Earth and Planetary Science Letters 236 (2005) 505 – 523
www.elsevier.com/locate/epsl
Lithosphere tearing at STEP faults: Response to edges of
subduction zones
R. Govers*, M.J.R. Wortel1
Department of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, Netherlands
Received 29 September 2004; received in revised form 21 March 2005; accepted 30 March 2005
Available online 20 June 2005
Editor: S. King
Abstract
Slab edges are a relatively common feature in plate tectonics. Two prominent examples are the northern end of the Tonga
subduction zone and the southern end of the New Hebrides subduction zone. Near such horizontal terminations of subduction
trenches, ongoing tearing of oceanic lithosphere is a geometric consequence. We refer to such kinks in the plate boundary as a
Subduction-Transform Edge Propagator, or STEP. Other STEPs are the north and south ends of the Lesser Antilles trench, the
north end of the South Sandwich trench, the south end of the Vrancea trench, and both ends of the Calabria trench. Volcanism
near STEPs is distinct from typical arc volcanism. In some cases, slab edges appear to coincide with mantle plumes. Using 3D
mechanical models, we establish that STEP faults are stable plate tectonic features in most circumstances. In the (probably rare)
cases that the resistance to fault propagation is high, slab break-off will occur. Relative motion along the transform segment of
the plate boundary often is non-uniform, and the STEP is not a transform plate boundary in the (rigid) plate tectonics sense of
the phrase. STEP propagation may result in substantial deformation, rotation, topography and sedimentary basins, with a very
specific time-space evolution. Surface velocities are substantially affected by nearby STEPs.
D 2005 Elsevier B.V. All rights reserved.
Keywords: geodynamics; Tonga trench; New Hebrides trench; Mediterranean region; Lesser Antilles trench; South Sandwich trench
1. Introduction
Near the majority of horizontal terminations of
subduction trenches, continual tearing of the litho* Corresponding author. Tel.: +31 302 534 985; fax: +31 302 535
030.
E-mail addresses: [email protected] (R. Govers),
[email protected] (M.J.R. Wortel).
1
Tel.: +31 302 535 074; fax: +31 302 535 030.
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.03.022
sphere is a geometric consequence [1]. With time,
the intersection between the subduction fault and the
transform segment propagates through the lithosphere,
thereby effectively increasing the area of the transform-like fault (Fig. 1). We refer to the tearing transform fragment as a Subduction-Transform Edge
Propagator (STEP) fault.
Following the identification of current plate boundaries, plate-tearing configurations were hypothesized
(bscissors type faultingQ [2], bhinge faultingQ [3,4]).
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R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
oceanic lithosphere
(a)
STEP
STEP
(b)
STEP
STEP
(c)
Fig. 1. Map view of a schematic evolution of a Subduction-Transform Edge Propagator (STEP). To highlight the characteristics of STEPs, we
assume that no east–west motion occurs between left and right boundaries of the region (rollers). (a) Subduction of the oceanic lithosphere
beneath the overriding plate is facilitated by trench rollback and back-arc extension (light grey area). Lithospheric tearing occurs along the
dashed line. Vectors represent relative velocities. (b) Later stage of development, where the STEP has propagated to the east. Back-arc extension
is assumed to have migrated east along with the trench. Principal lithospheric strain rates are indicated by arrows. Non-zero block rotation rates
occur along the STEP fault segment from the trench to the west end of the back-arc extension zone. (c) Alternative, where relative motion
between the overriding and subducting plate is possible at the left boundary. No in-plate deformation is necessary, except at plate boundaries.
The STEP fault acts like regular transform plate boundary.
Tearing has been occurring in the following regions in
the last couple of million years (Fig. 2); the northern
termination of the Tonga trench, the southern end of
the New Hebrides trench, the northern and southern
ends of the Lesser Antilles trench, the northern end of
the South Sandwich trench, the southern end of the
Hikurangi trench (North Island, New Zealand), the
southern end of the Vrancea trench (Carpathians), on
both ends of the Calabria trench (Sicily), both ends of
the Hellenic trench, and possibly the Gibraltar arc.
This list is not complete in that it does not include
collision examples.
From their inventory of seismicity in STEP
regions, Bilich et al. [5] find a bconspicuous
scarcityQ of strike-slip earthquakes. This finding is
consistent with the nature of STEP faults, as
explained next (Fig. 1a–b). A STEP fault allows subduction to continue. The amount of transform motion
is not necessarily uniform along the STEP fault. To
illustrate this point, consider a tectonic setting like the
Central Mediterranean, where relative motion between
continental Europe and Africa has nearly come to a
halt and subduction of the Ionian slab is accompanied
by slab roll-back. In such bland-locked basinsQ [6], the
overriding plate shows back-arc extension in response
to the movement of the trench. Relative horizontal
motion across the STEP fault only occurs up to the
back-arc domain. STEP propagation acts like a wave
traveling through the landscape; at any location along
the STEP path, strains and rotations build up until it
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
Fig. 2. Current STEP regions. Lines schematically indicate active plate boundary zones. Rotation symbols show the regionally observed sense of motion on transform faults, or
paleomagnetic rotations about a vertical axis.
507
508
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
has passed by. Accurate timing of deformation and
paleomagnetic rotation is therefore crucial to detect
STEP propagation geologically. In the alternative
case that rigid motion of the overriding plate is possible (Fig. 1c), strike-slip motion will occur across the
STEP fault that in this case behaves more like a
classical transform plate boundary. Whether the overriding plate can rigidly trail the trench during rollback,
or even actively overrides the subducting plate without
deforming internally, depends on boundary conditions
(left side of Fig. 1a–c) and internal strength.
Schellart et al. [7] use analogue models to study the
geological consequences of trench retreat for the North
Fiji back-arc basin. Implicit in their experimental approach is the assumption that the STEP fault at the
southern end of the New Hebrides trench propagates
easily. Prescribed saloon door kinematics on the edge
of the basin are shown to reproduce the observed
structural development within the overriding plate.
Return flow around slab edges is addressed in some
recent model studies. Kincaid and Griffiths [8] show
that return flow depends on velocities of subduction,
slab-steepening, and rollback. Vice versa, the kinematics of subduction are significantly affected by the presence of a slab edge, resulting in a geometry of edgebounded subduction zones that differs from the typical
concave shape towards the overriding plate [9–11].
We first review some of the main characteristics of
existing STEP regions. The inferred range of plate
tectonic settings in which STEPs occur, inspires endmember dynamic models of STEP regions. These
models are intentionally simple, and without a focus
on any specific region. We will show that STEP faults
are stable in that, once a STEP geometry exists, it will
grow upon itself except in relatively extreme cases.
Another outcome of the models is that very particular
patterns of surface deformation result from tear propagation, which may be geodetically and geologically
detectable. Finally, we predict some first order geological imprints from the models.
2. Characteristics of STEP regions
2.1. North Fiji Basin (Fig. 2a)
Following the classical work by Isacks et al. [2,3],
Millen and Hamburger [12] show that seismicity and
focal mechanisms are indicative of progressive downwarping and tearing of the Pacific plate as it enters the
northernmost segment of the Tonga subduction zone.
Dip-slip faulting along shallow (18–57 km) near-vertical planes that are oriented parallel to the slab edge is
inferred from large (5.6–7.5 mb) earthquakes. Sinistral strike-slip activity on the STEP fault tapers towards the northwestern most end of active back-arc
extension [13]. Montelli et al. [14] image a deeply
rooted plume at the northern end of the Tonga subduction zone. Lavas from the northern Tonga islands
are interpreted to contain geochemical imprints of the
Samoa plume by Wendt et al. [15]. Smith et al. [16]
conclude that seismic anisotropy and He isotopes
from the Samoa hotspot are indicative of return flow
around the slab edge.
The New Hebrides Trench marks the eastward
subduction of the Australia plate beneath the North
Fiji basin. Near the southern end of the trench we
expect a STEP fault. No shallow (b50 km depth)
tear faulting mechanisms were recorded in the period 1977–2002 in this region, so it is more difficult to identify the exact location of the active
STEP fault. The Hunter fracture zone likely represents the inactive portion of the STEP fault. Consistent with the STEP fault behavior discussed in
Fig. 1, (sinistral strike-slip) seismic moment release
dwindles to zero along the STEP fault towards the
ENE [17].
2.2. Mediterranean subduction zones (Fig. 2b)
The Mediterranean region currently hosts multiple
STEPs. Paleogeographic reconstructions show that the
Calabria trench has been retreating ESE at a rate of 30
mm/yr during the last few million years (e.g., [18]).
The southern edge of the Ionian slab is currently
imaged near Sicily [19], where Carminati et al. [20]
infer a STEP-like plate boundary. In Fig. 2b, the line
north of Sicily schematically indicates the 100 km
wide dextral shear zone between the Ustica-Eolie
line in the Tyrrhenian basin, and the Kumeta-Alcantara line in N-Sicily [21]. Clockwise paleomagnetic
rotation observations on Sicily [22,23] are consistent
with dextral strike-slip along the STEP fault. The
current southern edge of the Calabria trench appears
to be propagating along a Mesozoic weakness zone,
the Malta Escarpment [24,25]. No M N 4 tearing
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
events have been recorded in this region. Seismic
strain is dominated by the convergence between
Africa and Europe in a direction that is approximately
perpendicular to the direction of trench retreat [26].
GPS velocities in the Central Mediterranean suggest
that retreat of the Ionian trench may currently have
halted [27]. This may be explained by detachment of
the Ionian slab as was suggested by Wortel and Spakman [28] on the basis of observed rapid uplift of
Calabria. However, regional uplift may also be influenced by the plume that was imaged below Sicily by
Montelli et al. [14]. The STEP is located close to
Mount Etna, Aeolian and Tyrrhenian volcanoes. The
geodynamic cause for their peculiar magma chemistry
evolution has been the topic of debate in the last
couple of years (plume [29], plate detachment [30],
lithosphere delamination [31], return flow [32]).
Civello and Margheriti [33] conclude that return
flow causes shear wave splitting in the south-central
Mediterranean.
The northern edge of the Ionian slab [19] may
require a STEP fault through the Lucanian Apennines.
The line in Fig. 2b indicating this fault zone is poorly
constrained. Fast (~258/0.5 My), recent, counterclockwise paleomagnetic rotations about a vertical
axis in southern Italy are consistent with such STEP
fault zone [24,34–37]. Catalano et al. [38] show that
sinistral strike-slip along NW–SE faults in the southern Apennines was principal the mode of deformation
during middle (0.8–0.2 Ma) and late Pleistocene (0.8–
0 Ma). Support for a STEP fault zone from seismicity
is lacking.
The Hellenic trench is also laterally bounded by
STEP regions. The Kephallinia fault zone in the west
should probably be considered as a STEP. In the
east, a STEP fault is located somewhere between
the eastern Aegean region near Rhodes [39] and
the Isparta angle [40]. The strike-slip character of
this boundary is evident from focal mechanisms. At
depth, the eastern edge of the Hellenic slab was
interpreted as a bvertical ruptureQ by de Boorder et
al. [41].
The narrow Vrancea slab is currently constrained
in the south by STEP faulting near the Intramoesian
Fault and in the north by a STEP near the Trotus
fault (e.g., [42]). Alternating periods of migration of
slab detachment along the Carpathians [28] and
STEP tectonics may have resulted in multiple gen-
509
erations of STEP faults along the north end of the
slab.
The Betic-Rif mountain belt has been proposed to
have resulted from westward subduction of a ~200
km wide slab [43,44]. Important building stones of
this scenario are paleomagnetic rotations on either
side of the Gibraltar arc, which are interpreted to
reflect STEP fault activity at lateral slab edges. Here
too, no plate tearing events have yet been identified.
2.3. Eastern Caribbean plate (Fig. 2c)
VanDecar et al. [45] tomographically image the
aseismic edge of the Lesser Antilles slab beneath
continental South America, showing that the Lesser
Antilles subduction zone is distinct from the Caribbean subduction zone further to the west. In the absence
of tear faulting seismicity in this region, we speculate
that the STEP fault zone approximately follows the
continental margin. Carlos and Audemard [46] argue
that the cumulative (geological) slip on major faults in
northern Venezuela is substantially less than what
would be expected on the basis of large scale reconstructions. Similarly, seismic moment release in the
southeast Caribbean is limited to the active STEP
region [47].
The edge at the north end of the Lesser Antilles
trench is geometrically complicated. Here, the existence of a microplate (Gonave, Hispaniola, and Puerto
Rico (GHPR) blocks) along the northern STEP
fault is a major complication that requires further
study.
2.4. Sulawesi, Indonesia (Fig. 2d)
The western end of the North Sulawesi trench
marks the edge of the slab [48]. Sinistral strike-slip
on the Palu fault on Sulawesi [49,50] is consistent
with this being a very active STEP fault.
2.5. Scotia-Sandwich plates (Fig. 2e)
Forsyth [4] was the first to examine focal
mechanisms along the Scotia and Sandwich plates.
The E–W trending fault at the north end of the South
Sandwich Trench qualifies as a STEP fault which
accommodates eastward rollback of the westward
dipping South America plate. The plate boundary
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R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
geometry along the southern Sandwich plate is not
well constrained; from the southernmost East Scotia
spreading ridge to the southern end of the South
Sandwich Trench the plate boundary does not have
a clear bathymetric expression and seismicity is
limited. It is however likely that this also is a
STEP fault. Beyond the STEP further to the east,
the South America and Antarctica plates are direct
neighbors along an east–west oriented transform
plate boundary (bSouth America–AntarcticaQ (SAA) ridge). This has the consequence that STEP
fault lengthening here occurs along a pre-existing
plate boundary.
Motion of the Scotia and Sandwich plates relative
to the South America and Antarctica plates is constrained by Thomas et al. [51]. Rates of transform
motion along the northern and southern boundaries of
the Scotia plate are on the order of 10% of the
convergence rate at the South Sandwich Trench.
This is consistent with the STEP fault model we
propose.
2.6. Summary
Observations of STEP characteristics vary by the
region. Where available, focal mechanisms at STEPs
are consistent with tear propagation. Seismicity rates,
geological offsets and paleomagnetic rotations yield
support to the notion that STEP faults are dissimilar
from transform plate boundaries. Volcanism near
STEPs is distinct from typical arc volcanism. In
some cases, slab edges appear to coincide with mantle
plumes.
STEPS occur in plate tectonic settings ranging
from the landlocked (Mediterranean) situation in
Fig. 1a and 1b to the open subduction (Caribbean)
setting of Fig. 1c. We next focus on first order deformation/kinematic expressions of STEPs in these endmember cases. The distinct volcanism and the relation
with plumes is not addressed here.
3. STEP model setup
We use a finite element (FE) method to solve the
mechanical equilibrium equation for three-dimensional displacements. The code was developed from TECTON version 1.3 (1989) [52,53]. Constitutive
equations are based on (compressible) elastic and
non-linear, incompressible viscous flow;
ėe xx ¼
1
r yy þ ṙ
r zz
r xx m ṙ
ṙ
E
þ
ėe yy ¼
1
r yy mðṙ
ṙ
r xx þ ṙ
r zz Þ
E
þ
ėe zz ¼
ðrE =g eff Þn1 2rxx ryy rzz
6g eff
ðrE =g eff Þn1 2ryy rxx rzz
6g eff
1
r zz m ṙ
ṙ
r xx þ ṙ
r yy
E
þ
ðrE =geff Þn1 2rzz rxx ryy
6geff
ėe xy ¼
1þm
ðrE =g eff Þn1
r xy þ
ṙ
rxy
E
2g eff
ėe xz ¼
1þm
ðrE =g eff Þn1
r xz þ
ṙ
rxz
E
2g eff
ėe yz ¼
1þm
ðrE =g eff Þn1
ryz
r yz þ
ṙ
E
2g eff
where e] ij are components of the strain rate tensor, a
dot indicates differentiation with respect to time, E is
Young’s modulus, m is Poisson’s ratio, r ij are components of the Cauchy stress tensor, g eff is the effective
viscosity, and r E is the effective stress, defined as
rE u
ffi
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1=3 r2xx þ r2yy þ r2zz rxx ryy rxx rzz ryy rzz þ r2xy þ r2xz þ r2yz
This formulation avoids problems related to maintaining incompressibility. We account for geometric
non-linearity due to large deformation through the
formalism of McMeeking and Rice [54]. Simplex
elements are used to derive the FE equations. Matrix
equations are solved using PETSc ([55], http://
www.mcs.anl.gov/petsc), a suite of data structures
and routines for the scalable solution of partial differ-
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
ential equations. We adopt the conjugate gradient
implementation of the Krylov subspace method and
Eisenstat preconditioning (a good introduction is
given by Freund et al. [56]) to solve the algebraic
equations iteratively.
As it is our goal to establish the first order response
to subduction in STEP regions, our model geometry is
a stylized form of the STEP regions described above.
The model rheology is linear visco-elastic; highly
viscous in the lithosphere, and low viscosity in the
asthenosphere. Horizontal dimensions of the mechanical model are 1100 750 km, which is substantially
larger than the elastic flexural parameter of the involved lithospheres (Fig. 3). Continental and oceanic
lithospheres are assumed to have 100 km and 80 km
thicknesses (L), respectively. In the models presented
here, the flexural rigidity of the lithosphere is uniform
(5.5 1024 Nm). The bottom of the model lies at 200
km or alternatively at 300 depth. The top of the curved
subducting slab is defined by an error function
pffiffiffi
xe R p 2
tan #erf
z ð xÞ ¼
e
2Rtan #
where z represents depth, x is horizontal distance
perpendicular to the trench, R is the radius of curvature, and # is the (deep) subduction angle. This representation has the benefit of a smooth variation of
Fig. 3. View through the 3D model geometry. The grey plates
labeled Africa and Ionian Sea (referring to the south-central Mediterranean plate tectonic setting) are part of a single Africa plate, part
of which is subducting. The transparent box outlines the overriding
plate of the Ionian subduction zone. The asthenospheric part of the
model is indicated by the sub-lithospheric transparent box.
511
curvature, so that (strain inducing) displacement discontinuities are avoided. Becker et al. [57] show that
the radius of curvature is of the order of the thickness
of the lithosphere at the trench. In the present study,
we assume R = 1.60L and # = 458. We modified the
slippery node technique [58] to allow for freely
deforming faults while minimizing overlaps or gaps.
Slippery nodes are used to represent the subduction
fault that extends throughout the assumed lithospheric
thickness of 80 km. The subduction fault is assumed
to be well-developed and therefore frictionless, consistent with inferences by Zhong et al. [59]. Similarly,
the STEP fault consists of slippery nodes and, for
simplicity, is assumed to be a vertical lithosphere
cutting feature. The influence of friction on the
STEP fault will be investigated.
We adopt a Maxwell rheology with uniform elastic
parameters (E = 1011 Pa, m = 0.25). Linear viscosity g eff
is selected to be 1024 Pa s in the lithosphere and 2–4
orders of magnitude lower in the asthenosphere,
depending on the model. We will show model evolution as function of multiples of the asthenospheric
Maxwell time s M, which is the shortest characteristic
response time in the models. As we restrict the total
integration time to a few hundred asthenospheric
Maxwell times, modeling results can be viewed as
the near-instantaneous response to body forces and
boundary conditions (BCs).
Flow in the model asthenosphere is assumed to
result from forcing within the model only, i.e., we do
not consider the effect of larger scale convective
motions. Lithosphere density is taken to be 3100
kg/m3, asthenosphere density 3050 kg/m3. Deviatoric
stresses resulting from gravity loading are assumed
to have relaxed so that gravity pre-stresses are hydrostatic, except in the slab. Pre-stresses are therefore
initialized with a 1-D layered density field not contributing to the initial forcing, and an anomalous
density field that excites sinking of the slab. This
approach has the benefit of including the full gravity
field while not introducing flow due to self-gravitation. In some of the models we assume that the slab
beyond the model domain exerts an explicit downpulling force, i.e., we assume that slab pull is not
locally compensated by shear on a slab segment or
by interaction with deeper phase boundaries. We use
Winkler normal pressure BCs along the bottom of
the model, causing a pressure increase of qgz for a
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R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
downward deflection of the interface by z meters, or
a decrease in case of an upward deflection.
Viscous resistance to horizontal flow along the
model bottom boundary is applied through Maxwell
BCs; this is a combination of a classical Winkler
spring element and a dashpot, in series [60]. Here,
the spring constant is proportional to the elastic shear
modulus, and the viscosity is equal to that of the
overlying asthenosphere. An initial Winkler force
consequently decays at a rate that is controlled by
the dashpot viscosity. Boundary conditions on vertical model boundaries vary, as discussed below. In
most of the models, Maxwell BCs act in a direction
perpendicular to the asthenospheric portion of these
boundaries.
We use GMSH version 1.53.2 [61], which is primarily based on a Delaunay algorithm, to generate a
three-dimensional tessellation by tetrahedral elements.
For visualization of the FE results, we use Data
Explorer (http://www.opendx.org).
3.1. STEP1: subduction model
We consider the instantaneous response of the
mechanical model to body forces and boundary conditions in order to predict the location and direction of
STEP propagation. Fault propagation itself is not
explicitly included in the dynamic model. Fig. 4
shows the resulting velocity field, normalized by the
highest velocity magnitude anywhere in the model. As
velocities scale linearly with the assumed asthenosphere viscosity and density anomaly, this is an efficient display of results for a variety of densities and
asthenosphere viscosities. The model result was computed for lithospheric lateral BCs that are fixed horizontally, except for the overriding model lithosphere
to allow for subduction (Fig. 1c). Perpendicular asthenospheric Maxwell BCs have a characteristic
decay time that is much larger than the Maxwell
time of the asthenosphere; effectively, lateral asthenosphere BCs simulate the compressible asthenosphere
beyond the model domain in this case (cf. [62]).
Sinking of the slab (particularly visible in top
and bottom views of the vertical velocity v z ) causes
slip along the subduction fault. At the trench, an
eastward pressure gradient from lateral density variations results in a suction force that drives the
overriding plate towards the trench. The top view
of horizontal trench-perpendicular motions (v x )
shows the overriding plate moving towards the
trench. Resistance to continued subduction near the
STEP is clear from the increase in subduction velocity with distance away from the STEP. Continuity
of normal stresses across the subduction fault result
in down-pulling of the overriding plate near the
trench. Subsidence of the lithosphere south of the
STEP fault, near the trench, is caused by viscous
coupling to the sinking slab. Uplift of the top
surface results from flexural bulging (elastic wavelength ~ 160 km) and asthenospheric flow. The realistic thickness of the model lithospheres has the
benefit of yielding equally realistic dimensions of
their contact surfaces. A consequence of the simple
rheology of the lithospheres is that their effective
elastic thickness is unrealistic, and we consequently
do not believe that either the horizontal wavelength
or the rates of predicted uplift due to flexure apply
to the Earth. However, the modeled phenomenon
that N–S flexural bulging of the subducting lithosphere also produces uplift of the non-subducting
lithosphere in the south is probably realistic. The
restricted mass flux through vertical asthenospheric
boundaries causes uplift of the lithosphere following
subduction; alternative models with more open lateral boundaries show decreased uplift rates and
higher subsidence rates. Non-zero friction on the
STEP fault smoothes the velocity discontinuity
across the fault.
In the asthenosphere, sinking of the slab results in
high and low pressure regions beneath and above the
slab, respectively. The bottom view of v y shows the
resulting slab-parallel horizontal flow from beneath
the slab towards its edge, and above the slab towards
the wedge. Experiments show that flow rate in the
asthenosphere decreases with increasing compressibility. Return flow is suppressed in cases where more
flow is allowed through lateral model boundaries.
Fig. 5 shows velocities at the model surface and
the predicted change in topography along a profile
south of and parallel to the STEP fault, assuming
that the STEP propagates at constant rate of 9 cm/
yr. Fig. 6 shows total effective strain rates which
maximize near the STEP end. Both figures illustrate
that relatively complicated surface deformation
fields may be expected in a region within about a
100 km distance of the STEP.
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
513
Fig. 4. Normalized velocities of bsubduction modelQ STEP1 (10s; j Y
m jmax ¼ 1 cm/yr), and finite element grid (light grey). North arrow is shown to facilitate model description.
Velocity magnitude, (a) top view and (b) bottom view. This and following panels; breakdown into individual components of the 3D velocity field at model boundaries. East
(positive)–west (negative) velocity component v x , (c) top view and (d) bottom view. In these and the following panels, black lines show zero velocity contours. South (positive)–north
(negative) velocity component v y, (e) top view and (f) bottom view. Up (positive)–down (negative) velocity component v z , (g) top view and (h) bottom view.
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
Fig. 4 (continued).
514
Topography (m)
1500
vz (mm/yr)
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
0.4
515
1000
500
0
0.2
-0.0
-0.2
-0.4
-1.05
1.05mm/yr
700
600
km
500
400
300
200
100
10 τ
η=2.1021
0
600
400
200
0
-200
-400
km
Fig. 5. Bottom panel; velocities of STEP1 model surface at integration time = 10 asthenospheric Maxwell times. Horizontal velocities are shown
as arrows, vertical velocity as color contours. The largest horizontal velocity, in the overriding plate, is 1 cm/yr. Surface intersections of the
subduction fault and STEP fault are shown as red lines. Middle panel; vertical velocities along the horizontal (black) profile line in the bottom
panel. Top panel; topography along the profile line resulting from STEP propagation (9 cm/yr) up to its location in the bottom panel.
Coulomb stresses can be used to predict the location and orientation of prospective faults. Fig. 7
shows Coulomb stresses in model STEP1 (friction
coefficient of 0.6). Stress magnitudes increase with
model time; the result shown is after 50 Maxwell
times. The location and timing of actual failure will
depend on the actual strength of the lithosphere,
which is infinite in our elastic model lithosphere.
Based on the modeled Coulomb stresses, STEP
propagation in a direction parallel to the assumed
STEP fault orientation is most likely. This mode is
similar to tear propagation in a tearing sheet of
paper. The results show that, alternatively, the slab
can break off along an inclined plane cutting the
slab. This will occur when the resistance to STEP
propagation/failure is larger than the resistance to
slab break-off. In reality, the total strength of the
(oceanic) subducting slab is most often equal to or
higher than the strength of the surface plate, especially when the surface plate involves continental
crust [63]. Consequently, we consider slab breakoff substantially less likely than incessant propagation of the STEP.
We investigated the sensitivity of the above
model response to variations in model geometry,
mesh density, forcing, boundary conditions and
visco-elastic material properties. The results of
STEP1 are found to be quite representative for
516
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0.37
45.46 nstrain/yr
700
600
km
500
400
300
200
100
10 τ
0
600
400
200
0
-200
-400
km
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Fig. 6. Effective total (i.e., sum of elastic and viscous) strain rates ėe E ¼ 1=2ėe Vij ėe Vij along the STEP1 model surface. The inset panel shows
principal horizontal strain rates in the boxed sub-region of the contour figure. Near the STEP, strain rates maximize and show a fanning pattern.
Coulomb stress (MPa)
90
80
70
60
50
ion
STEP propagat
Fig. 7. Coulomb stresses larger than 50 MPa in the STEP1 model lithospheres (50s). Coulomb stresses maximize along the STEP fault edge.
The expected orientation of fault propagation is indicated by the densely hatched vertical fault plane. Coulomb stresses are also significant along
a plane cutting the subducted lithosphere. Whether the STEP will propagate or slab break-off will occur, depends on the relative strength of the
subducted oceanic lithosphere and the lithosphere that needs to break for STEP propagation.
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
the investigated first-order models. One exception is
the (lack of boundary conditions which allow) free
horizontal motion of the overriding plate. This will
be addressed in a following model.
3.2. STEP2: land-locked basin model
The second model we consider is inspired by, but
not aimed to reproduce, the tectonics of the southcentral Mediterranean around Sicily. Importantly, and
possibly distinct from the situation beneath southern
Italy, we assume that the slab is continuous to predict
the response to density sinking of the slab. Moreover,
boundary conditions in the model do not allow east–
west motion of the westernmost overriding lithospheric boundary, which is more extreme than the southcentral Mediterranean, where there is little relative
motion between points on the 108 and 208 meridians,
but where intermediate back arc extension facilitates
subduction of the Ionian slab. Similar horizontal
stretching of our model overriding plate is not possible. Our extreme choice of locked boundary conditions (similar to Fig. 1a and b), is motivated by our
objective to investigate the entire spectrum of STEPdominated responses. Fig. 8 is a display of model
velocities similar to that of Fig. 4. The inability of
the overriding plate to move substantially is the most
striking difference. Instead, the velocity field is dominated by vertical sinking of the slab and overriding
plate-without significant slip along the (frictionless)
subduction fault. This illustrates that, in a land-locked
basin situation, subduction is only possible if simultaneous back-arc extension occurs. This implies that
there is a critical subduction velocity below which
subduction stops entirely; at lower speeds, the rate of
conductive cooling of the extending lithosphere is
higher than the rate of advective heating, so that the
overriding plate strengthens with time. In the model,
relative motion along the STEP fault is also negligible. Coulomb stresses near the STEP are accordingly
small, and slab break-off is predicted. Flow velocities
in the asthenosphere are higher than lithosphere velocities, and the flow pattern is different in that flow
towards the subduction wedge does not occur.
Fig. 9 shows the model surface velocity field (cf.
Fig. 5). The fanning pattern of horizontal velocities
closely follows the pattern of subsidence and uplift.
The predicted topography along the profile is subdued
517
relative to Fig. 5; the vertical velocity wave of the
middle panel that migrates with time to the east (left)
through the landscape produces topography in front of
the STEP, and subsidence its wake.
Surface strain rates are displayed in Fig. 10 (cf.
Fig. 6). Most important is the different pattern; due to
the lack of fault slip, maximum strain rates at the
surface do not occur near the STEP. Principal strain
rate directions follow the fanning pattern defined by
vertical velocity gradients.
4. Discussion
Once established, STEPs are stable features in
that they continue to propagate as long as the lithospheric strength is less than or equal to the slab
strength. As subduction results from instability of
the cold and strong boundary layer, STEPs will
mostly be stable as seems to be evident from the
length of STEP faults in Fig. 2. However, subduction
of progressively younger lithosphere may result in a
combination of substantial slab pull and a weak
shallow slab causing slab break-off rather than continued STEP propagation.
STEPs propagate in a direction opposite to the
subduction direction in lithosphere with laterally uniform horizontal properties. Horizontal variations in
strength in front of the STEP, most prominently passive
margins, may focus and redirect the direction of STEP
propagation. We expect that STEPs mostly will follow
passive margins. As passive margins have generally
rugged shapes in map view, the trench will lengthen
and shorten at the STEP accordingly (e.g., the southern
ends of the Calabria and Lesser Antilles trenches).
Conductive heating near the slab edge will be more
efficient, affecting both the body forces and the
strength of the slab. Assuming that the slab is instantaneously exposed to asthenospheric temperatures
below 100 km, a slab subducting at a rate of 4 cm/yr
at a 458 angle will be affected to about 10 km horizontal distance from the edge when arriving at 200 km
depth. Except for very slowly subducting slabs, horizontal heat flow therefore has limited effects.
The scarcity of tear fault mechanisms in the centennial catalog is remarkable. We speculate that this is
caused by alternating periods of STEP propagation
and loading. Immediately after a STEP propagation
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R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
Fig. 8. Normalized velocities of bland-locked basin modelQ STEP2 (10s; j Y
m jmax ¼ 0:25 cm/yr), and finite element grid (light grey). North arrow is shown to facilitate model
description. Velocity magnitude, (a) top view and (b) bottom view. This and following panels; breakdown into individual components of the 3D velocity field at model boundaries.
East (positive)–west (negative) velocity component v x , (c) top view and (d) bottom view. In these and the following panels, black lines show zero velocity contours. South (positive)–
north (negative) velocity component v y, (e) top view and (f) bottom view. Up (positive)–down (negative) velocity component v z , (g) top view and (h) bottom view.
519
Fig. 8 (continued).
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
vz (mm/yr)
Topography (m)
520
600
400
200
0
-200
-400
0.2
-0.0
-0.2
-0.4
-1.22
1.22 mm/yr
700
600
km
500
400
300
200
100
10 τ
η=2.1021
0
600
400
200
0
-200
-400
km
Fig. 9. Bottom panel; velocities at the STEP2 model surface at integration time = 10 asthenospheric Maxwell times. Horizontal velocities are
shown as arrows, vertical velocity as color contours. The largest horizontal velocity, in the overriding plate, is 0.8 mm/yr. See caption of Fig. 5
for further details.
event or period, the shallow slab dip near the STEP
may be too gentle. Depending on the sinking velocity,
it subsequently requires some time before the STEP is
reloaded. The observed tearing seismicity at north
Tonga implies that this region is in the propagation
mode.
STEPs are what Bilich et al. [5] refer to as twoplate convex regions, or closed corner transitions [64],
in their global characterization of subduction-tostrike-slip transitions. This nomenclature reflects the
perspective of an observer on the subducting plate.
Indeed, many of the hypothesized STEP regions have
rounded corners, and two-ocean plate boundaries specifically have a characteristic convex shape. Model
STEP1 alludes to a cause for this observation; resis-
tance to subduction near the STEP resulting from the
extra work that is required to break the lithosphere.
Duggen et al. [65] imply that two propagating
STEPs bounding the Strait of Gibraltar may result in
bands of neighboring lithospheric mantle to be
dragged along with the subducting lithosphere (beneath northern Africa and southern Spain). Depending
on the negative buoyancy of the lithospheric mantle
and the viscous coupling to the overlying crust this
appears a viable process, which may also occur in
other STEP regions. Subsequent decompression melting of infilling asthenospheric may explain the plume
petrologic signature of STEP-volcanoes. However,
return flow as found in model STEP1 may already
suffice to explain this observation.
R. Govers, M.J.R. Wortel / Earth and Planetary Science Letters 236 (2005) 505–523
0.13
521
7.29 nstrain/yr
700
600
km
500
400
300
200
100
10 τ
0
600
400
200
0
-200
-400
km
Fig. 10. Effective total strain rates along the STEP2 model surface. See caption of Fig. 6 for further details.
Our models do not include the dynamic effects of
phase transitions [66,67], interaction of the slab with
the upper-lower mantle discontinuity [68,69], and
(larger scale) convection not excited by model slab
subduction. Our first order description of the geometry, rheology and forcing in STEP regions allows us to
identify the base pattern of surface responses at such
features. Future work will focus on regional models to
investigate whether STEP evolution explains available observations. To this end, we will assimilate in
our models knowledge of geometry (seismology, potential field data), temperature (heat flow, seismology)
to define rheology and body forces, and pre-existing
regional faults.
5. Conclusions
We identify a dozen or so STEP-type edges of
subduction zones, where continued subduction
requires tearing of the lithosphere. Observational support for STEP propagation varies strongly per region,
partly because of the remoteness or inaccessibility of
the area, but also because lithosphere tearing events
seem to occur infrequently. STEPs are basically different from transform plate boundaries, although they
may sometimes mimic the kinematic behavior of
transforms. Once established, STEPs are stable features in that they continue to propagate in most circumstances. A STEP propagating through the
landscape has the potential to produce kilometerscale topography and major sedimentary basins. Geologically recorded deformation and rotation at points
along the STEP path are expected to exhibit a, yet
undocumented, very specific temporal variation.
Acknowledgements
This research was performed as part of the ISES
program. The work was partly supported by the
EUROMARGINS Program of the European Science
Foundation, 01-LEC-EMA22F WESTMED project.
Reviews by Michael Hamburger, Ray Russo and
Wouter Schellart are greatly appreciated.
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