SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-9-1187-2018Oblique rifting: the rule, not the exceptionOblique riftingBruneSaschabrune@gfz-potsdam.dehttps://orcid.org/0000-0003-4985-1810WilliamsSimon E.https://orcid.org/0000-0003-4670-8883MüllerR. Dietmarhttps://orcid.org/0000-0002-3334-5764GFZ German Research Centre for Geosciences, 14473 Potsdam, GermanyInstitute of Earth and Environmental Science, University of
Potsdam, 14476 Potsdam-Golm, GermanyEarthByte Group, School of Geosciences, University of Sydney, Sydney, New South Wales
2006, AustraliaSydney Informatics Hub, University of Sydney, Sydney, New South Wales,
AustraliaSascha Brune (brune@gfz-potsdam.de)26October201895118712063July201816July201812October201815October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/9/1187/2018/se-9-1187-2018.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/9/1187/2018/se-9-1187-2018.pdf
Movements of tectonic plates often induce oblique
deformation at divergent plate boundaries. This is in striking contrast with
traditional conceptual models of rifting and rifted margin formation, which
often assume 2-D deformation where the rift velocity is oriented
perpendicular to the plate boundary. Here we quantify the validity of this
assumption by analysing the kinematics of major continent-scale rift systems
in a global plate tectonic reconstruction from the onset of Pangea breakup
until the present day. We evaluate rift obliquity by joint examination of
relative extension velocity and local rift trend using the script-based plate
reconstruction software pyGPlates. Our results show that the global mean rift
obliquity since 230 Ma amounts to 34∘ with a standard deviation of
24∘, using the convention that the angle of obliquity is spanned by
extension direction and rift trend normal. We find that more than ∼70 % of all rift segments exceeded an obliquity of 20∘
demonstrating that oblique rifting should be considered the rule, not the
exception. In many cases, rift obliquity and extension velocity increase
during rift evolution (e.g. Australia-Antarctica, Gulf of California, South
Atlantic, India-Antarctica), which suggests an underlying geodynamic
correlation via obliquity-dependent rift strength. Oblique rifting produces
3-D stress and strain fields that cannot be accounted for in simplified 2-D
plane strain analysis. We therefore highlight the importance of 3-D
approaches in modelling, surveying, and interpretation of most rift segments
on Earth where oblique rifting is the dominant mode of deformation.
Introduction
The relative motion of Earth's tectonic plates often causes oblique
deformation at divergent plate boundaries. This is primarily due to the fact
that irregularly shaped plate boundaries generally do not align with
small-circles of relative plate movement and that changes in plate motion
additionally lead to time-dependent plate boundary obliquity
(Díaz-Azpiroz et al., 2016; Philippon and Corti, 2016). Traditionally,
rift evolution and passive margin formation have been investigated using 2-D
conceptual and numerical models assuming an alignment of relative plate
motion and plate boundary normal. These studies yielded major insights into
first-order subsidence patterns (McKenzie, 1978; White, 1993), described key
phases controlling the architecture of rifted margins (Lavier and Manatschal,
2006; Huismans and Beaumont, 2011; Brune et al., 2016) and provided insight
into the fault evolution during rifting (Ranero and Pérez-Gussinyé,
2010; Brune et al., 2014; Bayrakci et al., 2016; Naliboff et al., 2017). The
applicability of these concepts and models is often rooted in the assumption
that rifts can be understood via plane strain cross sections orthogonal to
the rift trend and that the direction of extension aligns with the
orientation of these cross sections. Many rifts and passive margins, however,
involve segments where the extension direction is not perpendicular to the
rift strike such that oblique, non-plane strain configurations occur
(Sanderson and Marchini, 1984; Dewey et al., 1998).
Oblique rift segments differ from classical orthogonal examples in several
major aspects:
In contrast to orthogonal rifts, the initial phase of oblique rifting is
characterized by segmented en échelon border faults that strike at an
angle to the rift trend. The orientation of these faults is controlled by the
interplay of inherited heterogeneities with far-field stresses, whereas
diverse modelling studies independently suggest a fault orientation that lies
in the middle between the rift trend and the extension-orthogonal direction
(Withjack and Jamison, 1986; Clifton et al., 2000; Corti, 2008; Agostini et
al., 2009; Brune and Autin, 2013). These large en échelon faults generate
pronounced relay structures (Fossen and Rotevatn, 2016) which act as a major
control on fluvial sediment transport (Gawthorpe and Leeder, 2000).
Seismic cross sections at rifted margins are often taken perpendicular to
the rift trend, which in most cases is also perpendicular to the coastline.
Considering that faults in oblique rifts do not strike parallel to the rift
trend, seismic profiles will observe a projection of the fault surface that
features a smaller dip angle than the actual 3-D fault, an error that might
cascade further into seismic restorations.
Besides these structural implications, oblique rifting
appears to be a major factor in governing the geodynamic evolution of
extensional systems. This has been shown via several pieces of evidence.
(i) Oblique rifting has been inferred to enhance strain localization by
enabling the formation of pull-apart basins and large-offset strike-slip
faults, for instance during the Gulf of California opening (van Wijk et al.,
2017; Umhoefer et al., 2018). (ii) Analytical and numerical modelling
suggests that the force required to maintain a given rift velocity is
anticorrelated with the rift's obliquity. The reason for this behaviour is
that plastic yielding takes place at smaller tectonic force when the
extension is oblique to the rift trend (Brune et al., 2012). (iii) At the
same extension velocity, oblique rifts deform with a certain rift-parallel
shear rate, which is balanced by a lower rift-perpendicular extension
velocity. This means that oblique segments of a particular rift accommodate
lithospheric and crustal thinning at a lower rate than orthogonal segments of
the same rift, a difference that effects the thermal configuration and
therefore the structural and magmatic evolution of each segment (Montési
and Behn, 2007).
Oblique rifting holds geodynamic implications on the global scale because
of its relation to toroidal plate motion, i.e. vertical axis rotation
components of plate movements and associated strike-slip deformation.
Toroidal motion is enigmatic from the perspective of plate-driving forces,
because its purely horizontal motion cannot be directly caused by buoyancy
forces in Earth's interior (Lithgow-Bertelloni et al., 1993; Bercovici,
2003). Oblique rifts feature strike-slip velocity components and therefore
contribute to toroidal motion, while orthogonal rifts are an expression of
purely buoyancy-driven (poloidal) flow.
The impact of rift obliquity on the structural architecture of
continental extensional systems varies between natural cases. This is mainly
due to rift variability in general, which arises from tectonic inheritance
(Manatschal et al., 2015; Morley, 2016; Hodge et al., 2018; Phillips et al.,
2018), or from along-strike changes in rheology, crustal configuration,
temperature, and rift velocity (e.g. Sippel et al., 2017; Molnar et al., 2017;
Brune et al., 2017a; Mondy et al., 2017).
Oblique rifting in presently active rifts can be easily deduced by combining
the local rift trend and GNSS-based surface velocities (Díaz-Azpiroz et
al., 2016). Prominent examples are the Main Ethiopian rift (Corti, 2008), the
Levant rift system including the Dead Sea rift (Mart et al., 2005), the Gulf
of California rift (Atwater and Stock, 1998; Fletcher et al., 2007), the
Upper Rhine graben (Bertrand et al., 2005), and the Cenozoic West Antarctic
rift (Rossetti et al., 2003; Vignaroli et al., 2015; Granot and Dyment,
2018). Structure and kinematics of past rift systems have been studied by
surveying obliquely rifted margins (Fournier et al., 2004; Lizarralde et al.,
2007; Klimke and Franke, 2016; Phethean et al., 2016) and transform
continental margins (Basile, 2015; Mericer de Lépinay et al., 2016;
Nemčok et al., 2016). However, quantifying rift obliquity of a specific
rift system through time is more difficult since the syn-rift velocity
evolution needs to be reconstructed from available geophysical and geological
data sets. Therefore, a global statistical analysis of rift obliquity through
geological time has been missing so far.
Here we strive to fill this gap by deducing the first-order obliquity
history of Earth's major rifts from the onset of Pangea fragmentation to
the present day. We first describe the applied methods and data sets, then we
focus on major individual rift systems that lead to the formation of the
Atlantic and Indian Ocean basins before we assess rift obliquity evolution
and average obliquity on a global scale.
Methods and dataRift kinematics
We quantify extension velocity using the global kinematic plate
reconstruction of Müller et al. (2016). This plate model integrates the
latest syn-rift reconstructions for the South Atlantic (Heine et al., 2013),
North Atlantic (Hosseinpour et al., 2013; Barnett-Moore et al., 2016),
Australia-Antarctica separation and India-Antarctica breakup
(Williams et al., 2011; Whittaker et al., 2013; Gibbons et al., 2015), and Gulf
of California opening (McQuarrie and Wernicke, 2005), among others (Fig. 1).
Global overview of rift obliquity and velocity. Rift obliquity is
measured as angle α spanned by the relative plate velocity vector and
the rift trend normal. Rift kinematics are deduced at sample points along the
present-day continental boundaries with a spacing of 50 km. Rift velocity
magnitude is represented by circle size (see scale in the lower right
corner), rift obliquity by circle colour (see colour bar in the middle). The
impact of rift obliquity on the rotation of largest and smallest horizontal
stress components (σHmax and σhmin,
respectively) is depicted at the bottom of the figure and is based on
relations discussed in Brune (2014). Rotations from Müller et al. (2016).
Continent–ocean boundaries from Brune et al. (2016).
Restoration of the relative position of continents prior to rifting in
the aforementioned regional studies is largely based on deriving the amount of
syn-rift extension from present-day crustal thickness (e.g. Williams et al.,
2011; Kneller et al., 2012). The kinematic evolution before breakup, i.e.
prior to the occurrence of oceanic fracture zones and oceanic magnetic
anomalies, has to be inferred via careful joint interpretation of several
geological indicators. Rift initiation for instance can be constrained
through the ages of syn-rift sediments and rift-related volcanism, which give
a minimum age for the beginning of rifting (e.g. Chaboureau et al., 2013;
Quirk et al., 2013). Later syn-rift kinematics can be inferred from seismic
tectono-stratigraphy and dating of rocks that have been drilled or dredged
within the continent–ocean transition, while additional information can be
derived from kinematic indicators at neighbouring plate boundaries (e.g.
Heine et al., 2013; Whittaker et al., 2013).
Rift trend
We define the rift trend as the general direction of a rift segment.
Considering a typical rift width of ∼100 km, the most suitable
wavelength to analyse rift trend variations is several hundred kilometres.
There are several possible proxies for the rift trend of past continental
rift systems: (1) the boundary between continental and oceanic crust, often
referred to as continent–ocean boundary (COB); (2) the general trend of the
present-day coast line or other bathymetric contours; and (3) rift-related
topographic highs (Osmundsen and Redfield, 2011). Note that these proxies do
not necessarily yield the same rift orientation and that all of them feature
certain disadvantages. Mapping the COB for instance requires seismic
refraction data and interpolation between individual profiles, and
considering COBs as sharp boundaries does not reflect the crustal complexity
in these areas, which mirrors the convoluted interplay of tectonic, magmatic,
and sedimentary processes. Coastlines and marginal bathymetry are affected by
eustatic sea-level variations and surface processes, as well as tectonic and
dynamic topography, while rift-related topographic highs do not exist
everywhere and if they do, they are submitted to erosion and post-rift local
vertical motions (Jeanniot and Buiter, 2018). Therefore, we employ a workflow
where we associate rift trend variations with the COB orientation (Fig. 2a).
We use COBs with deliberately simplified geometries designed to capture the
regional geometry of boundaries between continental and oceanic crusts,
without a detailed assimilation of local data reflecting fine-scale
deviations from these regional trends (see description in Brune et al.,
2016). This makes our analysis particularly representative for intermediate
and final rift stages since the COB trend is indicative of the rift
orientation during and after necking of the mantle lithosphere (Le Pourhiet
et al., 2017; Ammann et al., 2017), and allows us to more accurately capture
the time dependence of kinematics within rifts that experienced protracted,
diachronous breakup. Due to the previously mentioned limitation in defining
COBs as single, distinct boundaries, we define two COB endmember sets that
reflect the earliest and latest possible breakup based on available seismic
refraction data (Brune et al., 2016). Figure 2a depicts both the early and
the late breakup COB data sets exemplified for the North Atlantic
illustrating that the overall direction of the rift trend is not affected by
the precise location of the COB.
Methods illustration. (a) Endmember sets of
continent–ocean boundaries (COBs) are based on seismic refraction data, here
exemplified for the North Atlantic. The latest breakup COB constitutes our
reference model while the earliest breakup COB is used for robustness tests.
(b) Kinematic analysis approach exemplified for Australia-Antarctica
rifting.
The employed reconstruction is using rigid plate polygons and does not
directly capture plate boundary deformation. Thus, in a pre-rift
reconstruction, present-day COBs from conjugate passive margins will show
significant overlap (Fig. 1), where the amount of overlap is a proxy for the
subsequent extension before breakup. As the plates move apart, the overlap
decreases, and the moment within the reconstructions where there is no longer
overlap in each segment defines the transition from rifting to seafloor
spreading. Within this methodology, the two central implications of how we
interpret COB geometries for our purposes is that they control the time of
breakup along the margins, and that they define the orientation of each rift
segment.
Rift obliquity
Using the COB orientation as a proxy for the rift trend, and accounting for
the local direction of relative plate motions, we calculate rift obliquity
for all points within an active rift at any time during post-Pangea
extension. This workflow is illustrated in Fig. 2b, where we first keep
Antarctica fixed in order to evaluate the rift obliquity at an Australian COB
location and secondly we fix Australia and estimate rift obliquity for a
point at the Antarctic COB. Note that the conjugate values for local rift
obliquity are very similar but not the same. We therefore average obliquity
values from conjugate margins during our statistical analysis. This analysis
is repeated in 1 million-year intervals until the conjugate COBs do not
overlap anymore and the tectonic formation of the rifted margin ends.
We apply our workflow in an automated way using the python library pyGPlates
that provides script-based access to GPlates functionality. GPlates is a free
plate reconstruction software (http://www.gplates.org/, 23 October 2018) that allows for reconstructing and analysing plate
motions through geological time (Müller et al., 2018). In the following,
we use a spacing of 50 km between individual sample points where we extract
relative plate velocity and obliquity. That spacing is dense enough to
capture the relevant changes in rift trend. We also tested smaller point
distances, which did not affect our conclusions.
The limitations of this analysis workflow coincide with the limits of plate
tectonic reconstructions in general. Many rifts and especially failed rifts
are not included in plate tectonic reconstructions yet, which somewhat biases
our study towards rifted margins. Future improvements in plate tectonic
reconstructions and in defining COBs will enhance our results; however, by
testing several endmember scenarios in Sect. 4 we can already anticipate
that our main conclusions will still hold even though the detailed values
might change.
In this study we follow the convention that defines the angle of obliquity as
the angle between extension direction and local rift trend normal. This means
that 0∘ represents orthogonal rifting while 90∘ stands for
strike-slip motion. Note that this definition follows many previous studies
(e.g. Fournier and Petit, 2007; Philippon et al., 2015; Brune, 2016; Zwaan
and Schreurs, 2017; Ammann et al., 2017), but is opposite to the convention
used in almost as many articles (e.g. Withjack and Jamison, 1986; Tron and
Brun, 1991; Teyssier et al., 1995; Clifton and Schlische, 2001; Deng et al.,
2018).
There is clearly a gradual transition from orthogonal rifting to oblique
extension, especially since individual fault evolution is subject to natural
variability. In this study, however, where we investigate the frequency of
oblique rifting, it appears to be useful to draw a line between orthogonal
and oblique rifting. In simplified model settings, previous studies suggested
that qualitative differences in the rifting style emerge when rift obliquity
exceeds 15 to 20∘ (Clifton et al., 2000; Agostini et al., 2009;
Brune, 2014; Zwaan et al., 2016). Keeping in mind that the specific value is
somewhat arbitrary, we will use 20∘ as the critical obliquity
separating orthogonal from oblique rifts.
Regional analysis of individual rift systems
In this section, our plate tectonic analysis is employed focussing on
individual post-Pangea rift systems (Fig. 1). In doing so, we relate the
structural history of each rift with its obliquity and velocity evolution.
South Atlantic rift
The orientation of the different South Atlantic rift segments has been
affected by reactivation of mobile belts, which formed during the pan-African
orogeny in the Neoproterozoic and early Paleozoic (Kröner and Stern,
2005). This reactivation of inherited weaknesses is an ubiquitous process of
the Wilson cycle that has also been evoked to explain the formation of the
present-day East African rift system (Daly et al., 1989; Hetzel and Strecker,
1994). It has been suggested that 65 % of the South Atlantic rift developed
with near-parallel orientation to the pan-African fabric (de Wit et al.,
2008) evidencing the strong control of tectonic inheritance on the South
Atlantic rift obliquity.
South Atlantic rift. (a) Tectonic reconstruction of
continental polygons. The magnitude of the local relative plate velocity
along the rift system is represented by circle size, rift obliquity by circle
colour. Relative velocities of continents with respect to a fixed south
Africa are depicted as grey vectors. All velocities are based on the global
reconstruction of Müller et al. (2016) and analysed using pyGPlates. Low
rift obliquity prevails in the central and southern segment of the South
Atlantic rift, whereas high obliquities and strike-slip motion dominates the
northern and southernmost segments. EqRS: Equatorial Atlantic Rift System,
SARS: South Atlantic Rift System. (b) Frequency of rift obliquity
in terms of rift axis length. The colour displays the integrated length of
all rift segments that deform at the same rift obliquity.
(c) Cumulative distribution of rift obliquity throughout the entire
rift event. Bar colour represents obliquity. (d) Time-dependent
frequency of rift velocity in terms of rift axis length. Colour shows
integrated length of all segments deforming at the same velocity. Note that
rift velocity and obliquity increase jointly starting at 120 Ma.
We find that low obliquity predominates in the central and southern segment
of the South Atlantic rift (Fig. 3). High rift obliquities are encountered in
the Equatorial Atlantic and in the southernmost shear zone that ultimately
develops into the Falkland-Agulhas fracture zone. Our analysis shows that the
South Atlantic initially features a wide range of rift obliquities (Fig. 3b)
and trends to higher obliquity after 120 million years ago (Ma) when only the
northern and southernmost segments are active. The frequency diagram of rift
obliquity displays a bimodal distribution with the 0–25∘ range
representing the southern South Atlantic while the 45–65∘ range is
dominated by Equatorial South Atlantic rift geometry (Fig. 3c). The overall
mean obliquity at 38∘ lies between these two peaks while 68 %
of the rift formed at moderate to high rift obliquity larger than
20∘.
The rotational rifting of the South Atlantic with an initial Euler pole close
to west Africa leads to higher rift velocities in the south than in the
north. The mean rift velocity of the entire rift system displays an initial,
slow phase lasting more than 20 million years, followed by rift acceleration
during a few million years and finally a fast phase of rifting prior to the
transition to sea-floor spreading (Heine et al., 2013; Brune et al., 2016).
This two-phase evolution can also be seen in alternative reconstructions
(Nürnberg and Müller, 1991; Torsvik et al., 2009; Moulin et al.,
2010; Granot and Dyment, 2015) although it is partitioned between individual
South American blocks for some studies (Brune et al., 2016). Interestingly,
the evolution of rift obliquity and velocity appears to be correlated, which
we will discuss in Sect. 5.
North Atlantic rift
The formation of the North Atlantic involved a protracted rift history
involving several major plates (Eurasia, Greenland, Iberia, North America).
Initial inherited weaknesses from the Caledonian orogeny have been
reactivated in episodic continental extension that is recorded in
Carboniferous to Permian basins (Doré, 1991; Lundin and Doré, 1997).
The Mesozoic is marked by extensive crustal thinning that lead to formation
and abandonment of major rift arms like the Rockall Trough, the Porcupine,
Orphan, Møre, and Faroe-Shetland basins (Skogseid et al., 2000; Faleide et
al., 2008; Peron-Pinvidic et al., 2013). However, Mesozoic rifting also
induced continental breakup in the Iberia-Newfoundland segment, the Bay of
Biscay, and the North Atlantic rift south of Greenland
(Féraud et al., 1996; Tugend et al., 2014; Nirrengarten et al., 2018).
Opening of the Labrador Sea and rifting between Greenland and Europe competed
for many tens of millions of years (Dickie et al., 2011; Hosseinpour et al.,
2013; Barnett-Moore et al., 2016) before the present-day northeast Atlantic
mid-ocean ridge formed in Eocene times (Gernigon et al., 2015; Gaina et al.,
2017) possibly due to the arrival of the Iceland hotspot (Coffin and Eldholm,
1992; Storey et al., 2007). Final separation between Greenland and Europe
took place along the sheared margin of the Fram Strait in Miocene times ∼17–15 Ma (Jakobsson et al., 2007; Knies and Gaina, 2008).
Multiple plate motion changes reflect the complex tectonic history of the
North Atlantic during the last 200 million years. These changes translate to
time-dependent rift obliquity in each of the major rift branches (Fig. 4).
Between 200 and 120 Ma, the rift branches east and south of Greenland deform
at 35 to 60∘ rift obliquity which leads to a pronounced peak within
this obliquity range in Fig. 4b and c. This changes in the Early Cretaceous
with the more northward movement of Greenland and generates almost orthogonal
rifting between Greenland and Europe until breakup. Due to the northward
propagation of sea-floor spreading, the Iberia-Newfoundland rift is excluded
from the later plate motion changes and hence formed during low extension
obliquity. The latest stage of continental rifting is marked by more than
30 million years of high-obliquity shear between northern Greenland and
northwest Europe.
North Atlantic rift. Several changes in plate motion mirror the
complex tectonic history of the North Atlantic during the last 200 million
years. A distinct increase in syn-rift obliquity occurs at 50 Ma prior to
final breakup between northern Greenland and northwest Europe. For
explanation of symbols and diagrams see caption of Fig. 3.
The absolute frequency of rift obliquity is marked by two peaks at 0 and
45∘ (Fig. 4c). While this bears some similarity with the South
Atlantic (Fig. 3c), the underlying reason for these two distinct peaks is not
linked to the different orientation of two rift branches (like in the South
Atlantic), but is a result of Greenland's plate motion change at around
120 Ma (Fig. 4a, b). Except for Iberia-Newfoundland and the Labrador Sea branches,
which evolved at low obliquity, most rift branches experienced moderate or
high rift obliquity during the entire rift history. More than 70 % of the
rifts involved an obliquity of more than 20∘ and the overall mean
obliquity of the North Atlantic rift amounts to 34∘ (Fig. 4c).
Rifting between India and Australo-Antarctica
Mesozoic rifting within eastern Gondwana led to continental fragmentation
beginning with the separation of India (together with Sri Lanka and
Madagascar) from Australia and Antarctica in the Middle-Jurassic to Early
Cretaceous (Powell et al., 1988; Gibbons et al., 2013). The timing and
kinematics of breakup and spreading between Australia and India is well
constrained (Williams et al., 2013; Whittaker et al., 2016) and although the
precise geometry of greater India is obscured by subsequent deformation
during India-Eurasia collision, the divergence between India and Australia is
thought to have involved a significant component of oblique motion along
greater India's northern margin recorded by the Wallaby–Zenith Fracture Zone
(Ali and Aitchison, 2014). Initial breakup between India and Antarctica is
recorded in the Enderby Basin becoming progressively younger to the west
(Gibbons et al., 2013; Davis et al., 2016) and involved significant ridge jumps that isolated the Elan Bank microcontinent
(Borissova et al., 2003).
Rotational rifting between east India and Australo-Antarctica with an Euler
pole close to the southwestern tip of India induces almost orthogonal rifting
between India and Antarctica, and predominantly oblique rifting between India
and west Australia (Fig. 5). After 135 Ma, continued high obliquity shear
along the northern Indian margin and Australia finally culminates in the
formation of the Wallaby–Zenith Fracture Zone, the northern boundary of the
Perth Abyssal Plain. At the same time, breakup propagates from east to west
along the east Indian margin, so that the mean rift obliquity is more and
more dominated by high-angle shearing.
India / Australo-Antarctica rift. Rift obliquity is dominated by two
major rift trends: the low-obliquity India-Antarctica branch and the highly
oblique India-Australia branch. For simplicity, we neglect low-velocity
relative motion between Antarctica and Australia, which will be discussed in
Fig. 6. WZFZ: Wallaby–Zenith Fracture Zone, PAP: Perth Abyssal Plain. For
explanation of symbols and diagrams see caption of Fig. 3.
The rift obliquity distribution in Fig. 5c reflects the existence of two
major rift trends – the low obliquity India-Antarctica branch of 0 to
25∘ and the highly oblique India-Australia branch with more than
45∘. The mean obliquity of the entire rift system amounts to
32∘ while more than 60 % of the rift evolved at obliquity angles
higher than 20∘. Prior and during large parts of the breakup, the
velocity and obliquity of the rift system are positively correlated.
Australia and Antarctica
The first phases of rifting between Australia and Antarctica began in the
Late Jurassic (Powell et al., 1988; Ball et al., 2013) but continental
breakup did not begin until the Late Cretaceous and progressed diachronously
from west to east. Within this rift system, the signatures of oblique
extension have been previously recognized along Australia's southern margin
(Willcox and Stagg, 1990; Norvick and Smith, 2001). Lithospheric breakup was complex
and protracted (Gillard et al., 2015, 2016), leaving unresolved questions
surrounding the nature of the crust in the continent–ocean transition and the
oldest interpreted seafloor spreading magnetic lineations. Reconstructions of
the rift kinematics must therefore incorporate other geological constraints
from along the Australia-Antarctica plate boundary system (Whittaker et al.,
2013; Williams et al., 2018). This reconstruction, in common with earlier studies
(Powell et al., 1988; Royer and Sandwell, 1989), comprises an oblique component of
divergence during the Late Cretaceous prior to a change in plate motion
direction in the early Cenozoic.
The first-order history of rift obliquity can be understood by considering
two distinct conditions. (i) Due to the concave shape of the southern
Australian margin and their Antarctic conjugates, there are two major rift
trends along the Australia-Antarctica rift (Fig. 6). (ii) According to the
plate tectonic reconstruction (Williams et al., 2011), there has been a
significant change in relative plate motion at around 100 Ma from a
northward to a northwestward-directed plate velocity. The two existing rift
trends explain the dichotomy in rift obliquity of 10–25 and 35–45∘
from the onset of rifting until 100 Ma (Fig. 6b). The plate motion change at
100 Ma, however, shifts the rift obliquity in both branches to higher angles
of 20–45 and 80–90∘, respectively.
Australia / Antarctica rift. A distinct change in plate motion takes
place at 100 Ma generating two discrete phases: (1) slow rifting at moderate
obliquity and (2) fast rifting at high obliquity. For explanation of symbols and
diagrams see caption of Fig. 3.
The velocity history displays a prominent increase at 100 Ma that
corresponds to the increased rift obliquity via a plate motion change. An
increase in the rate of plate divergence in the Late Cretaceous is
corroborated by structural restoration studies based on seismic profiles
(Espurt et al., 2012) and the rate of divergence is similar to that
interpreted from initial seafloor spreading anomalies (Tikku and Cande, 1999;
Whittaker et al., 2013). Interestingly, after more than 20 million years of
fast divergence, the relative plate velocity inferred from magnetic anomalies
decreases again. If correct, this decrease cannot be related to
Australia-Antarctica plate boundary dynamics, since at this time the rift
system only consists of the last remaining continental bridge between
Tasmania and Antarctica while the majority of the plate boundary already
transitioned to sea-floor spreading.
A further noteworthy aspect of Australia-Antarctic divergence is the failure
of rifting between Tasmania and the southeast Australian mainland. Extension
in the Bass and Gippsland basins occurred predominantly in the Early
Cretaceous (e.g. Power et al., 2001). In the Late Cretaceous, rifting between
Australia and Antarctica localized between western Tasmania and Cape Adare,
where breakup eventually occurred. The higher obliquity of the successful
plate boundary compared to the failed rift arm in Bass Strait may explain why
this rift was favoured, similar to the successful opening of the Equatorial
Atlantic in favour of a Saharan ocean during South Atlantic formation (Heine
and Brune, 2014).
Gulf of California
The Gulf of California constitutes the youngest rift system in our analysis,
which is why its temporal evolution is known in much greater detail than the
previous examples. The first phase of the Gulf of California rift is
closely linked to the greater Basin and Range extensional zone.
Tectono-stratigraphy and dated rift-related magmatic rocks show that the
onset of continental extension must have occurred before the mid-Miocene
(Ferrari et al., 2013; Duque-Trujillo et al., 2015). This first phase of slow rifting
is marked by a wide rift style characteristic of the present-day Basin and
Range Province.
An increase in both rift velocity and rift obliquity has been
suggested as the underlying reason for basin-ward localization and finally
the transition to sea-floor spreading in the southern Gulf of California
(Bennett and Oskin, 2014; Darin et al., 2016; van Wijk et al., 2017). This change in
plate motion at ∼12 Ma has been explained by the final cessation of
subduction in this region so that most of the relative plate motion between
the Pacific and North America had to be taken up by transform motion between Baja
California and the North American mainland
(Atwater and Stock, 1998; Oskin and Stock, 2003).
The transition from orthogonal to highly oblique rifting is captured by the
tectonic reconstruction (McQuarrie and Wernicke, 2005) our plate model is
built on (Fig. 7a). According to that reconstruction, the transition occurs
during a time frame of less than 10 million years between 20 and 10 Ma
(Umhoefer, 2011). That transition is mirrored in our analysis by a gradual
increase in rift obliquity from less than 10∘ prior to 18 Ma to
20–25∘ between 18 and 12 Ma up to 40–90∘ from 12 Ma until
the present day (Fig. 7b).
Gulf of California rift. The initial rift phase is characterized by
slow, predominantly orthogonal extension and associated with a wide rift. An
increase in rift obliquity enhances localization between 20 and 10 Ma
inducing continental breakup. The obliquity and velocity of this rift
increases almost proportionally. For explanation of symbols and diagrams see
caption of Fig. 3.
A striking feature of the Gulf of California rift is that throughout the
existence of this plate boundary the velocity evolved almost proportional to
the rift obliquity (Fig. 7b, d) hinting at a causal relationship between
these two variables. Observations from the Gulf of California rift
corroborate the idea that localization within the lithosphere is enhanced by
obliquity-related formation of pull-apart basins and associated
energy-efficient strike-slip faults (Bennett and Oskin, 2014; van Wijk et
al., 2017). We discuss the relation of rift obliquity and extension velocity
in more detail in the Discussion (Sect. 5).
Global analysis
In this section we evaluate global rift obliquity since the onset of Pangea
fragmentation in terms of temporal and spatial variability. Analysing all
rift systems of our global plate tectonic model during the last 230 million
years, we test the robustness of our study by additionally considering
the impact on passive margin area and by employing an alternative set of
continent–ocean boundaries.
The extent of major rift systems varied through time (Fig. 8a), with a
pronounced peak between 160 and 110 million years ago when many rifts of the
Atlantic and Indian Ocean were simultaneously active. Figure 8b illustrates
that almost all angles of obliquity are represented at any given time.
Interestingly, obliquities in the range between 70 and 85∘ seem to be
under-represented while almost pure strike-slip systems are an ubiquitous
feature. This finding might be explained by the fact that the transition from
normal faulting to strike-slip faulting in ideal materials occurs at obliquities around
70∘ (Withjack and Jamison, 1986). We therefore speculate that once
major continental strike-slip faults form, the plate boundary adjusts to a
velocity-parallel configuration entailing the formation of a transform margin
(Gerya, 2013; Le Pourhiet et al., 2017; Ammann et al., 2017), which also explains the
relatively high peak at 90∘ obliquity (Fig. 8b, c).
Global analysis of rift obliquity. (a) Variations of major
rift system activity. Note that many rifts of the Atlantic and Indian Ocean
were simultaneously active between 160 and 110 Ma. (b, c) Rift
obliquity in terms of rift length for the reference model employing
late-breakup COBs (see Sect. 2.2 and Figs. 1 and 2 for more details).
(d, e) The reference model analysed in terms of margin formation
rate (i.e. rift segment length multiplied with segment-orthogonal velocity
component). (f, g) An alternative model employing early-breakup
COBs. All three models result in global mean rift obliquities of ∼30∘ and oblique rifting for more than ∼70 % illustrating the
robustness of our results.
In our reference model, we compute a mean global obliquity of 34∘
with a standard deviation of 24∘ since the inception of Pangea
breakup. We also find that 69 % of all rifts deform in oblique rift mode,
i.e. with obliquities exceeding 20∘ (Fig. 8c), illustrating that
oblique rifting appears to be the rule on Earth rather than the exception.
So far, we analysed the frequency of obliquity with respect to rift length,
where transform margins and rifted margins are given the same relative
importance. Now we focus briefly on the surface area that is generated during
passive margin formation, which is also a proxy for the size of sedimentary
basins. We note that it is the rift-orthogonal velocity component which
leads to lithospheric stretching and generates passive margin area, while the
rift-parallel component merely induces an along-strike offset. Hence we
compute how many square kilometres of passive margin are generated per
million year by multiplying the rift length of each small rift segment with
its segment-orthogonal velocity component. The resulting distribution
reflects the relative importance of rift obliquity weighted by passive margin
area, which essentially leads to a shift towards lower obliquity values.
Nevertheless, we find an average obliquity of 29∘ and a total of
67 % of passive margin area affected by obliquity angles larger than
20∘ (Fig. 8d, e).
We test the impact of an alternative set of continent–ocean boundaries
(Fig. 2), which represents the continent-ward endmember and hence stands for
an earlier breakup time (Sect. 2.2). While the total rift length is reduced
with respect to the reference model, we find that the first-order pattern of
obliquity evolution is not affected (Fig. 8f, g). Also, the mean rift
obliquity of 32∘ and the rift length percentage affected by oblique
deformation (70 %) is almost identical to the late-breakup endmember.
Global map of mean rift obliquity. For each rift point we display
the time-averaged rift obliquity illustrating the prevalence of oblique
rifting since Pangea fragmentation. Note that temporal changes in rift
obliquity cannot be visualized in this plot.
Finally, we map the time-averaged obliquity at each rift element (Fig. 9).
The advantage of this approach is that one can easily identify each point's
rift obliquity that dominated the tectonic evolution, however, one has to
keep in mind that changes in rift obliquity are not visualized. Figure 9
shows that only a few rifts exhibit a pure rift-orthogonal extension velocity
such as the Labrador Sea, the east Indian margin and some locations in the
North Atlantic and South Atlantic. Instead, many rifted margins feature moderate rift
obliquity between 20 and 40∘ like the west Iberia margin, the Red
Sea, as well as the central and southern segment of the South Atlantic. To a
large extent, however, the dominant rift obliquity exceeds even 40∘,
for instance in the Gulf of Mexico, the Equatorial South Atlantic, the Gulf
of Aden, the east African margins, the West Antarctic rift, the Tasman Sea
and also the sheared margins of the Fram Strait, Madagascar, Patagonia, and
Western Australia.
Discussion
Previous studies quantified the present-day plate boundary obliquity in
general terms and on long wavelength by considering extensional,
compressional, and transform plate boundary types. Woodcock (1986) noted that
59 % of all present-day plate boundaries feature obliquities larger than 22
degrees. A more detailed recent study found even higher obliquity by showing
that 65 % of present-day plate boundaries exhibit >30∘ obliquity
(Philippon and Corti, 2016). In the latter study, this result was further
decomposed by plate boundary type illustrating that 73 % of rifts and
mid-ocean ridges extend at more than 30∘ obliquity. While these
previous studies did not focus on rift obliquity, they nevertheless are
consistent with our results by highlighting that oblique plate boundary
deformation constitutes the rule and not the exception.
Jeanniot and Buiter (2018) evaluated the margin width of transtensionally
formed rifted margins and thereby estimated the rift obliquity of 26 major
rift segments worldwide. In general, their approach is very similar to ours:
they defined linearized rift segments that follow the trend of
continent–ocean boundaries, additionally accounting for coastlines and
topographic highs, and they estimated the direction of relative plate
divergence using the GPlates graphical user interface. They find a weak
positive correlation between obliquity and width of a rift system; however,
at highly oblique margins this relationship breaks down and these margins are
not only significantly narrower than orthogonal margins but they also
exhibit large-offset transform faults. Our approach differs from Jeanniot and
Buiter (2018) in that our linearized rift segments are considerably smaller
(of the order of several hundred kilometres), that we employ a script-based
approach using the python interface of GPlates and that we include a few more
regions, but most importantly that we explicitly focus on the temporal
evolution of rift obliquity. Nonetheless, for the time-averaged obliquity
(Fig. 9) we find a very good correspondence with their results, which
illustrates the robustness of both approaches with respect to the exact
definition of the rift trend and the linearized rift segment length.
The dynamics of oblique rifting have been previously investigated using
numerical and analogue modelling methods. In these experiments, rift
obliquity is imposed through oblique lateral boundary conditions
(Schreurs and Colletta, 1998; Brune, 2014; Zwaan et al., 2016), boundaries with a velocity
discontinuity (Tron and Brun, 1991; Bonini et al., 1997; Le Calvez and
Vendeville, 2002; Le Pourhiet et al., 2012), an oblique arrangement of weak
elongate zones (van Wijk, 2005; Corti, 2008; Agostini et al., 2009; Ammann et
al., 2017; Brune et al., 2017b; Balázs et al., 2018), or offset weak seeds
(Allken et al., 2012; Gerya, 2013; Le Pourhiet et al., 2017). The majority of these
studies focus on emergent fault patterns and their evolution, and relate the
existence of oblique rifts and rifted margins to tectonic inheritance,
segment linkage, rift propagation, and changes in extension direction. Some
models, however, were designed to investigate the required driving force
during oblique extension (Brune et al., 2012) and the competition between
simultaneously active, neighbouring rift systems (Heine and Brune, 2014).
These models suggest oblique rifting as a mechanically preferred type of
continental extension and we speculate that this could be a reason for the
unexpectedly high percentage of oblique rifts and rifted margins in our
analysis. The underlying cause for preferentially oblique rifting has been
addressed by means of analytical modelling suggesting that plastic yielding
takes place at less tectonic force when the relative plate velocity is
oblique to the rift trend (Brune et al., 2012). This process exerts
additional control on rift strength that is otherwise governed by
thermo-rheological properties, strain localization, and inherited weaknesses
(Buck, 2015; Burov, 2015; Brune, 2018). As a consequence, during rift competition, oblique
rifting should prevail over orthogonal rifting if all other rift parameters
are similar. This appears to be the case for the west African and Equatorial
Atlantic rift systems, which were active at the same time until localization
along the more oblique Equatorial Atlantic rift induced the failure of the
West African rift (Heine and Brune, 2014). A similar example is continental
rifting between Australia and Antarctica, where early extension formed failed
rift basins within the present-day Bass Strait
(e.g. Norvick and Smith, 2001) that competed with the more oblique and eventually
successful rift between Tasmania and Antarctica.
In many cases, we find a correlation between the obliquity and the velocity
of a rift (Figs. 3–7). This result can be understood when considering that
the effective rift-perpendicular velocity that leads to lithospheric thinning
is smaller than in neighbouring, purely orthogonal rift segments (Montési
and Behn, 2007). Therefore, oblique segments need more time to reach breakup.
This is why final continental rupture of highly oblique examples, such as
(1) the Fram Strait opening in the North Atlantic, (2) shearing between the
Patagonian shelf and south Africa, (3) the separation of Tasmania and the
Antarctic Cape Adare region, or (4) rifting between Greater India and west
Australia postdate continental breakup in neighbouring segments by several
tens of millions of years. This means that the average obliquity of an entire
rift system increases after the beginning of breakup because the orthogonal
segments tend to break early and do not contribute to the mean rift obliquity
anymore. Since rifting typically starts slowly and accelerates prior to
breakup (Brune et al., 2016), the long-lived oblique segments often witness
the final, fast stages of rifting. This argument applies to the final rift
stages of the South Atlantic and North Atlantic and the separation of India from
Australia but it does not explain the correlation between obliquity and
velocity prior to breakup of major rift segments, such as seen for
Australia-Antarctica at 100 Ma (Fig. 5) or the Gulf of California at 18 to
5 Ma (Fig. 7). In both situations, a change in the direction of plate
divergence induced higher rift obliquity and simultaneously the rift velocity
increased. This might be explained by the aforementioned argument that oblique
rifting requires less tectonic force, which leads to a higher rift velocity
at constant extensional stress (Brune et al., 2012). We suggest that the
change in extension direction sparked a significant loss in rift strength
(Brune et al., 2016), which ultimately generated a speed-up of Baja
California relative to North America and of Australia relative to Antarctica.
Oblique rifting is closely linked to toroidal plate motion, i.e. the spin of
plates and associated strike-slip deformation. The concept of decomposing
Earth's plate motions into toroidal (plate boundary parallel) and poloidal
(plate boundary perpendicular) components is motivated by the insight that
toroidal motion does not affect the buoyancy configuration of Earth's mantle.
It is hence not directly driven by mantle convection and in a homogeneous
plate-mantle system, energy consumption due to toroidal motion should
therefore be minimized (O'Connell et al., 1991). Large lateral rheological
contrasts within and between Earth's plates have been invoked to explain some
part of the toroidal motion component (Becker, 2006; Rolf et al., 2017).
However, an additional part of the toroidal plate motion might be due to rift
obliquity: since oblique rifting reduces rift strength, it favours the
development of oblique plate boundaries, which enhances large-scale plate
rotation and associated toroidal surface velocity components.
Lithgow-Bertelloni and Richards (1993) showed that the toroidal component of
plate motions slowly declined since 120 Ma, despite large variations in the
poloidal component of plate velocities. Notwithstanding significant
uncertainty, the ratio of toroidal to poloidal velocities appears to be
especially high between 120 and ∼80 Ma. This period corresponds to a
distinct decline in the lengths of rifts involved in Pangea breakup (Fig. 8,
and Brune et al., 2017c). Hence we speculate that the toroidal to poloidal ratio
could be higher during continental breakup because of the rift characteristic
to favour oblique motion, a process that has less impact on plate motions
once the continents become dispersed.
Conclusions
In this study we evaluated the rift obliquity of major continent-scale rift
systems by analysing a global plate tectonic reconstruction from the onset of
Pangea fragmentation to the present day. We find that the classical 2-D
assumption where the extension direction is perpendicular to the rift trend
is not justified in most cases. Instead, the majority of rift systems leading
to continental breakup during the last 230 million years involved moderate to
high rift obliquity. Approximately 70 % of all rift segments involved a
distinct obliquity higher than 20∘, while the global average in terms
of rift obliquity is 34∘. This high contribution of oblique
deformation can be explained through the generally irregular shape of plate
boundaries, possibly related to tectonic inheritance, and by the concept of
obliquity-dependent plate boundary strength. Oblique deformation generates
intrinsically 3-D stress and strain fields that hamper simplified tectonic
interpretation via 2-D cross sections, models and seismic profiles. Our
results indicate that oblique plate boundary deformation should be considered
the rule and not the exception when investigating the dynamics of rifts and
rifted margins.
The data used in this study are accessible as supplementary material to previous publications:
for the rotation file we refer to Müller et al. (2016)
and for continent–ocean boundaries we refer to Brune et al. (2016).
SB and SEW conceived the plate tectonic analysis and implemented the workflow.
SB, SEW, and RDM discussed and integrated the results. The paper was written by SB with contributions
from all authors.
The authors declare that they have no conflict of
interest.
Acknowledgements
This research has been funded by the German Academic Exchange Service (DAAD),
project no. 57319603. Sascha Brune was supported through the Helmholtz Young
Investigators Group CRYSTALS (VH-NG-1132). Simon E. Willliams and R. Dietmar
Müller were supported by Australian Research Council grant IH130200012. We
thank two anonymous reviewers and editor Federico Rossetti for their
constructive and motivating comments that significantly helped to improve
this manuscript.The article processing charges
for this open-access publication were covered by a Research
Centre of the Helmholtz Association.
Edited by: Federico Rossetti
Reviewed by: two anonymous referees
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