Anatomy of the North Anatolian Fault Zone in the Marmara Sea,
Western Turkey: Extensional Basins Above a Continental Transform
Ali E. Aksu, Tom J. Calon,
Richard N. Hiscott,
Department of Earth Sciences, Centre for Earth Resources Research,
Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada
Institute of Marine Sciences and Technology, Dokuz Eylül
University, Haydar Aliyev Caddesi No. 10, Inciralt,
Izmir 35340, Turkey
Although it straddles an area of extreme earthquake risk, the origin of the Marmara Sea transtensional basin has been enigmatic. Recently acquired high-resolution seismic profiles and earthquake hypocenter locations show the crustal architecture to be characterized by a negative flower structure, bounded by two west-trending sidewall faults that are linked to a single vertical to steeply south-dipping master fault that extends to depths of >30 km. The negative flower structure has a complicated architecture consisting of relatively intact detached basinal blocks, separated by southwest-trending ridges which serve as strike-slip transfer zones between the basins. The basins and ridges are rotating counterclockwise, accommodated by the southward retreat of the southern sidewall of the flower structure as crustal material is passed from its eastern to western end along the transtensional strike-slip zone. This new interpretation provides a better context for understanding seismicity in the region and for understanding complexities of fault segmentation in large transtensional basins along continental transforms in zones of tectonic escape.
In the January issue of GSA Today, Reilinger et al. (2000) explained
the inevitability of destructive earthquakes along the North Anatolian transform
fault of northern Turkey as a consequence of the westward tectonic escape of the
Aegean-Anatolian Plate from a collision zone between the converging African and
Eurasian plates (Fig. 1, inset). They pointed to the lack of a detailed map of
faults crossing the locally deep (>1200 m) floor of the Marmara Sea (Fig. 2A)
as an impediment to establishing the precise mechanics of faulting and earthquake
generation. This region is of critical concern because devastating earthquakes
over the past 100 years have progressed westward along the plate boundary toward
the Marmara Sea region (Reilinger et al., 2000). Because of poor constraints on
fault geometry, conflicting tectonic interpretations have been proposed for the
deep basins of the Marmara Sea and associated seismicity (Fig. 2C and 2D). Comparisons
of existing models show that separate groups of authors have advocated different
locations for fundamental strike-slip faults, contrasting asymmetries for adjacent
strike-slip basins, and different linkages with faults on land. This high level
of uncertainty as to the first-order geometry of structures makes it impossible
to confidently evaluate the seismicity of the Marmara Sea area.
The Marmara Sea region is also an important place for understanding the nature
of transform plate boundaries. The North Anatolian transform fault forms the northern
boundary of the Aegean-Anatolian plate and accommodates its westward escape by
dextral strike-slip movement (Fig.
1, inset). The Marmara Sea is located on the transform fault, at a place where
a notable southwestward swing occurs in the velocity field of the Aegean-Anatolian
plate and where a broad zone of faults swings gradually to the southwest to connect
the North Anatolian transform fault to the Saros-Ganos fault (Figs. 1
and 2). Global positioning
system measurements constrain the horizontal velocity field of the Aegean-Anatolian
plate relative to a fixed Eurasia (Reilinger et al., 2000), demonstrating a counterclockwise
rotation of the Aegean-Anatolian plate and a progressive southwestward increase
in plate velocity in the Aegean region (Fig.
Published tectonic models have failed to properly explain the origin of the
Marmara Sea because of poor seismic coverage and insufficient use of available
earthquake data. For example, cross-sectional plots of the locations of earthquake
hypocenters beneath the deeper areas of the Marmara Sea (Fig.
2B) show that the steep marginal fault scarps enclosing the deep basins are
not fundamental crustal-scale faults (i.e., none of these are main strands of
the North Anatolian transform fault). Instead, the plate boundary fault lies directly
beneath the axis of the Marmara Sea, where it is buried by a structurally complex
zone of rhombohedral to elongate basins and ridges. This observation, combined
with new maps of bathymetry (Fig.
2A) and fault traces (Fig.
1) that we have prepared from closely spaced seismic profiles (Fig.
2E), allows us to rule out origination of the Marmara Sea as either a pull-apart
basin (Fig. 3A) or a
transform-parallel strike-slip basin (Fig.
3B), and shows that it is instead a rather unconventional negative flower
structure with complex internal geometry (Fig.
3C). Mann (1997) formulated a general model for the formation of large transtensional
basins in zones of tectonic escape emphasizing the hybrid nature of such basins
in terms of both pull-apart and transform-normal extensional styles. We believe
that this notion is directly applicable to the Marmara Sea.
Bathymetry provides a first-order data set for inferring the positions of surface
faults, the geometry of uplift and subsidence, and the interaction of faulting
and sedimentation. The Marmara Sea is a 3035-km-wide and 150-km-long, west-trending
depression that consists of steep-flanked basins and ridges (10°30°
slopes) nestled between a 35-km-wide shelf dominated by eroded Tertiary
bedrock in the north and an ~30-km-wide shelf in the south (Fig.
2A). There are five deep depressions within the central zone of basins and
ridges. Westernmost basins 1 and 2 (Tekirda
and Central Marmara Basins of Wong et al., 1995) are elongate, southwesttrending
rhombohedral depressions deeper than 11001200 m. Easternmost basin 3 (Çnarck
Basin of Wong et al., 1995), at >1200 m depth, is a west-northwesttrending
elongate depression. Basins 4 and 5 are significantly shallower features. Basin
4 (~800 m deep) is perched on the broad southwest-trending ridge separating
basins 2 and 3, whereas basin 5 is a shallow (~370 m deep), crescent-shaped
depression perched high on the southern slope of basin 3 (Fig.
4A). The three ridges that separate basins 14 (b,
g and d in Fig. 1) have water depths shallower
than 600 m. The flanks of the ridges are generally segmented by steps, creating
a rugged and terraced appearance. The west-trending
ridge a (Figs. 1
and 4A) separates basins
3 and 5; here, the seafloor rises ~100 m above the floor of basin 5, then
quickly descends to basin 3.
UPPER CRUSTAL FAULT ARCHITECTURE
Faults were imaged seismically on 40 in.3 airgun profiles and precisely
transferred to a base map (Fig.
1). The upper crustal architecture in the Marmara Sea is characterized by
an intricately linked fault system with two long west-trending boundary faults
called sidewall faults. The zone between is referred to as the principal deformation
zone. The sidewall faults are actually zones of narrowly spaced faults that dip
steeply toward the axis of the principal deformation zone. They show close
correlation with bathymetry. The principal deformation zone swings gradually to
a west-southwest trend in the western Marmara Sea, the northern sidewall fault
linking to the Saros-Ganos fault via a set of faults along the western margin
of basin 1 (Fig. 1).
The southern sidewall fault appears to link to the west into a relay of southwest-trending
faults with normal throw extending to the eastern Dardanelles. To the east, the
two sidewall faults converge in western Izmit Bay, linking with the main northern
strand of the North Anatolian transform fault. The architecture of the principal
deformation zone thus displays an overall elongate tapered shape (Fig.
1). Basin 5 (Figs. 2A
and 4A) is considered
to be an out-step zone of the southern margin of the principal deformation zone,
bounded by an arcuate fault zone that merges with the southern sidewall fault
both to the east and west, and likely at depth (Fig.
1, section AA').
The principal deformation zone consists of a shingled array of four basins
and three ridges oblique to the trend of the sidewall faults (Figs. 1
and 2A). Basins 1, 2,
and 4 and their bounding ridges a, b,
and d are arranged in an en echelon pattern, controlled
by the southwest trend of the ridges. The western margin of basin 1 is defined
by steep southwest-trending faults with considerable normal throw. This fault
zone occupies a position similar to that of the marginal faults of the ridges.
To the northeast, the fault zone links with the northern sidewall fault; to the
southwest, it follows the northern shoreline of the Dardanelles and does not merge
with the southern sidewall fault. Furthermore, the zone is not cut by the Ganos
fault as was suggested by Okay et al. (1999) (Fig.
2D). The 1015-km-wide ridges are cut by narrowly spaced, southwest-trending,
high-angle faults, many of which extend to the seafloor, creating a rugged topography.
The boundaries between the ridges and adjacent basins are prominent fault scarps.
The Pliocene to Quaternary basinal strata converge dramatically onto the flanks
of the ridges (Fig. 4B).
Normal-sense drag on the faults suggests upward propagation of fault tips to the
surface (Fig. 4B). The
convex-upward internal stratal architecture of ridges b, g,
and d is attributed to pervasive faulting concentrated
in narrow zones, normal throw increasing toward the edges of the basins. The linkage
of the faults in the ridges with the sidewall faults is poorly resolved because
of the spacing of the seismic grid. However, the ridge-margin faults clearly bend
in a clockwise sense toward the sidewall faults, compatible with dextral displacement
Crustal blocks containing basins 1, 2, and 4 have well-defined rhombohedral
shapes with aspect ratios of ~2.3:1. The internal structure of these blocks
is defined by a central graben with south-southwest-trending normal faults that
dip both northward and southward (Figs. 3C
and 4B). Each basinal
depocenter lies oblique (20°25°) to both the sidewall faults and
the ridge-margin faults. The depocenters are truncated by the ridge-margin faults,
and their tapered ends are strongly dissected by faults. The setting of basin
3 is fundamentally different in that the depocenter is almost completely enveloped
by the sidewall faults of the principal deformation zone. Only at its western
edge is this block bounded by the southwest-trending faults of the eastern flank
of ridge d (Fig.
1). Basin 5 is an asymmetric half graben developed above a north-dipping listric
normal fault with its associated rollover anticline (Figs. 1,
section AA' and 4A).
This fault is interpreted as a footwall splay of the southern sidewall fault.
PULL-APART, TRANSFORM-NORMAL EXTENSION, OR SOMETHING DIFFERENT?
All previous tectonic models correctly note the fragmentation of the Marmara
Sea into small crustal blocks, but adhere to classical models of pull-apart basin
formation along releasing bends or stepovers within an east-trending dextral strike-slip
system (Fig. 2C and 2D)(Ergün
and Özel, 1995; Wong et al., 1995; Okay et al., 1999). Okay et al. (1999)
considered basin 1 to be a flat-bottomed, negative flower structure that is detached
at the base of the Pliocene to Quaternary sediments and is oriented transverse
to the main stem of the North Anatolian transform fault. This requires that the
North Anatolian transform fault cross the Marmara Sea, merging with the northern
sidewall fault (Fig. 2D)
to form a releasing bend as it curves toward the Saros-Ganos fault. Eastward in
their model, the North Anatolian transform fault swings along a restraining segment
coinciding with the western margin of ridge b, an inferred
push-up swell. Wong et al. (1995) and Ergün and Özel (1995) recognized
five blocks, consisting of three rhomb-shaped basins and two intervening transpressional
push-up structures aligned with a southwest trend oblique to the main dextral
North Anatolian transform fault (Fig.
2C). In other models, the northern and southern sidewall faults are considered
to be normal fault segments, allowing subsidence of the basins, whereas the ridges
form arrays of linking oblique-normal faults accommodating rotation between the
blocks (e.g., engör
et al., 1985, their Fig. 12).
The detailed geometry of the fault network in the principal deformation zone
and the location of earthquake hypocenters beneath its axis reveal a negative
flower or tulip structure. The tulip structure is the suprastructure of the principal
deformation zone, extending only to depths of ~45 km (Fig.
1 cross sections and Fig.
2B). It links below ~5 km into a single vertical to steeply south-dipping
stem, extending to depths of at least ~30 km (Fig.
2B). The tulip structure delimits an area of major Pliocene to Quaternary
subsidence, with an aspect ratio of 5.5:1, that is in line with the buried master
fault. The root of the tulip structure is thus a prominent, doubly plunging depression
of the tipline of the buried master fault, situated across the central part of
the Marmara Sea at a depth of ~45 km. Contrary to the earlier models
of Okay et al. (1999) and Wong et al. (1995), all faults show considerable normal
throw and upward fault-tip propagation, suggesting that the entire negative flower
structure is in a state of wholesale crustal extension. We propose that this extension
is partitioned between the basins and ridges. The basins represent relatively
intact detached blocks, whereas ridges serve as strike-slip transfer zones between
the basins linking the prominent sidewall faults.
The occurrence of a highly elongate, in-line negative flower structure above
a centrally positioned buried master fault precludes an origin as a classic pull-apart
basin. This conclusion is supported by the internal architecture of the flower
structure where various fault elements are oriented exactly opposite to the geometry
expected for a right-handed, releasing strike-slip system (Fig.
3A and 3C). Whereas the dimensional aspect ratios of ~3:1 for the individual
basins are compatible with the ratios observed for classic pull-apart basins (Mann
et al., 1983), the overall ratio of ~6:1 for the in-line flower structure
as a whole is anomalous. The geometry of an in-line negative flower structure
conforms better with models of basin development related to transform-normal extension
(Fig. 3B; Ben-Avraham
and Zoback, 1992). However, the presence of faults oblique to the sidewall faults
is not a feature of such models.
This segment of the North Anatolian transform fault has been in transtension since at least 5 Ma (Armijo et al., 1999), acting as a relatively "soft" transform margin, where deformation is distributed across a linked network of strike-slip and extensional faults. We propose that the east-trending normal faults delineating basin 3 record this extension in the region where the strike-slip system feeds crustal material into the flower structure. Conversely, basins 1, 2, and 4 record the progressive feed-through and counterclockwise rotation of the crustal material that has progressively slipped into the zone of transtension since the Pliocene. The rotation of the crustal blocks is allowed by the southward retreat of the southern sidewall of the flower structure, as exemplified by basin 5, and is accommodated by strike-slip along the faults in the ridges. The ridges and basins act like rotating domino blocks within the envelope of the flower structure above the centrally located master fault. It is noteworthy that microseismicity in the Marmara Sea region is concentrated in swarms, situated along the western and eastern edges of the principal deformation zone (Crampin and Evans, 1986) where the greatest displacement incompatibilities should occur.
It is clear that the highly anomalous, intricate architecture of the Marmara
Sea flower structure creates a challenging kinematic problem, particularly as
to how transtension is geometrically accommodated along the buried master fault
and how seismicity is partitioned between the master fault (seismic slip) and
the principal deformation zone (predominantly aseismic slip). Further, the fault
patterns that we describe here point to an alternative deformation style and architecture
for transtensional basins that is not represented in existing literature (Fig.
3C), and that may be an important element of shallow-level continental transform
systems and microplate sutures.
We thank Erol Izdar (director, Piri Reis Foundation for Maritime and Marine
Resources Development and Education), Orhan Uslu (director, IMST), and the officers
and crew of the RV Koca Piri Reis. Funding and in-kind support were provided
by the Natural Sciences and Engineering Research Council of Canada, the Piri Reis
Foundation, and the Geological Survey of Canada. We thank Kevin Burke, Mousumi
Roy, and Karl Karlstrom for their reviews of the manuscript.
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Manuscript received March 2, 2000; accepted April 12, 2000.
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