Geomorphic evidence for tilting at a continental transform: The Karamursel Basin along the North Anatolian Fault, Turkey

Geomorphology 119 (2010) 221–231

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Geomorphic evidence for tilting at a continental transform: The Karamursel Basin along the North Anatolian Fault, Turkey Alessandro Sorichetta a, Leonardo Seeber b, Andrea Taramelli c,⁎, Cecilia McHugh b,d, Milene Cormier e a

Dipartimento di Scieze della Terra “Ardito Desio”, Università degli studi di Milano, Via Mangiagalli 34, 20133, Milano, Italy Lamont Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964, USA ISPRA High Institute for Environmental Protection and Research, via di Casalotti, 300, Rome, Italy d School of Earth and Environmental Sciences, Queens College of City University of New York., 65-30 Kissena Blvd., Flushing, NY 11367, USA e Dept. Geological Sciences, Univ. of Missouri, Columbia MO, USA b c

a r t i c l e

i n f o

Article history: Received 17 March 2008 Received in revised form 26 March 2010 Accepted 29 March 2010 Available online 7 April 2010 Keywords: Tilting Uplift SRTM DEM River profiles Drainage divides

a b s t r a c t Continental transform boundaries are characterised by regional transcurrent faults that locally slip obliquely and spawn rapidly subsiding and tilting basins. A type example is the North Anatolian Fault (NAF) that accounts for the westward motion of the Anatolian “platelet” relative to Asia at about 25 mm/yr. Many of the basins along the NAF are asymmetric half grabens that border a strand of the NAF on the extensional side of a fault bend and tilt obliquely toward the fault. A prominent example is the 17 km-long Karamursel half graben south of the NAF in Izmit Gulf, one of the starved basins flooded by the Marmara Sea. A tilt rate of 3°/10 kyr has been proposed for the submerged part of the basin within 2 km south of the NAF, on the basis of a tilted early Holocene paleo-shoreline. Very rapid tilt and subsidence have been reported for similar basins along the NAF, but the locus of tilt shifts relative to the basin and is short lived at any one place. We find evidence of recent tilting from surface flow patterns on the steep flank of the basin above the southern coastline, up to 6–8 km from the NAF. Drainage divides between 13 rivers mark a northward 8–10° tilted sub-planar surface that extrapolates down into the basement below the progressively tilted sediments of the basin. River profiles are only slightly concave below this surface. We interpret this as the bevelled surface that preceded tilting, and the immature comb-like drainage to be symptomatic of recent tilt. On the north flank of the basin, the drainage is equally steep but markedly different. It suggests a backward eroding fault scarp. Northward drainage into the Black Sea suggests a subtle but regional northward tilt of the Kocaeli Plateau, possibly a flexural response to unloading along the fault. Along the western NAF, rapid progressive tilting of sediments is typical of submarine transform basins. They provide ground truth for investigating the geomorphic effects of rapid tilting on land. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Continental transform boundaries tend to concentrate motion on a rapidly slipping master fault. These faults do not create relief at a regional scale, but they can produce relief locally. While pure transcurrent motion can create relief by offsetting pre-existing landforms, many foci of relief along transform boundaries have been related to local secondary tectonics that include a vertical component of deformation (Mann, 2007). The North Anatolian Fault system is a continental transform with active basins and ridges (Barka and Kandinsky-Cade, 1988; Wesnousky, 1988; Koral, 2007; Gürbüz and Gürer, 2008). Localised uplift and/or subsidence along strike-slip faults are often associated with anomalies in fault geometry. A bend where the local ⁎ Corresponding author. Tel.: +39 06 61570477, +39 06 50074096 (Office); fax: +39 06 61570543. E-mail address: [email protected] (A. Taramelli). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.03.035

fault strike differs from the average strike of the fault occurs as a common geometric asperity. These bends are often the result of lothologic and structural anomalies on one side of the fault and thus are fixed to that side. They therefore move at the speed of the transform relative to the other side of the fault and cause rapid but transient uplift or subsidence on that side (Seeber et al., 2004). If this occurs, uplift may turn to subsidence very rapidly and can shift faster than geomorphic processes can reach equilibrium with tectonic forcing. Continental transforms may therefore offer opportunities to study transient geomorphic responses to short tectonic events. We test this hypothesis along the north branch of the North Anatolian continental transform fault (NAF) in Izmit Gulf at the western end of the composite rupture in the catastrophic 1999 earthquakes (Fig. 1). Izmit Gulf, on the western side of the Marmara Sea (Fig. 1), is a 50 km-long linear depression along the NAF. Recent swath bathymetry surveys and shallow-penetration seismic profiles (Polonia et al., 2002; Kuşçu et al., 2002; Cormier et al., 2006) show that the Izmit segment of the NAF is continuous and much straighter than originally

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Fig. 1. Tectonic framework for the eastern Mediterranean area (A). The Eurasian and African plates are converging slowly and the Arabian plate is moving north faster than Africa and colliding with Eurasia. The Anatolian platelet is being squeezed out to the west and is also being pulled by rollback subduction along the Hellenic arc. The dextral North Anatolian Fault (NAF) and sinistral Eastern Anatolian fault (EAF) are continental transform faults accommodating most of this motion. In the Marmara Sea area (B), the NAF has prominent bends and is flanked by many basins and highs (concentric ellipses). West of 31E longitude, the NAF bifurcates into a northern and a southern branch (NAF-N and NAF-S). The dotted trace marks the two M7.4 and M7.2 earthquake ruptures in 1999. The box centred on Izmit Gulf and the Kocaeli Plateau marks the area in Fig. 2A.

postulated, generally following the bathymetric axis of the gulf. Despite this first-order linearity in map view, several starved basins and ridges characterise the gulf. The string of active basins along the Izmit segment resembles in morphology and structure the Marmara trough, even if smaller in size, and is characteristic of the NAF elsewhere in northwestern Turkey. In the gulf and to the west, these basins are starved of sediment and the transform is mostly submerged. In some of the basins, such as the central (Karamursel) basin in Izmit Gulf, the fault tracks the steeper submarine edge of the basin's floor in a typical border-fault configuration. It has long been accepted that basins grow along transcurrent faults as a result of geometric complexities on these faults (Mann, 2007 and references within). The nature of this coupling remains unclear, however, in part because its effects tend to be time transgressive and thus 4-dimensional. The large step-overs and pull-apart geometries originally postulated for the NAF in Izmit Gulf (e.g., Barka and Kandinsky-Cade, 1988) were not found, though the fault trace displays subtle bends that have been proposed to modulate vertical motion along the fault (Cormier et al., 2006).

The purpose of this work is to examine landforms around Izmit Gulf so as to test and expand upon results obtained by Cormier et al. (2006) from the submarine data. We qualitatively compare drainage morphology on opposite flanks of the gulf and regionally on the Cocaeli Plateau to the north. Compared to the drainage on the plateau, the drainage along the gulf is clearly immature. This is in agreement with exceptionally rapid vertical tectonics along the NAF. We also show that drainage profiles into the gulf from the north and from the south are systematically different, despite similarities in drainage areas and overall gradient. This drainage asymmetry is consistent with the deformation across the fault that is inferred from submarine morphology and structure, namely northward tilt toward the fault on the south side and relative uplift on the north side (Kuşçu et al., 2002; Polonia et al., 2002, 2004; Cormier et al., 2006). Landform analysis for the south flank of the gulf supports the hypothesis that an originally planar surface was tilted and dissected. Our strategy is to compare stream profiles on this surface with profiles of the water divides separating them. In the early dissection of the hypothetical tilted surface, rivers and divides are expected to mark

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loci of the greatest and the least erosion, respectively (Phillips, 2002). All river profiles are remarkably similar and only slightly concave upward, which suggests equally immature drainage (Hack, 1973). The divides fit closely to the same inclined plane above but close to the rivers. Finally, the plane is extrapolated below the coastline and matches the tilted surface inferred to the sediment floor in the basin. While none of these observations are conclusive individually, the combined evidence supports the hypothesis that the ongoing tilting of the southern flank of the basin is continuous from the submarine environment onto land. 2. Regional tectonic context The Marmara Sea region of northwest Turkey is traversed by the NAF, a 1500 km-long continental transform that separates the Asian plate from the westward moving Anatolian platelet (Fig. 1; e.g., Reilinger et al., 1997; Şengör, 1979; Flerit et al., 2004; Şengör et al., 1985; Armijo et al., 1996, 1999; Saroglu, 1988). The 150 km-long Marmara segment is the only portion of the NAF that has not yet ruptured. The velocity field inferred from GPS measurements shows that Anatolia is a relatively rigid block. Most of the motion is confined to the NAF, where the slip rate is about 25 mm/yr (Noomen et al., 1996; McClusky et al., 2000). Toward the Aegean extensional area in the west, however, the NAF branches into a multi-stranded system. According to geodetic data, the northernmost branch — the fault examined in this study — is consistently accommodating more slip than all the other branches combined, with a total of about 20 mm/yr accommodated in our study area (Figs. 1 and 2; Meade et al., 2002; Flerit et al., 2003). The NAF developed during the Plio-Pleistocene along pre-existing Paleogene sutures, and probably re-activated persistent weaknesses in the continental lithosphere (e.g., Şengör et al., 1985). The evolution of this young structure may include shifts from one pre-existing fault to another and externally imposed changes due to instabilities in the tectonic processes driving Anatolia. Not surprisingly, estimates of the onset and accumulated displacement in the current regime of the NAF vary widely: Armijo et al. (1999) estimated an onset time of 5 Myr and a displacement of 80 km, Seeber et al. (2004) estimated an onset time of 1.4 Myr and a displacement of 28 km, Le Pichon et al. (2001) estimated an onset time of 200 kyr; and Yaltirak et al. (2002) a displacement of 40 km,. The issue may be moot, however, because both age and displacement may vary among different strands of the NAF. The Izmit Gulf and its basins may be younger than the NAF and the larger basins in the Marmara Sea, and may represent an eastward propagation of transtension along the NAF (Şengör et al., 1985). 2.1. Basin formation in Izmit Gulf Izmit Gulf is a narrow, 40 km eastward embayment of the Marmara Sea reaching into Asia. The northern branch of the NAF enters the sea at the eastern end of Izmit Gulf and continues westward as a predominantly submarine fault along the Marmara Sea. It continues across the northern Aegean Sea to its terminus offshore Greece. Through the entire marine segment, the lithosphere remains continental. NAF-related basins occur in the Marmara Sea, to the west where they are sediment starved, and also to the east where they are intramountain. High relief, therefore, characterizes the trace as well as the top of the basement along the NAF in northwest Turkey. NAFrelated basins in the Marmara Sea span a wide range of sizes. The three main basins are several tens of kilometres long and have maximum water depths of about 1.2 km (Fig. 2). Three smaller basins are recognised along the Izmit Gulf (e.g., Cormier et al., 2006). The focus of this study is the largest and deepest of these basins, the Central or Karamursel Basin. This basin is about 15 km long, 5 km wide and has a maximum water depth of 215 m (Fig. 3).

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Before the plethora of new data following the 1999 earthquake, the sea-floor geometry of the NAF beneath the Izmit Gulf and other submarine segments was mostly uncharted. Structural models for the Izmit Gulf based on the sparse data tended to be mostly concept driven and ranged from classic pull-aparts (Barka and KandinskyCade, 1988) to negative flower structures (Okay et al., 2000). Nearly complete high-resolution bathymetric coverage and densely spaced (50–100 m) shallow-penetration seismic profiles in Izmit Gulf (Polonia et al., 2002; e.g., Fig. 4) were critical for a sinoptic structural interpretation of the shallow portion of the NAF in Izmit Gulf (Cormier et al., 2006). The fault trace exhibits an extensional jog and possible pullapart in the easternmost portion of the gulf but, despite pronounced bathymetric relief, it is surprisingly continuous along the rest of the gulf. Rather than side steps, bends along the NAF seem to play a major role in several of the transform basins. The high-resolution data displayed significant evidence of tectonic activity, including numerous faults and tilted and/or folded strata, as well as evidence of sea-floor rupture, such as earthquake-related mass wasting and homogeneities (McHugh et al., 2006). The superposition of gravity-induced collapse on tectonic deformation is primarily responsible for any remaining subtle ambiguities regarding the geometry of the main fault near the sea floor (e.g., Kuşçu et al., 2002; 2005). The western portion of the 1999 main shock rupture in Izmit Gulf has been interpreted from seismological data to be vertical and purely dextral (Delouis et al., 2000; Li et al., 2002; Çakir et al., 2003). These data, however, tend to average kinematics over large portions of the fault at mid-crustal depths where most of the seismic moment is released, and therefore tend to be insensitive to oblique motion in the upper few kilometres of the crust. Bends on the NAF are generally subtle, yet they are systematically related to the vertical component of motion. This is seen in both the bathymetric data and the growth structure (Cormier et al., 2006). Portions of the fault with an oblique-extensional orientation relative to the average strike of the fault tend to be continuous, single stranded, and to coincide with the axis of maximum subsidence. The instantaneous subsidence rate at a given place depends on local fault geometry, and this rate can be expected to change in time as the dextral motion moves a given location to different fault geometries. Total subsidence is determined by the accumulated effect of previous kinematics and the deepest part of the basins, where syntectonic sediment reaches the greatest depth, is generally the location where the basin has been subsiding for the longest amount of time. In contrast, the fastest subsidence usually occurs where subsidence is just beginning near the bend and may be short lived at any given place. Both transtension and subsidence seem to be accommodated by the main strand of the transform, which is thus inferred to be significantly non-vertical and to slip obliquely. The fault kinematics described here lead to asymmetrical tilting, which is characteristic of basins along the NAF and other transforms (Okay et al., 2000; Mann, 2007; Seeber et al., 2010). 2.2. Morphology and tectonic deformation in the Karamursel Basin Cormier et al. (2006) identify a paleo-shoreline/shelf that they associate with the last glacial low-stand at the eastern end of the Karamursel Basins (Figs. 3 and 4). This paleo-horizontal surface was formed when the basin was lacustrine and sea level was below the sill-depth of the basin. The most recent episode lasted several tens of thousands of years and ended 10 kyr BP (Çağatay et al., 2000). Steady water depth during the relatively long lacustrine interval and rapid rise of the water in the early Holocene account for the strong geomorphic signature of the shoreline and its preservation by rapid transgression. Thus, the paleo-shoreline provides a key marker to evaluate tilting and subsidence accounting for the basin. The shoreline shows that the northern side of the fault has not tilted nor subsided and is relatively stable, except for a shallow-rooted gravity collapse of the scarp near the fault. In contrast, the southern side of the fault has

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Fig. 2. Study area landforms around the narrow, 50-km long Izmit Gulf of the eastern Marmara Sea. Topography is from SRTM data and is rendered as if illuminated from the NW. The numbers refer to the rivers that were analysed in each of three groups: flowing north through the Kocaeli Plateau into the Black Sea (A); flowing south into Izmit Gulf (A); and flowing north into the Izmit Gulf (shown at higher resolution in B). The same numbering is used in Fig. 5. Note that most of the Kocaeli Plateau drains northward.

tilted 3° toward the fault and has subsided locally 90 m since the drowning of the shoreline 10,000 years ago. This interpretation implies an average Holocene tilt rate of 0.0003°/yr and a subsidence rate of 9 mm/yr near the fault. During the same time period, the fault accumulated more dextral motion, in the range between 200 m (from extrapolating current geodetic strain; McClusky et al., 2000) and 110 m (from a channel offset 25 km west of the tilted shoreline; Polonia et al., 2002). An oblique fault slip on the steep shallow part of the NAF can cause these rates. This is where the motion vector deviates from horizontal between 39° and 24°. Oblique motion and extremely rapid subsidence are typical of the youngest portions of the transform basins along the NAF (Seeber et al., 2004, 2006; Pantosti et al., 2008). The onset of subsidence is thought to be progressively younger to the east, reaching the present at the easternmost structural expression of the basin. The basins are, therefore, generally

older than the onset of subsidence in most parts of the basin and the age is better represented by the along-strike dimension of the basins. The oblique motion on steep faults tends to create a steep topography that leads to gravitational instability and collapse. A good example of this is seen in the submarine morphology of the Karamursel Basin, which stems from both tectonics and slumping. Depth profiles across the basin are consistently asymmetric with respect to the fault and the turbiditic floor of the basin is consistently south of the fault (e.g., Fig. 4). This relationship suggests that the NAF serves as the border fault of the basin. Instead of a steep scarp, however, the northern side of the fault exhibits a broad semicircular amphitheatre facing south and is characterised by a bumpy (“washboard”) morphology (Cormier et al., 2006). This feature has been interpreted to be the gravitational collapse of the footwall scarp along the NAF and en-echelon anticline ridges cover the fault at the compressional toe of this collapse (Figs. 3 and 4).

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Fig. 3. Submerged and exposed terrain of the Karamursel Central Basin in Izmit Gulf. The straight coastline on the southern flank of the basin is associated by many authors with a fault (e.g. Barka and Kandinsky-Cade, 1988), but it can also be accounted for by tilting a planar surface. Lines upstream of this coast mark water divides that separate drainage basins flowing north into Izmit Gulf. The zero-order divide (see text) marks the upstream border of this drainage and is the outer and thickest line. White lines of intermediate thickness mark first-order divides, which are inferred to have been the least degraded by erosion. The thinnest lines mark higher order divides. The numbering of first-order divides is the same as in Fig. 6.

Fig. 4. Oblique view of the eastern half of the Karamursel Basin (about 5 km N–S and 7 km E–W, located in Fig. 3; from Cormier et al., 2006). High-resolution bathymetry displays details of basin morphology. The shelf/shoreline of the early Holocene lacustrine stage is generally 60 m below current sea level. On the southeast side of the basin, this shoreline is tilted by 3° and is dropped by 90 m. This implies an average subsidence rate of 10 m/ka and tilt rate of 0.3°/ka (Cormier et al., 2006). The fault trace is clearly expressed in the eastern part of the basin, but in the western part the fault is blind and is manifested by a series of en echelon folds, one of which is visible in the foreground.

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These are clearly transpressional structures and may otherwise seem inconsistent with a transtensional basin. However, a similar gravitational footwall collapse and related transpressional folding characterise the area of fastest subsidence in the Western Marmara (Tekirdag) Basin (e.g., Seeber et al., 2004). The oblique slip and ground failure were dramatically demonstrated in both 1999 main shocks, where surface rupture along the southern coast between the Karamursel and Ģolcuk basins (Fig. 2A) included a large north-side-down component and a submarine slide (Cormier et al., 2006). The linear southern coastline and the sharp submarine edge of the southern shelf are prominent features of the Karamursel Basin (Figs. 3 and 4) and they have been interpreted as the manifestations of a north-dipping normal fault that is antithetic to the NAF (e.g., Barka, 1996). But, northward tilting in the basin is inconsistent with a twosided graben. Furthermore, a linear coastline is expected along a tilted planar surface. The north-facing slope below the shelf break is only 15° (note vertical exaggeration in Fig. 4) and may be steeper than the underlying pre-basin bevel surface because of sedimentation along the shelf break (Cormier et al., 2006). This paper provides further negative evidence for an active fault along the southern flank of Karamursel Basin by showing that the flank of the valley, which is predepositional below sea level and pre-erosional above sea level, is still a continuous surface. 3. Strategy and methods 3.1. Topographic attributes from DEMs The comparison between stream and watershed networks is central to our geomorphologic analysis and we use a new approach for deriving these topographic attributes from digital elevation models (DEMs). In this analysis, we use the Shuttle Radar Topography Mission (SRTM) DEM data Version 1 (Farr et al., 2007). In the original format, these data have a resolution of 3-arc-seconds in geographic coordinate system — WGS84 datum. Assemblage and local interpolation was performed by importing tiles into ArcInfo 9.× (©ESRI) using an Arc-Macro Language procedure (Taramelli and Barbour, 2006). The final grid, covering the Marmara Sea region, was projected in the UTM zone 32 N, which has a resolution of approximately 90 m × 90 m over the study area. 3.2. Uniform uplift versus tilt The classic peneplanation theory deals with the uniform uplift of a formerly bevelled area that produces a plateau. Initially, drainage will be energised only around the steep edges of the plateau and rivers will develop typical “two tier” profiles, with a steep central section below a knickpoint. These knickpoints will remain at the plateau elevation and migrate upstream toward the interior of the plateau as erosion deepens the valleys and eventually affects the entire drainage system (Phillips, 2002 and references within; Westaway et al., 2004; Bonow, 2005; Bonow et al., 2006). A plateau formed by uniform uplift can be considered as a special case of an inclined surface formed by uniform tilting (Süzen et al., 2006). Part of this surface may be uplifted and its outer limits may develop drainage and erosion patterns indistinguishable from uniform uplift. The peneplanation theory, however, is inadequate to account for drainage in the interior of this tilted area, which experiences an immediate increase in stream energy and erosion throughout the area. Initially, river profiles will be nearly linear on top of the tilted surface. The rivers will erode down into this surface, forming progressively deeper valleys. Their profiles will become increasingly more upwardly concave until they reach equilibrium asymptotically (Hack, 1973). Rivers that reach the sea without crossing the edge of the tilted area will not develop knickpoints. We propose that the profiles of otherwise similar rivers flowing into Izmit Gulf from the north and south fit into two distinct groups that represent the response to plateau uplift and tilt, respectively. We

test the tilt hypothesis by comparing the morphology of the southern flank both above and below sea level, where tilting is recorded in the syntectonic sediment. We find continuity between the basement below the sediment and the pre-erosional surface above sea level as the hypothesis requires. 3.3. Drainage divides as anchors of the envelope surface An “envelope” is defined as the smoothest possible surface that matches the topography along water divides but is above topography everywhere else. Such a surface was conceived in an attempt to reconstruct pre-incision landforms (Stearns and Thurber, 1965). The streams can be ordered in a hierarchy according to their position in the drainage system (Shreve, 1966; Strahler, 1980) and drainage divides separating these streams can be similarly ordered. Using this hierarchy, an envelope can be anchored solely to water divides up to a particular order (Mark, 1979, 1981, 1988; Werner, 1972a,b, 1982, 1988, 1991, 1993; Wharton, 1994; Rana, 2006). We map drainage divides from the DEM and we order them according to “specific elevation”: the higher the elevation the lower the order (see below). We use only first-order divides to construct the envelope. Rather than contouring these divides to construct the surface, however, we test the hypothesis that the tilted pre-erosional surface is planar and we simply project them along the linear southern coastline of Karamursel, which we identify as the strike of this surface (Fig. 6). This test is successful as the projection reveals the divides to be nearly co-planar. If erosion is minor and these divides are indeed close to the preerosional planar surface, they imply nearly uniform tilt. This hypothesis incorporating youthful drainage is supported by a small gap between the envelope-marked by the divides-and the sub-envelope (Stearns and Thurber, 1965) that is marked by the river profiles. The rivers are nearly linear and perpendicular to the coast and thus their profiles can be superimposed on the divides as projected in Fig. 6. Our strategy follows that of Pavich (1989), who considered drainage divides and argued for oceanward tilting of the Appalachian piedmont in southeast North America. The North American passive margin setting is qualitatively similar to the setting in the Karamursel Basin, but the tilting in Karamursel is at least an order of magnitude shorter based and four orders of magnitude faster. Furthermore, the geomorphic response to the tilting in Karamursel is still in transition whereas the piedmont is in equilibrium. 3.4. Hierarchy of drainage divides A representation of a drainage system incorporates a finite number of hierarchical upstream branches into progressively smaller tributaries (Shreve, 1966; Strahler, 1980; Mark, 1988; Rana, 2006). “Stream order” refers to the position of a stream in this hierarchy and each stream in the system is classified according to the number of tributaries it possesses. We propose a similar classification for drainage divides. In this classification, a higher numeric order for a drainage divide is designed to reflect a higher average elevation. The “zero-order” divide is the most regional divide and bounds the drainage basins we mapped on the south flank of the basin. Branching points along this zero order divide connect it to the sea via higher order divides. To identify further branching down these ridges, we must distinguish between them. “First-order” identifies drainage divides that have the greatest “specific elevation integral”, i.e., the greatest area under their longitudinal profile divided by their length (average elevation): 0

∫ hdL = L; L

where h is the elevation along the profile and L is the horizontal length of the profile. The same procedure is used to identify the “second-order” profiles branching off from the first order divides, and so on. In the case

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of a tilted planar paleo-surface, the average elevation of these drainage divides reflects the degree of erosion: more erosion occurs for lower elevation and thus they have a higher order. The less degraded drainage divides are the closest to the first-stage basin at the onset of the river erosion process and thus are the best candidates to anchor a planar envelope surface. 3.5. Lithology and structure of the drainages Distinct geologic settings are found across the Izmit Gulf. This is due to the displacement accumulated by the NAF and because the current plate boundary reactivated the Eocene–Oligocene aged Intra-Pontide suture (Şengör and Yilmaz, 1981). The Istanbul terrane north of the NAF and the Sakarya terrane to the south have distinct rock assemblages. The suture zone along the NAF contains typical ocean–affinity assemblages dominated by flysh. The north- and south-draining drainage systems we examined on the Kocaeli Plateau north of the NAF are both on the Istanbul Terrane and are mostly on deformed Mesozoic rocks. The drainage system south of the NAF is almost entirely on the Eocene– Oligocene melange of the suture zone. The differences in lithologic assemblages and deformation histories of the rocks involved in these drainages are unlikely to systematically bias geomorphic development. The melange suture rocks are dominated by flysh, which is a rock type containing lithologies notoriously weak to erosion. Differences in lithology and structure are likely to play only a minor role in differentiating the drainages on the opposite sides of the gulf as compared to the effect of differences in the deformation patterns across the NAF. Erosional weakness on the south flank of the gulf is the one factor that stands out and that is expected to hasten maturing of the drainage in the area, thus reinforcing the geomorphic evidence that the drainage south of the NAF is very young. 4. Results and discussion 4.1. Longitudinal river profiles Longitudinal profiles of the main rivers were mapped in three distinct drainages within the longitude range of the Karamursel basin: into the Izmit Gulf from the north and from the south, and northward across the Cocaeli Plateau into the Black Sea (Fig. 5). Profiles display different characteristics in each of these three drainage areas, and we attribute them to differences in the deformation patterns on the opposite sides of the NAF in Izmit Gulf. Despite similarities in length, steepness, and immaturity, the drainages into the Izmit Gulf from the north and the south are clearly asymmetric. The southern rivers have remarkably similar and overlapping profiles, while the profiles of the northern rivers differ from each other (Fig. 5). The southern profiles all exhibit the same subtle upper facing concavity (Fig. 5A), while the northern profiles are two-tiered and convex-upward, with a bench and downward steepening at an elevation of 300–400 m. All the northern profiles with altitudes higher than 300 m show similar benches (knickpoints), except for number 8, which may be too short. Lower benches are present in the shorter rivers and none of the southern rivers exhibit such knickpoints. The northern rivers that flow into the gulf exhibit immature profiles typical of plateau uplift (Fig. 5; Kirby et al., 2003). The highest bench probably marks the plateau edge, while lower benches may be paleoshorelines that are now elevated above sea level. The current shoreline is about 5 km north of the NAF and has formed a large bench or shelf. The pronounced retreat of the topographic front from the fault can be partly ascribed to coastal erosion and shelf widening. Coastal erosion is often more effective than erosion by short drainages, and therefore it can lead to hanging valleys. Gravitational footwall collapse is likely to be an additional cause for the northward retreat of the scarp. A steep south-facing scarp caused by oblique slip on the NAF would be expected to form in the sediment starved

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Karamursel basin. The collapse of this scarp caused by uplift of the north side toward the subsiding south side of the basin is suggested by the “washboard” morphology of the seafloor north of the fault, the active compressional folds forming along the trace of the fault (Fig. 4; Cormier et al., 2006) and by similar examples of footwall collapse elsewhere in the Marmara Sea (Seeber et al., 2004). Uniform uplift, however, cannot account for the Kocaeli Plateau rivers that flow northward into the Black Sea. These rivers are much longer than those flowing south and display relatively mature equilibrium profiles, but their lower reaches are remarkably linear and steep (Fig. 5A). Rather than the uniform plateau uplift that may have affected regionally northwest Anatolia (Westaway et al., 2004), the multifaceted drainage asymmetry across the Kocaeli Plateau instead suggests northward tilting (Yildirim and Emre, 2004). This tilting is subtle and more broad than the tilting along the southern flank of the gulf. Both instances of tilting are related to the NAF, even though the mechanisms we propose are radically different. 4.2. Linear doming along the NAF A chain of basins along a strike-slip fault characterises fault-zone morphology in Izmit Gulf, the Marmara Sea, and elsewhere along the NAF. The flanks of these transform basins tend to be narrow and have shorter drainage channels flowing into them than the drainage flowing away from the basins on the outer side of the water divides ringing the Marmara Sea. This morphology suggests that regional doming compensates both subsidence and mass deficit in the basins along the NAF (Paluska et al., 1989; Okay and Okay, 2002). This doming may be a flexural response to unloading along the fault (Sorichetta et al., 2005). Further, doming centred on the NAF in Izmit Gulf can account for northward tilting of the Kocaeli Plateau, which was otherwise tectonically stable during the Quaternary (Fig. 2; Emre et al., 1998). South of the gulf, however, the crustal wedge between the northern and southern branches of the NAF is part of the plate boundary zone and is much more tectonically active than the Kocaeli Plateau. Therefore, any regional doming in response to subsidence localised along the faults would probably be masked by local tectonics (Fig. 2). 4.3. Progressively tilted turbidites Cormier et al. (2006) show a 3° tilted submarine surface that they interpret as the marker for the shoreline-shelf of lake Karamursel that was flooded and drowned by rising sea level about 10 kyr ago. We test the implied and astonishingly rapid tilting by applying it to the progressively tilted turbidite sequence within the basin (Fig. 7). Assuming a tilt rate, the present dip of a horizontally deposited turbidite specifies its age. Applied to the profile across the depocenter shown in Fig. 7, this simple concept yields a sedimentation rate of 8 mm/yr for a tilt rate of 3°/10 kyr, which is comparable to the deposition rate of 5 mm/yr estimated from the stratigraphy in a core on the western side of the basin (McHugh et al., 2006). Conversely, if we assume the core-derived 5 mm/yr deposition rate, we obtain a tilt rate 2°/10 kyr. Comparing this with other Marmara Sea basins, the vertical component decreases from the causative fault bend and the 3°/10 kyr is likely to apply only to the eastern part of the basin where the tilted shoreline was observed. The growth structure in the eastern part of the basin, therefore, requires a very high tilt rate. In Fig. 7, eleven clear reflectors are imaged above the layer dipping at 0.8°. This layer is 2.5 or 4 kyr old according to the tilt and sedimentation rates, respectively. Their average interval is 230–360 years. According to cores from the basin, these reflectors are interpreted as turbidites and they probably derive from mass wasting on the steep flanks of the basin that was triggered by large earthquake ruptures along the NAF through the basin (McHugh et al., 2006). The shallowest layer shown in Fig. 7 is significantly tilted and therefore is likely to precede the 1999 earthquake. This observation is consistent with the lack of seafloor

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Fig. 5. Profiles of rivers affected by vertical tectonics along the NAF (Fig. 2). Of the rivers north of the NAF (A), the rivers flowing into the Black Sea (thick and gray) are much longer than the ones flowing into Izmit Gulf (thin and black). The profiles of rivers flowing into Izmit Gulf from the south (B) are slightly concave and overlap closely, except for 1, which is affected by the Hersek Peninsula. Rivers reaching the gulf from the north (some are in B for comparison) are similarly steep but tend to be convex and more irregular. Vertical scale is the same, but horizontal scale is 10× larger in A than in B.

rupture through the entire basin during the latest large earthquake (Cormier et al., 2006). The 230–360 year average interval is longer than the recurrence of damaging earthquakes in the Izmit Gulf but is consistent with these data if only a subset of earthquake ruptures reach the seafloor and trigger large turbidites.

Fig. 6. Two parallel profiles combining submarine morphology and structure of the Karamursel Basin (located in Fig. 3; from Cormier et al., 2006). First-order water divides are also projected and are presumed to be the least degraded by erosion (numbering as in Fig. 3). These divides cluster on the same sub-planar surface, which is slightly steeper toward the east than the west. Divides 9 and 10, the furthest to the east, may be affected by footwall uplift. Extrapolated downdip, this surface closely matches the inferred structural floor of the basin. The average river profiles (dashed) are from Fig. 5 and can be displayed in this projection because the rivers are linear and normal to the projection.

4.4. Asymmetric tilting and half grabens after bends along the NAF Submarine and land morphology along the Izmit Gulf is indicative of rapid differential vertical motion that has been ascribed to the oblique slip associated with bends on the NAF (Cormier et al., 2006). The Karamursel Basin is a typical oblique half-graben associated with bends along the fault (Seeber et al., 2010). It is classified as a halfgraben because the south side of the fault is subsiding and tilting towards the fault; it is called oblique because current subsidence is fastest at the eastern end of the basin near the fault bend at Golcuk (G in Fig. 2A). Subsidence is then asymmetric along the fault as well as across the fault. The submarine shoreline, which is dipping at 3°, is mapped up-dip to a depth of about 50 m and 0.5 km from shore without evidence of a rollover (Fig. 15 in Cormier et al., 2006). Very rapid tilting can then continue updip above sea level. The uplifted shoreline sediments indicate a rapid uplift of the subaerial flank of Karamursel Basin (Paluska et al., 1989). If the tilting is continuous across the shoreline, the drainage on the exposed flank of the basin is expected to be young and erosion limited. Immature river profiles shown in Fig. 5 qualitatively confirm the presence of limited erosion. We therefore postulate that some of the original surface may be preserved along the water divides, where the least amount of erosion is expected (Fig. 3). The water divides fit within ∼100 m to two planar surfaces normal to the profile in Fig. 6. One surface is dipping to the west at 8° and one is dipping to the east at 10°, separated roughly midway along the southern shoreline (Fig. 3). These surfaces also fit the inferred tilted basement that makes up the floor of the basin (Figs. 6 and 7). Furthermore, the subtly steeper surface derived from the eastern divides correlates with the deeper eastern part of the basin (divides 5–10 in Fig. 3). This coherent variation in tilt and in the depth of the basin may be associated with subtle changes in the strike of the NAF (Cormier et al., 2006). In summary, the drainage morphology on opposite flanks of the Karamursel Basin in the Izmit Gulf suggests plateau uplift on the north

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Fig. 7. CHIRP profile across the depocenter in the eastern part of the Karamursel Basin (located in Fig. 3; from Cormier et al., 2006). If the tilt rate is 3°/10 kyr, the bed dipping 0.8° is 2.5 kyr old. This implies a depocenter sedimentation rate of 8 mm/yr, which is in the expected range, and an average time between reflectors of two centuries, which is similar to the repeat time of destructive earthquakes on this segment of the NAF. The inset compares turbidite thickness with the associated tilt increment; both are presumably the result of a local large rupture on the NAF. The correlation between tilt and thickness suggests that both tilt deficit and unstable sediment accumulate steadily since the last event. The floor of the basin dips 0.16°, suggesting that the 1999 earthquake tilted this part of the basin, but did not trigger a significant turbidite.

flank and northward tilting on the south flank. This is consistent with the tectonic model proposed on the basis of submarine data. The southern flank of the Karamursel Basin is controlled by a paleohorizontal surface that is tilted quasi-uniformly from the NAF to an elevation of 600–800 m for at least 7–8 km south (Fig. 3). The rapid tilt rate inferred from a tilted shoreline and progressively tilted turbidite sequences pertains only to the eastern end of the basin. Because a fundamental characteristic of transform-bend related basin is to be time transgressive in their growth pattern (Seeber et al., 2010), other parts of the basin may have begun tilting earlier, and may currently be tilting at a slower rate or not at all. Drainage on the north side of the basin exhibits two-tier profiles with knickpoints at 300–400 m asl, which may mark the Kocaeli Plateau before the NAF. This elevation is likely to combine pre-NAF elevation of the plateau and vertical motion associated with unloading along the fault. An obvious task that is beyond the scope of this work is to explore the possible subsurface structure and transform kinematics that could produce the asymmetric surface deformation pattern observed both in the submarine environment and on land. 5. Conclusion 1. Rapid tectonic tilting is common along the North Anatolian continental transform (NAF) and produces characteristic syntectonic signatures in erosional landforms and in sediments. A tilting bevelled surface leads to a basin and a ridge that may be below and above sea level, respectively. Prominent submarine paleohorizontal markers, such as turbidites and shorelines are progressively tilted. On land, drainage on a rapidly tilting surface develops parallel narrow river basins characterised by steep lower reaches. The sub-envelope marked by these rivers is only slightly depressed relative to the envelope and the pre-erosional surface marked by drainage divides separating them. 2. Izmit Gulf is a linear depression along the NAF composed of distinct basins. Patches of oblique slip on the master fault are primarily responsible for the vertical tectonics forming these basins along the transform. The structural asymmetry of these basins relative to the NAF is clearly expressed by submarine morphology as well as by the drainage system on opposite sides of the gulf. The south flank of

the gulf, corresponding to the Karamursel Basin, shows the effects of basin formation by tilting toward the NAF. The drainage on the Kocaeli Plateau north of the gulf is symptomatic of regional doming centred on the NAF. Long rivers flowing north across the plateau display relatively steep lower reaches suggesting gentle northward tilt toward the Black Sea. In contrast, rivers flowing south into the gulf have short two-tier profiles with prominent knickpoints 300–400 m asl. They probably mark progressive plateau uplift and a retreating scarp controlled by coastal erosion and gravitational footwall collapse into the basin. 3. The drainage divides of rivers flowing north into the Karamursel (Central) Basin in Izmit Gulf narrowly define two planar envelopes dipping 8° and 10°, respectively, on the western and eastern halves of the basin. These planes project downward as the base of the sediments in the submarine parts of the basin and are thus interpreted to mark the pre-erosional and pre-depositional surface. The rate of tilting near the fault bend thought to be responsible for the basin is constrained by 10 kyr old shoreline dipping 3° north. At the depocenter, ∼2 km west of the shoreline, a tilt rate of 2°/10 kyr is derived from an independent measure of the sedimentation rate (5 mm/yr). Tilt rate may decrease rapidly to the west and vanish or reverse within the basin. The age of the basin can be more directly related to the length of the basin (e.g., 20 km/20 mm/yr = 1 Myr) than to tilt rate. 4. Our results raise challenging issues regarding basin formation and their geomorphic expressions along the NAF and probably along other continental transform. The rapidly tilting flanks of these basins provide natural laboratories to study the development of drainage on rapidly tilting surfaces, both on land and under water. Acknowledgments We thank M. Materazzi, and the referees for their critical reviews that helped to improve the manuscript. We also thank MARMARA2001 scientific parties and Alina Polonia for the bathymetric data. This work was supported by the following grants: Borsa di studio per attività di perfezionamento all'estero — II bando anno 2003, Università degli Studi di Camerino; NSF OCE 03-28119, OCE 03-27273, OCE 09-28447,

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and OCE 03-28119; Fondazione Cassa di Risparmio di Foligno Award #1, Università degli Studi di Perugia — Corso di Laurea in Protezione Civile. When this study began A. Sorichetta was affiliated with the CUNY Queens College's School of Earth and Environmental Sciences and the University of Camerino, and A. Taramelli was affiliated with the LDEO of Columbia University and with the Department of Earth Sciences of the University of Perugia.

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