SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-9-821-2018The seismogenic fault system of the 2017 Mw 7.3 Iran–Iraq earthquake:
constraints from surface and subsurface data, cross-section balancing, and
restorationThe seismogenic fault system of the 2017 Mw 7.3 Iran–Iraq earthquakeTavaniStefanostefano.tavani@unina.ithttps://orcid.org/0000-0003-3033-5314ParenteMarianohttps://orcid.org/0000-0002-3755-1207PuzoneFrancescoCorradettiAmerigohttps://orcid.org/0000-0002-5174-0653GharabeigliGholamrezaValinejadMehdiMorsalnejadDavoudMazzoliStefanohttps://orcid.org/0000-0003-3911-9183DISTAR, Università degli Studi di Napoli “Federico II”, Naples,
ItalyN.I.O.C., Tehran, IranStefano Tavani (stefano.tavani@unina.it)20June2018938218318March201819March20184June201812June2018This 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/9/821/2018/se-9-821-2018.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/9/821/2018/se-9-821-2018.pdf
The 2017 Mw 7.3 Iran–Iraq earthquake occurred in a region where the
pattern of major plate convergence is well constrained, but limited
information is available on the seismogenic structures. Geological
observations, interpretation of seismic reflection profiles, and well data
are used in this paper to build a regional, balanced cross section that
provides a comprehensive picture of the geometry and dimensional parameters
of active faults in the hypocentral area. Our results indicate (i) the coexistence of thin- and thick-skinned thrusting, (ii) the reactivation of
inherited structures, and (iii) the occurrence of weak units promoting
heterogeneous deformation within the palaeo-Cenozoic sedimentary cover and
partial decoupling from the underlying basement. According to our study, the
main shock of the November 2017 seismic sequence is located within the
basement, along the low-angle Mountain Front Fault. Aftershocks unzipped the
up-dip portion of the same fault. This merges with a detachment level located
at the base of the Paleozoic succession, to form a crustal-scale fault-bend
anticline. Size and geometry of the Mountain Front Fault are consistent with
a down-dip rupture width of 30 km, which is required for an Mw 7.3
earthquake.
Introduction
On 12 November 2017, a Mw 7.3 earthquake struck the northwestern
portion of the Lurestan region of the Zagros Belt, at the boundary region
between Iran and Iraq (Fig. 1). This earthquake had a thrust fault plane
solution with a 351∘ striking and 16∘ dipping nodal
plane. The other nodal plane has a strike of 122∘ and a dip of
79∘. The P axis plunges 33∘ toward 223∘,
whereas the T axis plunges 54∘ toward 18∘ (Fig. 1)
(source: USGS, https://earthquake.usgs.gov; last access: 18 June 2018). These parameters
indicate SW-directed co-seismic slip along a low-angle thrust, such a
direction being nearly perpendicular to the strike of the Zagros Belt and of
its main thrust systems. The hypocentre is located at a depth of ca. 20 km
where, according to preliminary teleseismic data, the slip was nearly 9 m
(Utkucu, 2017). Coherently with SW-directed motion along a gently dipping
thrust, interferometric synthetic aperture radar (SAR) data show a NW–SE to NNW–SSE-elongated
displacement field (Fig. 2). Consistently, the maximum surface deformation
(reaching ca. 90 cm of uplift; Kobayashi et al., 2018) is shifted some tens
of kilometres southwestward of the epicentre of the main shock. Forty-five Mw
> 4 aftershocks followed during the next 30 days in a
N–S-elongated, 50 km × 150 km area located to the west of the main shock (Fig. 2). Aftershocks lined up along the Mountain
Front Flexure (Figs. 1, 2), as most of the major earthquakes of the last 50 years (Berberian, 1995; Talebian and Jackson, 2004) have been, a major tectonic lineament of the area. However,
the instrumental seismic record indicates that this structure had never
produced a Mw > 7 earthquake in the last decades. Identifying the
fault or fault segment activated during the seismic event and defining its
dimensional parameters are thus essential for the assessment of the seismic
hazard (Wells and Coppersmith, 1994).
Tectonic sketch map of the Zagros Mountains,
showing epicentre and moment tensor of the
12 November 2017 Mw 7.3 earthquake
(source: USGS, https://earthquake.usgs.gov/).
Elevation map (source: ESDIS) showing the main structural features of the Lurestan
region and earthquake distribution (source: USGS, https://earthquake.usgs.gov/). Mw> 4
earthquakes of the November 2017 sequence are reported in white; pre-2017
Mw> 5 earthquakes are reported in yellow. The Sentinel 1 co-seismic interferogram
(11 November, 15:00 UTC to 17 November 2017, 14:59 UTC; http://sarviews-hazards.alaska.edu/Event/34/; last access: 18 June 2018) is also shown as an overlay.
In seismically active fold and thrust belts (FTBs), where the earthquake
dataset is not sufficiently robust to constrain the geometry of active
faults, deep cross sections built using balancing techniques (Dahlstrom,
1969; Hossack, 1979) have been successfully used to improve the knowledge of
the seismogenic structures, as carried out in, e.g., the Los Angeles area
(Shaw and Suppe, 1996; Davis et al., 1989), Taiwan (Yue et al., 2005;
Mouthereau and Lacombe, 2006), and the Longmen Shan FTB (Wang et al., 2013).
In the Zagros FTB, many of the largest earthquakes are associated with major
reverse faults affecting the Precambrian basement (e.g. Jackson, 1980;
Berberian, 1995; Talebian and Jackson, 2004), which are included in almost
all the published balanced cross sections across the belt (Blanc et al.,
2003; Molinaro et al., 2005; Mouthereau et al., 2007; Vergés et al.,
2011). Despite being located more than 200 km away from the epicentral area,
these cross sections suggest that the seismogenic structure of the Mw 7.3 earthquake could be related to the Mountain Front Flexure, which
extends across the aftershock area of the November 2017 earthquake (Figs. 1, 2). The flexure, across which a marked variation of both topography and
structural relief occurs (Falcon, 1961), is commonly interpreted as produced
by a large underlying basement thrust, namely the Mountain Front Fault. This
structure is thus a candidate for the seismogenic fault of the recent
Mw 7.3 earthquake.
Geological observations of faults and folds affecting Meso–Cenozoic rocks
exposed in the epicentral area are reported in this study. These
observations were integrated with the interpretation of near-vertical
seismic reflection profiles calibrated with well logs, allowing us to
produce a detailed and well-constrained geological cross section reaching a
depth ranging from 2 to 5 km. The section was then completed at depth by
using the balancing technique (e.g. Dahlstrom, 1969; Hossack, 1979). Our
results indicate that the November 2017 seismic activity is attributable to
the Mountain Front Fault, for which, using the balancing technique, we
reconstructed 10 km of cumulative displacement in the hypocentral area.
Geological map of the NW portion of the Lurestan region (source: National Iranian Oil
Company and original field mapping) showing (i) November 2017 earthquakes; (ii) traces of near-vertical
seismic profiles and wells used to constrain the geological
cross section of Fig. 6 (profiles shown in Figs. 4 and 5 are in black); (iii) magnetic basement depth
(Teknik and Ghods, 2017), and (iv) a trace of the geological section in
Figs. 4 and 5. The inset shows the stratigraphic succession of the area, with
thicknesses for the Mesozoic to Cenozoic stratigraphic units computed from original
field data. Thickness for the Paleozoic to Lower Triassic is taken from the literature
on the geology of Iraq (Jassim and Goff, 2006). The supposed trace of the Khanaqin
fault is from Lawa et al. (2013).
Geological background
The NW–SE-striking Zagros mountain belt formed due to the continental
collision between the Arabian and Eurasian plates (Berberian and King, 1981;
Alavi, 1994, 2007; Argand et al., 2005; Mouthereau et al., 2006; Vergés
et al., 2011). The present-day northward motion of Arabia relative to fixed
Eurasia is about 2 cm yr-1 (Vernant et al., 2004). This is partitioned between
right-lateral motion along NE–SW-striking faults and NE–SW-oriented
shortening (Blanc et al., 2003; Vernant et al., 2004; Talebian and Jackson,
2002, 2004), which in the Zagros belt is about 5–10 mm yr-1 (Vernant et al.,
2004). The belt is bounded to the NE by the Main Recent Fault and Main
Zagros Thrust (Fig. 1), forming the suture zone that separates terrains
derived from the Mesozoic conjugate margins of the Neo-Tethyan Ocean. The
Zagros FTB, to the SW of the suture, involves units originally pertaining to
the Arabian continental margin (Ziegler, 2001; Blanc et al., 2003; Sepehr
and Cosgrove, 2004; Ghasemi and Talbot, 2006; Mouthereau et al., 2012;
English et al., 2015). Within the Zagros FTB, the High Zagros Fault, a major
structure striking NW–SE, separates the imbricate zone to the NE, where
intensely faulted and folded units are exposed, from the Folded Belt to the
SW (Blanc et al., 2003; Karim et al., 2011; Vergés et al., 2011). The SW
boundary of the Zagros FTB is the Mountain Front Flexure, corresponding to a
basement and topographic step that divides the belt from its foreland basin
to the SW (Falcon, 1961). The flexure is commonly interpreted as being
underlined by a thick-skinned basement structure (e.g. Berberian, 1995;
Blanc et al., 2003; Vergés et al., 2011), although many researchers have
also proposed a thin-skinned geometry (McQuarrie, 2004; Hinsch and Bretis,
2015). The flexure has a sinusoidal shape, defining salients and recesses
along the belt. The seismic sequence of the November 2017 earthquake is
located at the boundary between two of them, namely the Kirkūk embayment and the
Lurestan arc (Figs. 1, 2). Folds and thrusts of the Folded Belt of the Kirkūk
embayment and of the Lurestan arc are NW–SE-striking, becoming locally
NNW–SSE-trending along the boundary between the two domains. There, a major
bend of the Mountain Front Flexure occurs (Vergés et al., 2011; Sadeghi
and Yassaghi, 2016; Koshnaw et al., 2017) (Figs. 2, 3). Indeed, the envelope
of NNW–SSE-striking en échelon folds along the Mountain Front Flexure in the
epicentral area of the November 2017 earthquake roughly runs N–S (Fig. 2).
This is interpreted as being associated with the occurrence of a
N–S-striking basement fault (i.e. the Khanaqin Fault; e.g. Berberian, 1995;
Hessami et al., 2001; Lawa et al., 2013; Allen et al., 2013) that should
currently act as a right-lateral fault. Folds in the Lurestan arc affect an
about 10 km thick sedimentary succession (Hessami et al., 2001; Ziegler,
2001; Homke et al., 2009; Vergés et al., 2011; English et al., 2015). In
detail, the uppermost Proterozoic basement of the Arabian plate in the
Lurestan region is overlain by a nearly 3000 m thick Paleozoic succession
dominated by continental clastic deposits (Jassim and Goff, 2006; Bordenave,
2008). The strong rheological contrast between the crystalline basement and
the overlying sedimentary cover makes the basement–cover interface a major
decollement horizon of the Lurestan region (e.g. Vergés et al.,
2011), despite the lack of evidence for the occurrence of Hormuz salt at
the base of the sedimentary pile of the study area. Permian rifting, related
to the opening of the Neo-Tethys Ocean (Berberian and King, 1981; Sepehr and
Cosgrove, 2004; Ghasemi and Talbot, 2006), led to the deposition of about 1 km of shallow-water carbonates (Chia Zairi Formation) (Jassim and Goff,
2006; Bordenave, 2008), with some tens of metres of shales at the base,
forming a mobile level sandwiched between two competent packages (Fig. 3).
With continuing passive margin subsidence, nearly 1800 m of Triassic–Lower
Jurassic shallow-marine carbonates and evaporites, with minor shales,
accumulated (Mirga Mir to Sekhaniyan Formation) (Jassim and Goff, 2006; Bordenave,
2008). This interval is essentially formed by competent units, with the
exception of the about 100 m thick Baluti and Bedu shale formations, at the top
and base of the Triassic succession, respectively. This is a remarkable
difference with respect to the Fars and Dezful embayment areas to the SE of
the Zagros Belt, where the dolostones and limestones of the Triassic Kurra
Chine Formation are substituted by the evaporite-dominated Dashtak Formation, which acts as a major decollement level there. A major late Early to Middle
Jurassic subsidence pulse led to carbonate platform drowning and deposition
of about 100 m of relatively deep-water limestones, marls, and black shales
and evaporites (Sargelu, Naokelekan, Barsarin Formation, Toarcian to Tithonian),
followed by 700 m of Cretaceous basinal limestones, shales, and marls (Garau,
Sarvak and Ilam formations) (Jassim and Goff, 2006; Bordenave, 2008). The closure
of the Neo-Tethys Ocean during the Late Cretaceous led to the formation of a
flexural basin, filled by a ca. 2 km thick Maastrichtian to Eocene
succession (Hessami et al., 2001; Homke et al., 2009; Vergés et al.,
2011; Saura et al., 2015), evolving from deep-marine marls and limestones to
a prograding wedge of deep-marine to continental clastic sediments. This
first foredeep infill is overlain by about 500 m of shallow-water carbonates
of the Shahbazan and Asmari Formation (Oligocene–lower Miocene), passing upward to
lower Miocene evaporites. Renewed shortening and thrusting from the late
Miocene to recent times led to the deposition of a younger foreland basin
clastic infill (Fig. 3) (Hessami et al., 2001; Jassim and Goff, 2006; Homke
et al., 2009).
NE part of the NE–SW-oriented geological section across the hypocentral area, with field
photographs illustrating the main structural features. A near-vertical seismic profile is
displayed below the cross section (vertical scale is roughly equal to the horizontal scale).
NE–SW geological cross section
In this paragraph we present a NW–SE-oriented geological section across the
study area. The section is divided into two portions. Figures 4 and 5
illustrate the NE and SW portion of the section, respectively (with a small
overlap area). Two seismic reflection profiles running at a low angle to the
geological cross-section trace are projected onto the section plane, and key
field observations along the NE portion of the section are also reported in
Fig. 4.
The High Zagros Fault to the NE of the study area intersects the cross
section of Fig. 4 in its northern portion. There, the major thrust fault
dips roughly parallel to the strata of both hanging-wall and footwall blocks
(i.e the cut-off angles are close to 0). Cretaceous strata in the footwall
are affected by the NW–SE-striking, tens of kilometres long thrusts of the Satiary
Thrust System. These thrusts have low (< 10∘)
hanging-wall and footwall cut-off angles (Fig. 4). Along the section, the
Garau Formation sits in the hanging wall of the thrust and the Ilam Formation lies in
its footwall. However, the geological map of Fig. 3 shows that the
Sehkanian Formation is the oldest exposed unit in the hanging-wall block and that
it is thrust on top of the Upper Cretaceous Gurpi Formation (see also the field
photograph of Fig. 4), which lies about 1000 m higher in the stratigraphic
column. This feature, coupled with the observed relationship between hanging-wall flat
and footwall flat, suggests displacements in the order of several
kilometres. In the footwall of the Satiary Thrust System, Upper Triassic to
Cretaceous strata are, as a whole, 20–30∘ NE-dipping for about 4 km, until they meet the tens of kilometres long Herta Thrust System. This
includes two 30∘ dipping thrusts (joining southeastward; Fig. 3)
showing very low cut-off angles and separating the Triassic Sarki Formation in the
hanging wall of the trailing thrust from the Sargelu and Garau formations in its
footwall (Fig. 4). The repetition of hanging-wall flat on footwall flat
geometries (Fig. 4) indicates a remarkable (i.e. several kilometres)
displacement also for the Herta Thrust System.
SW part of the NE–SW-oriented geological section across the hypocentral area. Near
vertical seismic profiles are displayed below the cross section (vertical scale is roughly
equal to the horizontal scale).
Near-vertical reflection seismic profiles in this northern area are affected
by significant noise; however, both the Satiary and the Herta thrust
systems are imaged at depth (Fig. 4), displaying very low cut-off angles,
which confirms their significant horizontal displacement. Folds associated
with the Herta and Satiary thrust systems are truncated by the High Zagros
Fault in the SE portion of the study area. This may be observed in the
eastern portion of the geological map of Fig. 3 and, in more detail, in
the photograph of Fig. 4, where the sub-horizontal High Zagros Fault
truncates an anticline exposing the Gurpi Formation in the limbs and the Ilam Formation
in the core. This observation constrains the relative timing of the development
of these structures, pointing to an out-of-sequence emplacement (or
reactivation) of the High Zagros Fault, which post-dates the development of
the Herta and Satiary fault systems. Moving to the southwest, the Marakhil
Anticline exposes the Geli Khana Formation in its core, and the seismic profile
indicates that the Paleozoic strata are folded as well. The Marakhil Fault,
bounding the anticline to the SW, has a high (> 60∘)
hanging-wall cut-off angle, typical of a reactivated (i.e. positively
inverted) extensional fault (e.g. Sibson, 1985; Williams et al., 1989). To the NE, the
fault flanks a roughly 5 km wide, gentle syncline affected by
low-displacement (i.e. < 100 m) reverse faults with both low (e.g.
the Qlaji Thrust) and high (e.g. the Bawrol Thrust) cut-off angles. In
detail, similarly to the Marakhil Fault, the Bawrol Thrust has a
hanging-wall cut-off angle typical of a positively inverted normal fault, the
original extensional activity of which post-dated the deposition of the
Sehkaniyan Formation. Indeed, syn-kinematic thickening of the Sargelu, Naokelekan,
and Barsarin formations (S-N-B in Fig. 4) observed across the Marzan
extensional fault, as well as wedging of the same formations in the hanging
wall of the Qlaji Thrust indicate that many of the previously illustrated
inverted faults (affecting Triassic and Jurassic strata) developed during a
Middle Jurassic extensional pulse. The Sheykh Saleh Anticline is another
major structure of this part of the Lurestan region. It separates an area to
the SW, where the oldest rocks exposed in the cores of the anticlines
(Gheytuleh, Azgaleh, and Miringeh anticlines) belong to the Upper Cretaceous
Ilam Formation, from an area to the NE where the oldest rocks exposed at the core
of the anticlines belong to the Triassic Kurra Chine and Geli Khana formations
(Fig. 3). The NE block has a structural relief of about 2 km. Despite the
significant noise affecting the seismic section, the Ilam and Sehkaniyan
formations are clearly imaged in the subsurface of the area SE of the Sheykh Saleh
Anticline (Fig. 5). Both formations are made of carbonates and are capped by
shales and marls of the Sargelu and Gurpi formations, respectively, this making
their top strongly reflective and recognisable. The first clear occurrence
of the top Sehkaniyan reflectors is underneath the southwestern limb of the
Gheytuleh Anticline, at about 1 s two-way time (TWT) (Fig. 5), entirely consistent with the
dip and thickness of the overlying stratigraphic units. These Sehkaniyan
reflectors are SW-dipping and become NE-dipping about 2 km to the SW, below
the syncline flanking to the SW the Gheytuleh Anticline. This coherence
between surface and subsurface geometries points to a roughly parallel
folding of the entire package overlying the Sehkaniyan Formation. About 1 km to the
SW, the top Ilam reflectors also become recognisable. Further to the SW,
starting from the Azgaleh Anticline area, reflectors are calibrated with
well logs and exposures of the top Ilam Formation. In this southwestern portion of
the section, the envelope of the top of the Ilam and Sehkaniyan formations
defines a 2–5∘ SW-dipping, regional-scale panel, with limited
decoupled deformation between the Mesozoic and Cenozoic units due to the
occurrence of a weak package comprised between the stiff Ilam and Asmari
formations. This shallow-dipping faulted and folded panel terminates at the
Miringeh Anticline, which displays an unfaulted forelimb. There the strata
of the entire Paleozoic to Cenozoic sedimentary succession are parallel and
form a 10 km wide SW-dipping monocline. In more detail, below the Miringeh
Anticline, a gentle unconformity occurs between the middle and upper
Paleozoic reflectors, evidencing the occurrence of middle Paleozoic
deformation. The above-mentioned monocline is bounded by two N–S-striking
anticlines cored by the Asmari Formation; below them, a repetition of the Mesozoic
reflectors is observed, which is produced by a back thrust. At the SW
termination of the seismic sections, the entire Paleozoic to Cenozoic
sedimentary succession becomes horizontal and forms a large-scale syncline.
(a) Balanced cross section along the direction of the geological section in Figs. 4 and 5,
showing projected main shock and detail of the co-seismic interferogram with a trace of the section. (b) Restored section.
Balancing the cross section
The cross section shown in Figs. 4 and 5 is completed at depth by
producing a geological solution (Fig. 6) in which line-length preservation
during folding and thrusting is assumed (e.g. Dahlstrom, 1969; Hossack,
1979). The balanced cross section is built along a direction oriented
49∘ N, which is perpendicular to the trend of major folds and
thrusts. These structures display negligible regional plunge along the
section, which allows us to use a vertical plane to build the section. This
also ensures the absence of remarkable out-of-plane motion and allows us to
directly compute the thickness of the exposed Mesozoic and Cenozoic units
along the section. The chosen section plane forms an angle of 17∘
with the 215∘ N striking and 78∘ dipping plane
containing the P and T axes of the of the 2017 Mw 7.3 earthquake, thus
representing a proper section to obtain insights on the seismogenic
structures.
Some lateral thickness variations, in the order of some tens of metres, are
observed for the package comprised between the Sargelu and Barsarin formations.
The Sehkaniyan and Sarki formations also display lateral thickness variations of
the same order of magnitude. In the Geli Khana and Kurra Chine formations we have
not observed any kind of growth structure, and the parallelism between
reflectors observed in the seismic line of Fig. 4 indicates that the
thickness of these formations can be considered roughly constant. These
observations indicate that, as a whole, a constant thickness can be used for
the almost 2 km thick package comprised between the base of the Geli Khana
Formation and the base of the Garau Formation. The overlying units are not continuously
exposed in the northern part of the section and, because of that, they are
not shown in the restoration. The Paleozoic units and the basement, for
which only limited and discontinuous information is available, are modelled
using 1 and 2 km thick layers, respectively. For the sake of simplicity,
thickness variations in upper Paleozoic units are first neglected and then
reintroduced after cross-section balancing. This is because the adoption of
constant thickness for the entire upper crust and of flexural slip folding
allowed us to assume line-length preservation. Coherently, the restored
cross section shows the cumulative length of Mesozoic, Paleozoic, and
basement layers. The trace of the faults in the restored section is obtained
by smoothing the polyline built by connecting the restored cut-off points.
This is done to avoid zigzag effects, and, in any case, smoothing is less
than 0.5 % of the original cut-off point position.
Coherently with field observations, in our reconstruction, thrusts to the NE
of the Marakhil Anticline are thin-skinned and have a displacement in the
order of some kilometres. They splay off from a basal decollement
located at the bottom of the Triassic sequence, namely within the Bedu
Shale, sandwiched between the competent Chia Zairi and Geli Khana–Kurra Chine packages. The Marakhil Anticline, on the other hand, is a deeply rooted structure,
associated with the Marakhil inverted normal fault, which is observed at the
surface (Fig. 4). The simple shallow geometry of this large wavelength fold
introduces a geometrical problem at depth, as two solutions can be applied
to model the deeper portion of the anticline. In the first one, the inverted
fault affects only the sedimentary cover, the core of the anticline is
filled by ductile material and the underlying basement is not involved in
faulting and folding. In the second solution, the inverted fault involves
also the basement. The lack of a sufficiently thick ductile layer at the
base of the Paleozoic sequence and the occurrence of a structural step
across the Marakhil Anticline are more compatible with the second,
basement-involved, solution. Following this structural model and keeping
constant the line length of both basement and cover, we solved the geometry
in the core of the anticline by assuming the occurrence of a footwall
shortcut of the inverted normal faults in the basement. This represents a
typical feature associated with the inversion of normal faults (e.g. McClay,
1989). In our solution, this shortcut transfers displacement from the main
reactivated fault to the base of the sedimentary cover. Low-displacement,
SW-verging reverse faults, and a major back thrust accommodate such a
displacement in the Mesozoic and Paleozoic strata. The Sheykh
Saleh Anticline to the SW shows a similar deep structure, which is even
better supported by the remarkable structural step occurring at this
location. Here, a positively inverted normal fault with a footwall shortcut
occurs in the basement. The footwall shortcut transfers displacement from
the main reactivated fault to the base of the sedimentary cover sequence.
Such a displacement is accommodated by folding and faulting of the
sedimentary cover, with the Paleozoic or Lower Triassic incompetent units
(i.e. the Bedu Shale Formation or the shaly level at the base of the Chia Zairi
Formation) promoting decoupling between Mesozoic and Paleozoic strata. In our
interpretation, a positively inverted normal fault bounds the
Miringeh Anticline to the NE too, producing the uplift of the crustal block in its
hanging wall and preventing the southward propagation of the deformation of
the sedimentary cover. Indeed, Paleozoic to Cenozoic strata in the crest and
in the wide, homogeneously dipping SE limb of this anticline are parallel,
unfolded, and unfaulted. The lack of second-order faults and folds to the SW
of the Miringeh inverted fault and their occurrence to the SW of the
Marakhil and Sheykh Saleh faults, both the latter faults being characterised
by a footwall shortcut, indicate that coupling between the basement and the
sedimentary cover is intimately linked with the shortcut development. The SW
limb of the Miringeh anticline is underlain by a basement low-angle thrust,
corresponding to the Mountain Front Fault, on which the main shock is
located (Fig. 6). The focal mechanism provided by the USGS indicates a
351∘ striking and 16∘ dipping thrust fault, and its
intersection with our 49∘ N striking vertical section gives
14∘ of apparent dip. Coherently, in our reconstruction the thrust
dips 15∘ at the hypocentral depth and becomes almost
sub-horizontal upwards, where it reactivates the basement–cover interface. A
back thrust splays from this upper flat, accommodating part of the
displacement transferred from the main ramp of the Mountain Front Fault and
forming a fishtail structure together with it, responsible for the surface
deformation observed from interferometric data. The position of such a
back thrust roughly coincides with the Khanaqin Fault (e.g. Lawa et al.,
2013) (Fig. 3), which accordingly must be downgraded to an accommodation
structure of the Mountain Front Fault.
An independent quality check of our reconstruction is provided by the top of
magnetic basement data (Fig. 6), computed according to the regional depth
map in Teknik and Ghods (2017). The depths of the crystalline basement
underlying the sedimentary cover and the top of the magnetic basement
obviously do not coincide, due to the heterogeneous nature of the magnetic
basement. However, their large-scale shape is similar, confirming the
occurrence of highs and lows predicted by our reconstruction. The restored
length of the section is 104 km, with a negligible maximum error of 1.5 %.
The total shortening is 20 km, 8 km of which are associated with the
thin-skinned Satiary and Herta thrust systems to the NE of the Marakhil
Anticline. As previously mentioned, these thrusts are truncated by the High
Zagros Fault, which in this area was active during the Late Cretaceous to
Paleocene interval (Karim et al., 2011; Vergés et al., 2011; Saura et
al., 2015). These thrusts also have anomalously high displacements compared
to the other structures along the section. For both reasons, the Satiary and
Herta thrust systems are interpretable as footwall splays of the High Zagros
Fault, probably merging with it to the NE, outside the section. Lower
displacements are instead associated with the Marakhil (2.5 km), Sheykh
Saleh (2.0), and Miringeh (1.0) faults, the amount of shortening
accommodated in the area between the Marakhil and Miringeh anticlines being
5.3 km. The remaining shortening is accommodated by the Mountain Front Fault
and associated structures.
Discussion
According to our reconstruction, the Mountain Front Fault has 9.7 km of
cumulative displacement at 20 km depth, where the main shock nucleated. The
displacement decreases upwards, becoming 5.8 km at the upper flat. About 1 km
of this is accommodated by the frontal back thrust, i.e. by the Khanaqin
Fault, while 4.3 km of shortening is transferred to the foreland structures
to the SW of our balanced cross section. Such an expected shortening in the
foreland is highly in agreement with data derived from cross-section
balancing in the Kirkūk embayment, where 5 km of shortening have been
proposed by Obaid and Allen (2017). The computed 9.7 km of displacement of
the Mountain Front Fault at the hypocentre are broadly consistent with the
13 km proposed for the same structures 200 km to the SE (Blanc et al., 2003;
Vergés et al., 2011). The earthquakes of the November 2017 seismic
sequence can thus be attributed to the movement of the Mountain Front Fault,
which forms part of a thrust system splaying from a mid-crustal
decollement (Vergés et al., 2011), similar to that documented in
other FTBs (Cristallini and Ramos, 2000; Lacombe and Mouthereau, 2002;
Butler et al., 2004; Lacombe and Bellahsen, 2016). The important occurrence
of reactivated extensional faults documented in this study suggests that the
mid-crustal decollement could represent a reactivated inherited
extensional decollement (e.g. Marshak et al., 2000; Tavani, 2012). The
Miringeh fault would be the innermost extensional fault associated with this
extensional decollement, and the Mountain Front Fault should be regarded
as a sort of crustal shortcut of the reactivated decollement.
Interferometric data show that the maximum surface deformation occurs at the
SW edge of the geological section (Fig. 6). This reveals that the co-seismic
displacement has induced slip along the shallower, near-horizontal, upper
flat located 20 km to the SW of the main shock, at the basement–cover
interface. Decoupling between the Mesozoic and Paleozoic successions and
between Paleozoic strata and the basement has strong implications in terms
of seismic potential. As already pointed out by Nissen et al. (2011),
decoupling at the base of the cover sequence implies vertically confined
faults, with a down-dip width smaller than 8 km. In fact, only four faults
affect the entire upper crust: the three major, steeply dipping inverted
normal faults splaying out from the basal decollement, probably
corresponding to the brittle–ductile transition, and the Mountain Front
Fault. The former, with their cross-sectional length of up to 25 km,
can generate a down-dip rupture width exceeding 8 km, required for an
Mw 6 earthquake (Wells and Coppersmith, 1994). On the other hand, the
Mountain Front Fault is the only fault on which a down-dip rupture width of
30 km, required for an Mw 7.3 earthquake, may occur.
Beyond their importance for seismic hazard assessments, the data illustrated
in this work have major implications in terms of a better understanding of
thrust tectonics in the Zagros Mountains. The occurrence of salients and
recesses is a common feature in fold and thrust belts (Marshak, 1988)
including the Zagros Mountains, where different mechanisms are invoked to explain the
occurrence of bends in the trace of the Mountain Front Fault (e.g.
Berberian, 1995; Talbot and Alavi, 1996; Bahroudi and Koyi, 2003; Allen and
Talebian, 2011; Navabpour et al., 2014; Malekzade et al., 2016, and
references therein). According to the scaling relationship of magnitude vs.
rupture area (Wells and Coppersmith, 1994), the rupture area for the
Iran–Iraq Mw 7.3 earthquake should exceed 103 km2. Therefore,
the low-angle Mountain Front Fault must extend into the area where the
Mountain Front Flexure runs roughly N–S (Figs. 2, 3). This, coupled with the
N–S clustering of aftershocks (Fig. 2) triggered by SW-directed co-seismic
slip along the low-angle thrust ramp, clearly points to the occurrence of a
lateral ramp beneath the N–S segment of the Mountain Front Flexure at the
boundary between the Kirkūk embayment and the Lurestan arc. As previously
mentioned, in our structural
reconstruction the N–S-striking Khanaqin Fault (e.g. Berberian, 1995; Hessami et
al., 2001; Lawa et al., 2013; Allen et al., 2013) becomes an accommodation structure of the Mountain Front
Fault. A further implication of our work concerns the role of structural
inheritance in the Zagros FTB. The age of rifting and passive margin
development is still a matter of debate in the tectonic puzzle of the area.
A Permian to Early Triassic age is commonly inferred for the onset of
rifting in the Zagros area (e.g. Berberian and King, 1981; Ghasemi and
Talbot, 2006). However, we observed extensional structures that developed
synchronously with the deposition of the Middle Jurassic Sargelu Formation, the
Marzan extensional fault (Fig. 4) being the most striking one. The
positively inverted Marakhil and Bawrol faults, affecting Upper Triassic and
Lower Jurassic units (thus younger than the main rifting event) also fit
well into an Early to Middle Jurassic extensional episode. Such an
extensional pulse could also explain the drowning of the long-lived
Triassic–Jurassic carbonate platform and the onset of deep-water conditions
in the area (Ziegler, 2001; Jassim and Goff, 2006; Bordenave, 2008).
Accordingly, for many of the inverted basement extensional faults, a
polyphase extensional history could be proposed, including a Permo-Triassic
development and a Middle Jurassic extensional reactivation. An even older,
middle Paleozoic origin can be inferred for some of these faults, based on
the occurrence of a middle Paleozoic unconformity seen in some seismic lines
(Fig. 5).
Conclusions
The integration of field data, near-vertical seismic reflection profiles,
and earthquake data allowed us to provide a comprehensive picture of the
geometry and dimensional parameters of the faults in the hypocentral area of
November 2017 seismic sequence at the Iran–Iraq border. The tectonic
framework of this area includes a likely mid-crustal decollement level
at a depth of ca. 20 km, from which high-angle, positively inverted normal
faults splay off. At its southwestern edge, the decollement ramps up to
form the Mountain Front Fault, which joins an upper
decollement level in the south located at the basement–cover interface. The
occurrence of multiple decollement levels in the sedimentary succession
promotes a partly decoupled deformation and limits the size of most of the
faults of the area. The main shock of the November 2017 Mw 7.3
earthquake nucleated in the basement, along the Mountain Front Fault.
Co-seismic slip unzipped the shallower portion of the fault to the SW, at
the basement–cover interface, and activated structures responsible for the
observed surface deformation.
Requests for obtaining the near-vertical seismic sections and well
data should be submitted to the National Iranian Oil Company.
The authors declare that they have no conflict of
interest.
Acknowledgements
We acknowledge the use of imagery from the Land Atmosphere Near-real time
Capability for EOS (LANCE) system, operated by the NASA/GSFC Earth Science
Data and Information System (ESDIS) with funding provided by NASA/HQ, and of
Copernicus Sentinel data 2017, processed by ESA. The geological cross
sections presented in this work were constructed using the Midland Valley
Move software. Requests for obtaining the near-vertical seismic sections and
well data should be submitted to the National Iranian Oil Company. We thank
two anonymous reviewers and Ralph Hinsch for helping improve an early
version of the manuscript.
Edited by: Mark Allen
Reviewed by: two anonymous referees
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