Lithospheric structure along the Queen Charlotte margin in western Canada: constraints from flexural modeling

 The Queen Charlotte fault zone along the western margin of Canada, a right-lateral transform, is part of the boundary between the Pacific and North America plates. Combining reflection and refraction surveys and flexural modeling, we place limits on the amount of underthrusting of the Pacific plate beneath the North America plate. Results from our two-dimensional elastic modeling suggest 10–15 km of underthrusting along the northern Queen Charlotte Islands, in agreement with the amount of underthrusting inferred from plate motion models. Received: 12 June 1995/Revision received: 9 December 1995     

Faulting, mass-wasting and deposition in an active dextral shear zone, the Gulf of Saros and the NE Aegean Sea, NW Turkey

Structural, mass-wasting and sedimentation processes along an active dextral shear zone beneath the Gulf of Saros and the NE Aegean Sea were investigated on the basis of new high-resolution swath bathymetric data and multi-channel seismics. A long history of dextral shearing operating since the Pliocene culminated in the formation of a NE-SW-trending, ca. 800-m-deep basin (the so-called inner basin) in this region, which is bordered by a broad shelf along its northern and eastern sides and a narrow shelf at the southern side. The western extension of the North Anatolian Fault Zone (the Ganos Fault) cuts the eastern shelf along a narrow deformation zone, and ends sharply at the toe of the slope, where the strain is taken up by two NE-SW-oriented fault zones. These two fault zones cut the basin floor along its central axis and generate a new, Riedel-type pull-apart basin (the so-called inner depression). According to the bathymetric and seismic data, these basin boundary fault zones are very recent features. The northern boundary of the inner depression is a through-going fault comprising several NE-SW- and E-W-oriented, overlapping fault segments. The southern boundary fault zone, on the other hand, consists of spectacular en-echelon fault systems aligned in NE–SW and WNW–ESE directions. These en-echelon faults accommodate both dextral and vertical motions, thereby generating block rotations along their horizontal axis. As the basin margins retreat, the basin widens continuously by mass-wasting of the slopes of the inner basin. The mass-wasting, triggered by active tectonics, occurs by intense landsliding and channel erosion. The eroded material is transported into the deep basin, where it is deposited in a series of deep-sea fans and slumps. The high sedimentation rate is reflected in an over 1,500-m-thick basin fill which has accumulated in Pliocene–Quaternary times.     

The western Svalbard margin (74°–80°N)

A regional study of the continental margin between the Senja and Molloy-Spitsbergen fracture zones reveals that the transition from continental to oceanic crust occurs in a narrow zone beneath the outer shelf and uppermost slope. The postulated continent-ocean boundary appears to be fault-related consisting of sheared and rifted segments. The marginal structures are compatible with a plate tectonic model in which the southern Greenland Sea opened along a northeasterly propagating plate boundary in the Eocene, whereas the northern Greenland Sea started opening in the early Oligocene. The main structure at the margin is the Hornsund Fault Zone which probably reflects an old zone of weakness rejuvenated in the Tertiary, first by shear and later by extensional movements. In the early Tertiary local transpressional and transtensional components along the plate boundary are associated with the Spitsbergen Orogeny, emplacement of belts of high-density oceanic crust and tectonism in the western Barents Sea. A complex volcanic rifted margin characterized by the Bjørnøya Marginal High links the predominantly sheared margin segments on either side. The main ridge-like segment of the Hovgaard Fracture Zone was originally part of the Spitsbergen margin. In a regional sense, the Hornsund Fault Zone demarcates the eastern boundary of the Tertiary sedimentary wedge which reaches a total thickness of more than 7 km. There appears to have been a considerable increase in deposition of sediments the last 5–6 my. Depocentres located seaward of the east-west fjord systems and submarine depressions indicate a relationship between late Cenozoic glaciations and high sedimentation rates.     

Strike-slip tectonic processes in the northern Caribbean between Cuba and Hispaniola (Windward Passage)

Marine geophysical data including Seabeam, seismic reflection, magnetics, gravimetry and side-scan sonar have been recently collected along the northern Caribbean strike-slip plate boundary between Cuba and Hispaniola, in the Windward Passage area. The analysis of this comprehensive data set allows us to illustrate active strike-slip tectonic processes in relation to the kinematics of the Caribbean Plate. We show that the transcurrent plate boundary trace runs straight across the Windward Passage, from the southern Cuban Margin in the west (Oriente Fault) to the Tortue Channel in the east. The Windward Passage Deep is thus not an active pull-apart basin, as previously suggested. The plate boundary geometry implies that the motion of the Caribbean Plate relative to the North American Plate is partitioned between a strike-slip component, accommodated by the Windward Passage active fault zone, and a convergence component, accommodated by compression at the bottom of the Northern Hispaniola Margin. On the basis of a correlation with onland geological data, an age is given to the stratigraphic sequences identified on seismic profiles. A kinematic reconstruction is proposed that follows the tectonic unconformities recognized at sea and on land (Late Eocene, Early Miocene, Middle Miocene and Late Pliocene). Each one of these tectonic events corresponds to a drastic reorganization of the plate boundary geometry. We propose to correlate these events with successive collisions of the northern Caribbean mobile terranes against the Bahamas Bank. During each event, the plate boundary trace is shifted to the south and a part of the Caribbean Plate is accreted to North America.     

Forearc structures and tectonics in the southern Peru—northern Chile Continental Margin

SeaMARC II side-scan images, bathymetry, and single-channel seismic reflection data along the southern Peru—northern Chile forearc area between 16° and 23° S reveal a complex region of morpho-structural, submarine drainage and depression patterns. In the subducting plate area, the NW—SE trending primary normal fault system represented by trench-paralleled scarps was incipiently formed as the Nazca Plate was bent in the outer edge and further intensified as the plate approached the trench. The NE—SW trending secondary normal fault system that consists of discontinuous and smaller faults, usually intersect the primary trench-paralleled fault system. Similar to the Nazca Plate, the overriding continental plate also shows two major NW—SE and NE—SW trending fault systems represented by fault scarps or narrow elongated depressions.The submarine drainage systems represented by a series of canyon and channel courses appear to be partly controlled by the faults and exhibit a pattern similar to the onshore drainage which flows into the central region of the coastal area. Two large depressions occurring along the middle—upper slope areas of the continental margin are recognized as collapse and slump that perhaps are a major result of increased slope gradient. The subsidence of the forearc area in the southern Peru—northern Chile Continental Margin is indicated by: a) drainage systems flowing into the central region, b) the slope collapse and slumps heading to the central region, c) the deepening of the trench and inclining of the lower slope terrace to the central region, and d) submerging of the upper-slope ridge and the Peru—Chile Coast Range off the Arica Bight area.The subsidence of the forearc area in the southern Perunorthern Chile margin is probably attributed to a subduction erosion which causes wearing away and removal of the rock and sedimentary masses of the overriding plate as the Nazca Plate subducts under the South American Plate.     

Morpho-tectonic evolution of the ?anakkale Basin (NW Anatolia): evidence for a recent tectonic inversion from transpression to transtension

Onshore and offshore seismic and geologic-morphologic evidence from the wider region of the ?anakkale Basin indicates that this area has been widely exposed to transpressional tectonism, which already commenced in the Pliocene. During this transpressional tectonism, the Gelibolu Fault and the Anafartalar Shear Zone on the Gelibolu Peninsula, as well as the Bozcaada-Biga Shear Zone on the Biga Peninsula were activated. As a consequence, the northern part of the Gelibolu Peninsula, and a broad zone between Bozcaada Island and the Karaburun Peninsula were uplifted to form the northern and southern boundaries of the ?anakkale Basin, respectively. This remained a low-elevation intermontane basin between these two highlands. The original morphology of the ?anakkale Basin may have developed as a coastal and shelf section of the large extensional Marmara Sea Basin at the end of the Late Miocene. This tectonic phase was followed in the Pliocene by the transpressional tectonism of the North Anatolian Fault Zone, which destroyed the initial morphology and formed the present V-shaped basin. The activity of the Gelibolu Fault and the Anafartalar Shear Zone along the northern boundary of the ?anakkale Basin ended in the late Pleistocene with the initiation of the northern segment of the North Anatolian Fault Zone. The tectonism along the northern boundary of the ?anakkale Basin thus shifted from a transpressional to a transtensional regime. Seismic data indicate that the Bozcaada-Biga Shear Zone continues to be active to the present day.     

Morpho-tectonic evolution of the Marmara Sea inferred from multi-beam bathymetric and seismic data

In an initial stage, the Sea of Marmara developed as a graben and, in due course, considerable volumes of sediments were deposited in this basin. Before 200 ka, a new fault (New Marmara Fault) cutting through the whole basin developed, which postdated large sub-marine land sliding in the western part of the basin. This mass movement created the Western Ridge. The initiation of this strike-slip fault indicates that the extensional stress regime was replaced by a new, shearing stress field. In the eastern part of the Marmara Basin, the New Marmara Fault consists of two branches. The northern one replaces the normal faulting at the bottom of the northeastern slope of the basin. As a result, this slope has been rejuvenated. The southern branch is located along the central axis of the basin, forming the major extension of the North Anatolian Fault Zone within the region. Two restraining bends were formed because of the counterclockwise rotation of that part of the Anatolian Block. This resulted the uplifting of the Eastern Ridge and the formation of the positive flower structure within the Tekirdag Basin. The establishment of the compressional regime around the Sea of Marmara also resulted in the northwest–southeast shortening of the initial Marmara Basin.     

Morphology of the Blanco Transform Fault Zone-NE Pacific: Implications for its tectonic evolution

The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to kill the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.     

The 16°40′ S triple junction in the North Fiji Basin (SW Pacific)

During the last three years, the North Fiji Basin (SW Pacific) has been intensively studied on three oceanographic cruises carried out by French, American and Japanese ships. One of the main goals of these cruises was to study by means of precise SeaBeam, SEAMARC II, seismic and magnetic surveys, the active spreading system and its associated hydrothermal processes. The North Fiji basin, bounded by the major Pacific and Indo-Australian plates, shows a complex polyphased tectonic evolution. One of the last phases of this evolution is the functioning since 3 Ma of a NS spreading center in the axial part of the basin. The tectonic instability of the area resulted in a permanent rearrangement of the ridge axis. Among others, the 16°40′ S triple junction is one of the major manifestations of such an instability. Sinistral strike-slip motion 1 Ma ago, along the North Fiji Fracture Zone induced the change in direction of two segments of the axis from NS to N15 and N160. The first segment is characterized by a typical spreading ridge similar to various parts of the EPR, while the second shows an atypical ‘en echelon’ fan-shape opening. The N15 and N160 ridges converging with the North Fiji Fracture Zone constitute the 16°40′ S Ridge-Ridge-Fracture Zone triple junction. The detailed morphologic and kinematic study of this junction allows us to understand one of the mechanisms of the deformation in the North Fiji basin.     

Recent tectonics of the Blanco Ridge,eastern blanco transform fault zone

Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the 150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector estimates of 43 earthquakes (M _w3.8) that occurred along the eastern BTFZ over the last 5 years reveal that the Blanco Ridge is a high-angle right-lateral strike-slip fault, with a small component of dip-slip motion, where the Juan de Fuca plate is the hanging wall relative to the Pacific plate. Furthermore, the Cascadia and Gorda basins are undergoing normal faulting with extension predominantly oblique to the transform trend. Seafloor submersible observations agree with previous hypotheses that the active transform fault trace is the elongate basin that runs the length of the BR summit. Brecciated and undeformed basalt, diabase, and gabbro samples were collected at the four submersible survey sites along the Blanco Ridge. These petrologic samples indicate the Blanco Ridge is composed of an ocean crustal sequence that has been uplifted and highly fractured. The petrologic samples also appear to show an increase in elevation of the crustal section from east to west along the Blanco Ridge, with gabbros exposed at a shallower depth farther west along the southern (Pacific plate side) BR ridge flank. Further supporting evidence for BR uplift exists in the seismic reflection profiles across the BR showing uplift of turbidite sequences along the north and south ridge base, and gravity and magnetics profiles that indicate possible basement uplift and a low-density zone centered on the ridge's Pacific plate side. The BR formation mechanism preferred here is first, uplift achieved partially through strike-slip motion (with a small dip-slip component). Second, seawater penetration along the fault into the lower crust upper mantle, which then enhanced formation and intrusion of a mantle-derived serpentinized-peridotite diapir into the shallow ocean crust, causing further uplift along the fault.     

埕北凹陷沙河街组构造-沉积响应特征与油气勘探意义

通过三维地震、岩芯、测井、录井资料,以“源-沟-汇”思想为指导,分析了埕北凹陷沙河街组构造断裂体系演化及其沉积响应特征,梳理了不同构造演化阶段沉积体系发育类型与时空展布。研究表明,沙河街组沉积期埕北断层、埕子口断层具有典型幕式构造演化特征。沙三段沉积期表现为强烈断陷幕,埕北断层活动强烈,陡坡带沿大型古沟谷发育扇三角洲沉积体系;西斜坡沿断槽型沟谷发育三角洲沉积体系;沙二—沙一段沉积期为走滑改造断陷幕,埕北断层活动明显减弱,埕子口断层活动相对较强,陡坡带和西斜坡沿物源通道均继承性发育三角洲沉积体系。勘探实践验证了砂体发育区为构造-沉积响应的结果,为研究区油气勘探指明方向。     

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a⁴⁰Ar/³⁹ Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.     

青东凹陷断裂特征及其对沉积的控制作用

通过对青东凹陷古近纪的断裂体系和主要同沉积断裂的活动和特征分析,表明区内主要发育NNE、EW、NW向三组断裂体系,NNE向的凹陷边界断裂青东1号断层和EW向的主要二级同沉积断层f1、f2控制了凹陷内次级构造单元的划分,凹陷由北向南依次划分为北部深洼、北部凸起、中部次洼、南部凸起、南部次洼、西南缘缓坡带以及东部陡坡带等7个次级构造单元。研究表明,凹陷内各洼陷的沉降中心在古近纪发生了明显的分异和迁移,这些变化受控于区内主要同沉积断裂的差异活动和演化,青东1号断裂和f3断裂在古近纪的持续活动共同控制了北部深洼的沉积充填,f1和f4断裂分别控制了中部次洼和南部次洼的发育;主要断裂的活动及其组合样式控制着湖底扇、扇三角洲、辫状河三角洲等粗碎屑沉积体系的发育。与生油洼陷相邻的主要同沉积断裂形成的断裂带具备形成砂岩油气藏的各种有利条件,其中东部盆缘断裂带是区内砂岩油气藏勘探最有利的区带。     

Tectonics related to the North Anatolian Fault in the Sea of Marmara: evidence from seismic reflection data

The North Anatolian Fault crosses the Sea of Marmara from east to west. Tectonic features of the Sea of Marmara were studied using multi-channel deep seismic reflection data. The northern branch of the North Anatolian Fault is active as a right lateral strike-slip fault zone and indicates both negative and positive flower structures. The North Anatolian Fault splays into two faults at the Sea of Marmara as a northern branch and north segment of the southern branch. The northern branch named the Main Marmara Fault extends in a complicated manner from the north of the Kapıdağı Peninsula to westward in the Sea of Marmara. The north segment of southern branch extends between the Gemlik and Bandırma gulfs in the south of the Sea of Marmara. In addition, uplift areas arose by compression and a push-up style in between the Kapıdağı Peninsula and the Main Marmara Fault. The North Anatolian Fault is characterized by a negative flower structure in basins and push-up style in uplift areas in the Sea of Marmara. An uplift area arose between the north segment of the southern branch and the northern branch of the North Anatolian Fault. The north segment of the southern branch of the North Anatolian Fault is a strike-slip fault and displays a pull-apart style in the seismic reflection data.     

Submarine canyons on the north of Chiwei Island:influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified： the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea （ECS） slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initia- ted during the middle Pleistocene and guided by F4 that is a N--S trending fault on the slope and F1, a large NW--SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE -trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by FS, a N-S trending fault, diverted and blocked by the submarine terrace.     

Submarine canyons on the north of Chiwei Island: influenced by recent extension of the southern Okinawa Trough

Based on new multibeam bathymetric data and about 300 km long single seismic profiles, three topographic units were identified:the canyons, fractural valley and submarine terrace on the north of Chiwei Island where is a structural transition zone between the southern trough and the middle trough. The Chiwei Canyon and the North Chiwei Canyon are two of the largest canyons in the East China Sea (ECS) slope. Topographic features and architectures of them are described. The study shows that both of them are originated along faults. The evolution and spatial distribution of topographic units in the study area are controlled mainly by three groups of faults which were formed and reactive in the recent extensional phase of Okinawa Trough. The Chiwei Canyon was initiated during the middle Pleistocene and guided by F4 that is a N-S trending fault on the slope and F1, a large NW-SE trending fault on the trough. The pathway migration from the remnant channel to the present one of Chiwei Canyon is the result of uplift of tilted fault block that is coupled to the recent extension movements of the southern trough. The submarine terrace is detached from the ECS slope by the NEE-trending fault. The North Chiwei Canyon, developing during the late Pleistocene, is guided by F5, a N-S trending fault, diverted and blocked by the submarine terrace.     

珠江三角洲的活动断裂与区域稳定性分析

珠江三角洲的活动断裂主要为弱活动断裂,分别属于NW,NE和EW向三组断裂,活动断裂是控制该三角洲断块运动和地震活动的关键。珠江三角洲主要为区域稳定性中等地区。     

Morphology and crustal structure of a small transform fault along the Mid-Atlantic Ridge: The Atlantis Fracture Zone

The Atlantis Fracture Zone (30° N) is one of the smallest transform faults along the Mid-Atlantic Ridge with a spatial offset of 70 km and an age offset of ~ 6 Ma. The morphology of the Atlantis Fracture Zone is typical of that of slow-slipping transforms. The transform valley is 15–20 km wide and 2–4 km deep. The locus of strike-slip deformation is confined to a narrow band a few kilometers wide. Terrain created at the outside corners of the transform is characterized by ridges which curve toward the ridge-transform intersections and depressions which resemble nodal basins. Hooked ridges are not observed on the transform side of the ridge-transform intersections. Results of the three-dimensional inversion of the surface magnetic field over our survey area suggest that accretionary processes are sufficiently organized within 3–4 km of the transform fault to produce lineated magnetic anomalies. The magnetization solution further documents a 15-km, westward relocation of the axis of accretion immediately south of the transform about 0.25 Ma ago. The Atlantis Transform is associated with a band of high mantle Bouguer anomalies, suggesting the presence of high densities in the crust and/or mantle along the transform, or anomalously thin crust beneath the transform. Assuming that all the mantle Bouguer anomalies are due to crustal thickness variations, we calculate that the crust may be 2–3 km thinner than a reference 6-km thickness beneath the transform valley, and 2–3 km thicker beneath the mid-points of the spreading segments which bound the transform. Our results indicate that crustal thinning is not uniform along the strike of the fracture zone. Based on studies of the state of compensation of the transform, we conclude that the depth anomaly associated with the fracture zone valley is not compensated everywhere by thin crust. Instead, the regional relationship between bathymetry and gravity is best explained by compensation with an elastic plate with an effective thickness of ~ 4 km or greater. However, the remaining isostatic anomalies indicate that there are large variations away from this simple model which are likely due to variations in crustal thickness and density near the transform.     

Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (Izmit) earthquake on 17 August 1999

Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avcılar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the ?ınarcık Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the ?ınarcık Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through B üy ük ?ekmece and K ü ? ük ?ekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthquake in the NAFZ may have extended through Avcılar. Indeed, Avcılar and İzmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avcılar and İzmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avcılar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.     

Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (İzmit) earthquake on 17 August 1999

Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avc?lar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the Ç?narc?k Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the Ç?narc?k Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through B üy ük çekmece and K ü ç ük çekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthquake in the NAFZ may have extended through Avc?lar. Indeed, Avc?lar and ?zmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avc?lar and ?zmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avc?lar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.