Wrangellia
Terrane of southern Alaska, Trop et al., 1999).
In similar transpressional subduction environments, significant strike-slip in the forearc
region has been shown to laterally separate arc complexes from associated forearc basins
(e.g. Alaska’s Kodiak Complex, Sample and Reid, 2003). In addition, periods of
magmatic hiatus are reported in numerous Mesozoic subduction complexes around the
Pacific Rim (e.g. Cretaceous Mexican Trench, (Solari et al., 2007) or Miocene Cascadia
(Dostal et al., 2008)) during which significant transverse motion may have occurred.
Therefore, the highly oblique transpression in the Tygerberg Formation implies the
potential for substantial forearc displacement. We note that due to the oblique setting,
matching of ~750 Ma belts likely does not apply to correlations of ~550 Ma belts,
especially when attempted on a Mesozoic (pre-Gondwana rift) tectonic map, as
demonstrated by the spatial mismatch of metamorphic and magmatic ages (Gray et al.,
2008, Figure 12).
As obliquity is characteristic of all the Neoproterozoic belts on the western margin of
southern Africa, it is unlikely that the current attempts to reconstruct the Cambrian
geometry of southwest Gondwana can be accurate when using a plate fit model based on
the mid-Mesozoic configuration. A large number of the attempts at reconstruction use
these Jurassic plate configurations (e.g. Bossi and Goucher (2004), Da Silva et al. (2005),
Frimmel et al. (1996), Frimmel and Frank (1998), Gaucher et al. 2004, Gresse (1995),
Gresse and Scheepers (1993) and Rozendaal et al. (1999) which all refer to Porada
(1979; 1989), which reuse the models of Smith and Hallam (1970) and Norton and
Sclater, (1979)). Therefore, future reconstructions of Cambrian (late Pan-African) plate
geometries must consider this additional complication.
Conclusions
The Tygerberg Formation was deposited in a high-energy, high deposition-rate,
tectonically deepening basin during the late Neoproterozoic to very early Cambrian.
These characteristics as well as the unconformable relationship to highly sheared, slightly
higher metamorphic grade underlying sedimentary units are consistent with deposition in
a forearc basin. Syn- to immediately post-depositional deformation occurred in an overall
regime of highly oblique transpression and rapid regional exhumation ended the life of
the basin. Although direct correlations with South American equivalents are not yet
possible and most attempts at regional reconstruction end at the Gariep Belt, we hope this
study might contribute an additional constraint toward filling the gaps in Pan-African
southern Gondwana.
Acknowledgements
In order to produce a high-density structural data set in a short time period and provide
for student training and research experience, we formed “Team Tygerberg Terrane”, an
informal group at the University of Cape Town Department of Geological Sciences.
Students from pre-Honours to MSc level joined the field team and participated in
preparation of this manuscript. Funding for our Robben Island field work, as well as
further expeditions of Team Tygerberg Terrane, was generously provided by the
Geological Society of South Africa: LE Kent Trust Fund. We thank Steven Jansen and
Sue Kirschner from the Chief Directorate: National Geospatial Information, South Africa
for permission to use the base image for the structural map (Figure 5). We are very
grateful to Richard Sherley (Avian Demography Unit, UCT) and Mario Leshoro
(Ecologist, Robben Island Museum) for their time and support for our initial
reconaissance work. Rabia Damon of the Robben Island Museum made our field work
possible and enabled the project to go forward. Richard Whiting facilitated our field
studies on the island by preventing conflicts with seabird breeding activities on Robben
Island. Prof. Alex Kisters and Dr. Francesca Meneghini provided valuable insights on the
regional tectonic history and structural models which informed the conclusions of this
work. Dr. Federico Farina, Prof. Ian Buick and Dr. Arnaud Villaros are gratefully
acknowledged for conversations on the Cape Granites. Stereonets were plotted using
Stereonet v. 6.3 X for Macintosh © 2005, and Professor Richard Allmendinger is
sincerely thanked for making this software freely available for non-commercial use. We
thank Prof.s Alex Kisters and Udo Zimmermann and Editor Jay Barton for constructive
feedback which greatly improved this manuscript.
Author Contributions
Rowe wrote the manuscript with contributions from van Rensburg. Backeberg compiled
the maps with contributions from Maclennan. All authors contributed to field data
collection and observations and completion of the manuscript.
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Figure Captions
Figure 1
Regional geologic map, Cape Town area, South Africa. Redrawn from Belcher and
Kisters (2003) with additional geology from Scheepers (1995). Sources for igneous ages:
1 = Scheepers and Armstrong (2002); 2 = Scheepers and Poujol (2002); 3 = Da Silva et
al. (2000); 4 = Jordaan et al. (1995); 5 = Armstrong et al. (1998). Da Silva et al. (2000)
also reported an age of 536 ± 5 Ma for the Robertson I-type granite (east of the study
area) which is the only recent data available for the I-type suite. Additional previous
dates by Rb-Sr or Pb-Pb are not used in this study. Inset: study location (grey box)
relative to African continent. Borders of South Africa, Namibia, Swaziland and Lesotho
are shown.
Figure 2
Generalized lithologic map of the rock type distributions and ma jor structures within the
Tygerberg Formation on Robben Island. Locations of field photos in Figures 3, 4 and 6
are indicated.
Figure 3
Features of sandstone-rich lithologic strata. A: Sub-horizontal bedding surface view of a
coarse sandstone bed on the southeast corner of Robben Island. Three major joint
orientations are suggested by white outlines on the lower left of the photo. Spheroidal
outcrop surfaces are created in situ by joint-controlled weathering processes. B: A thin
convolute bed in the tan sandstones, NW corner of Robben Island. Brunton compass is 7
cm across casing and indicator points toward North. Bedding contacts indicated by solid
white line. Pressure solution cleavage cuts bedding at a high angle and refracts gently to a
slightly steeper orientation within convolute sandstone bed. Modification of convolute
bedding forms qualitatively hint at the degree of shortening along pressure solution
cleavage. C: Cleavage spacing and orientation changes across lithologic variation in
sandstone units near the northwest corner of Robben Island. Pen is 14 cm long and points
north. Bedding cuts horizontally across photo (solid white lines) and cleavage is locally
sub-perpendicular, cutting vertically across photo. Changes in bedding lithology are
annotated on the right side of photograph with variation from clay rich toward coarse
sand toward the WNW. Corresponding changes in cleavage spacing are annotated on the
left side of the photo and slight changes in orientation are suggested by the dashed white
lines. D: Interbedded shales and greywacke siltstones outcrop beautifully in the axial
region of a north-plunging anticline near the northwest corner of the island.
Figure 4
Features of dark grey slates. A: Combined ripples in dark grey slates, Jan van Riebeeck
Quarry. B: Rip-up clasts of light-coloured siltstone in fine dark grey slates, Jan van
Riebeeck Quarry. Clasts are weakly oriented with long axis approximately horizontal
across photograph. Median axis is approximately up-down. Clasts are highly flattened
perpendicular to cleavage in the third dimension. C: Fine-scale planar lamination and
convolute slump textures in dark grey slates, Jan van Riebeeck Quarry. Diameter of pen
is 12 mm. D: Pressure solution cleavage in dark grey slates, Jan van Riebeeck Quarry.
Left edge of photo is annotated to show scoured bedding surfaces (sub horizontal, solid
white lines) and pressure solution cleavage (dashed white lines). Gradations within each
bed correspond to changes in orientation and spacing of cleavage creating anastomosing
geometries. General regions of closely and broadly spaced cleavage (relatively speaking)
are indicated. Repetitive graded beds result in closely spaced cleavage transitioning
toward increasing spacing upsection. Bedding is upright as shown by the subtle
crossbedding. E: Anastomosing slaty cleavage seen intersecting the plan of the bedding
surface, Jan van Riebeeck Quarry. Pen is 12 mm in diameter and points toward the
northwest. Dashed white lines on left side of photo annotate cleavage-bedding
intersection lines showing gently wavy, anastomosing geometry. F: Stereonet plot of
axial planar pressure solution cleavages from all across Robben Island (n=107).
Figure 5
Structural map of the Tygerberg Formation outcropping on Robben Island. Aerial
photography acquired in 2008 generously provided by Chief Directorate: National
Geospatial Information (DMC job 3318C 2008 127). Inset A: Cleavage fanning in mesoscale cylindrical fold. Stereonet shows π-diagram solution for fold axis from bedding
measurements. Star symbol = fold axis (15/122). Inset B: Adjacent but oppositely
plunging folds separated by cryptic bedding-parallel wrench surface. Bedding is
continuous from each axis out to the wrench fault, the position of which is restricted to a
1.5m-wide limb-parallel zone with minor bedding deflection and no noticeable brittle
damage zone on either side. Wreck of the 60m Sea Challenger for scale. C: Stereonet plot
of meso-scale fold axes across Robben Island (n=18). Each fold axis was calculated using
a minimum of 4 bedding attitudes representing both limbs, although spatially detailed
data has been omitted from the map (Figure 5) for clarity. D: Block diagram shows the
sense of rotation on a typical Mode II fault on the west coast of Robben Island.
Figure 6
Occurrence of quartz veins and breccia cements on Robben Island. A: Intrafolial quartzcemented breccia extended along pressure solution fabric. Length of pen = 14cm. B:
Hanging wall asymmetric fold pair adjacent to small SW-vergent quartz-cemented
brecciated thrust zone. C: Late transtensional quartz veins crosscut pressure solution
fabrics. Aligned fibres and C-axes of subhedral crystals show direction of vein opening
(black arrows), which is consistent across different vein orientations D: Stereonet plot of
late transtensional tabular quartz veins from all across Robben Island (n=37). Extension
direction as recorded by orientation of quartz fibres shown with black squares.