Abstract The margins of the Gulf of Oman Basin range from convergent at the north to translation at the west and east, and passive at the south. The basin's northern margin has been a site of continuous subduction since Cretaceous time, which has led to the creation of an 800 km long and 650 km wide accretionary wedge, most of which is above sea level. Strata in the centre of the Gulf of Oman Basin display minor deformation resulting from the northward tilting of oceanic crust. A basin-wide unconformity dividing these strata in two was the result of erosion during Early Oligocene time when bottom water circulation was enhanced during a climatic deterioration. The morphology of the basin's south margin is due to Early Triassic rifting, deposition during Jurassic-Early Cretaceous time, early Late Cretaceous ophiolite obduction and Late Cretaceous-Cenozoic deposition. The western side of the accretionary wedge, along the north side of the Gulf of Oman Basin, is in sharp contact with the western translation margin. Structures along this margin are the result of post-Eocene convergence of the Lut and Central Iran microplates. The eastern end of the accretionary wedge, however, is not in contact with the eastern transform margin, but is separated from it by a north-trending trough. The landward extension of this trough is defined by the north-trending Las Bela Valley. The eastern side of the accretionary wedge turns northward at 65°30'N along the west side of the trough and becomes aligned with the north-trending Ornach-Nal Fault along the west side of the Las Bela Valley. Similarly, the Murray Ridge complex turns northward at 25°N and becomes aligned with the north-trending Surjan Fault on the Las Bela Valley's east side. The Ornach-Nal and Surjan faults merge at the apex of the Las Bela Valley with the north-trending Las Bela-Chaman Structural Axis. Differences between the eastern and western sides of the accretionary wedge may be due to the presence of the Ormara microplate on the eastern end of the wedge, a plate that is being pushed ahead of the Arabian plate. The morphology of the Murray Ridge complex is the result of transtension and secondary compression along the Indian-Arabian plate boundary. We infer that most of the relief of the Murray Ridge complex resulted from a change in plate geometry in Early Miocene time. Subsequent tectonic Pliocene-Quaternary events have enhanced this relief.

Like the W. M. Davis construction of Cape Cod published in 1896, this special paper suggests that the Cape was formed by glacial deposition during the late Pleistocene and by marine and aeolian processes during the Holocene. It differs, however, from the Davis model in several significant ways. For example, Davis proposed that the lower Cape extended 4 km east of its present location, and that only 4,000 yr were needed for the lower Cape to attain its present form. This study indicates that the glacial Cape extended as far as 7 km east of its present position, and that it took about 9,500 yr for the Cape to attain its present morphology. Davis also believed that the detritus eroded from the sea cliffs on the east side of the lower Cape was transported northward to form the Provincetown Hook. On the other hand, this book indicates that, as a result of the subaerial exposure of Georges Bank from 9,500 to 6,000 yr ago, littoral drift to the north was inhibited and sediment was transported southward to fill a depression at the Cape's elbow. Since Georges Bank became submerged about 6,000 yr ago, however, littoral drift shifted partly to the north, leading to the construction of Provincetown Hook, a process that is still taking place today. The bulk of material (86%) eroded from the cliffs along the east side of the lower Cape during the last 6,000 yr was transported northward. Of the remainder, 7% was used in the construction of the beaches and offshore bars fronting the cliffs, and the remaining 7% was incorporated into the spits and barriers south of the cliffs. During the last 70 yr, the eastern Cape cliffs have been retreating at an average rate of 0.8 m a -1 , a rate enhanced by a relative rise in sea level of about 2 to 3 mm a -1 .

Abstract Earth's surface morphology is primarily the result of interaction between plates moved by seafloor spreading and/or intraplate tectonic/magmatic processes. Once the youthful endogenetic (tectonic/magmatic) terranes are isolated from inter/intraplate influences as a result of long-continued lateral migration or changes in geometry, exogenetic processes (erosion and deposition) subdue and reduce the original relief. The rate of this modification and the nature of the geologic processes involved in it are controlled by climate, which may change with time or with migration of the plates across climatic zones, and by oscillations in sea level. Whether a terrane can reach an old-age stage in the geomorphic cycle depends upon its isolation from plate activity long enough for non-tectonic processes to complete its degradation. The low relief of ancient terranes in Precambrian shields is a clear indication that the geomorphic cycle can come to completion.

Abstract A multichannel seismic-reflection survey of the Newfoundland basin southeast of the Grand Banks was conducted to investigate the rift-drift history of the basin and to examine the nature and location of the continent-ocean boundary. The data suggest that the continent-ocean boundary is marked by the “J” magnetic anomaly (M1-M0), which separates crust having a relatively smooth magnetic field to the west from higher amplitude anomalies to the east. Basement at the boundary is characterized by a set of large volcanic ridges that were constructed simultaneously with the adjacent Southeast New-foundland Ridge and the J-Anomaly Ridge. The ridges appear to have formed from magma derived from a mantle plume then located beneath the Southeast Newfoundland Ridge. The Newfoundland basin west of the interpreted continent-ocean boundary exhibits two distinct structural provinces in basement. A large (60 × 400 km) rift basin, the Salar basin, occurs beneath the continental slope and upper rise and is filled with evaporites that are probably Late Triassic to Early Jurassic in age. Seaward of the Salar basin, a zone of complexly faulted basement blocks with intervening, probably synrift, sedimentary and vol-canic fill extends to the J anomaly. Both the blocks and fill are capped by a high-amplitude, relatively flat seismic unconformity. The unconformity pinches out at the J anomaly and appears to corre-late with the “U,” or Avalon, breakup unconform-ity on the adjacent Grand Banks. Late Triassic rifting formed the Salar basin and caused an unknown amount of extension in adja-cent continental crust to the east. A second phase of Late Jurassic through Early Cretaceous rifting reac-tivated parts of the western margin of the Salar basin and probably caused substantial extension and thinning of continental crust to the east. The rifting culminated in broad uplift or doming of this extended crust, erosion of the breakup unconform-ity, and Barremian-Aptian emplacement of volcanic ridges marking the continent-ocean boundary at the J anomaly. All crustal elements (extended continental crust, J-Anomaly ridges, oldest oceanic crust) appear to have experienced unusually rapid subsidence, much faster than age/depth relationships of normal oceanic crust, following continental breakup.

Abstract Seismic refraction measurements indicate that the transition from rifted continental crust to oceanic crust takes place at a water depth of over 3000 m northwest of Cuba between the Florida and Campeche escarpments. Along the eastern flank of the Mississippi Embayment, the transition occurs at about 2500 m deep and on the embayment itself inboard of the coast. Off south Texas the boundary between the rifted continental crust and oceanic crust is near the shelf’s edge, off Mexico the boundary is on the continental slope, and off Campeche Bank the boundary is about 100 km northwest of Campeche Escarpment. In the northern Gulf an oceanic crustal high may lie beneath the upper continental slope. This high served as a foundation for a Mesozoic reef. Maximum sediment accumulation took place along the contact between the rifted continental crust and oceanic crust.

Abstract Echo-sounding data collected during the 1969 Atlantis II cruise to the Black Sea show four distinct physiographic provinces: shelf, basin slope, basin apron, and abyssal plain. The Danube fan, a relict sedimentary feature, crosses the latter three provinces. The basin slopes are highly dissected where erosion is dominant and smooth where deposition is prevalent. Submarine canyons are common in the highly dissected parts of the slope. Small topographic features similar to lower-continental-rise hills found off the eastern United States occur near the Bosporus. Two types of such features were found: blocklike features and conical to asymmetrical features. The blocklike features are the result of slumping. The conical to asymmetrical mounds are of undetermined origin; they may be channel divides or levees, or they may be due to slumping or bottom currents.

Abstract Gravity, magnetic, and seismic reflection data indicate that recent subsidence in the Black Sea has been associated with faulting along the southern, eastern, and northern parts of the basin. In contrast, the western margin appears to have been tectonically quiet during subsidence. A large, positive Bouguer anomaly is typical of most of the Black Sea, and free-air anomalies are usually negative. Local patterns in the gravity field suggest a separate structure for the eastern and western parts of the basin. Along most of the near-coastal parts of the sea, local variations of the gravity anomalies correlate with topography and subsurface structures displayed by the seismic profiles; in deeper water and in the southeast corner of the basin, there is no obvious correlation. The residual magnetic field generally parallels the surrounding Caucasus and Pontic Mountains, suggesting that these structures extend partly into the Black Sea. The southern and eastern slope of the Black Sea is characterized by a shallow basement deeply entrenched by numerous channels. In contrast, the basin slope along the northwestern side is relatively smooth, indicating little tectonic activity and a long period of sedimentation. The transition from the basin slope to basin apron on the south and east side of the Black Sea is generally abrupt; slumps and slides commonly are present at the base of the slope. In the eastern part of the basin the transition from basin slope to apron is more gentle. Most of the basin apron and the abyssal plain in the center of the Black Sea are characterized by horizontal layers that can be traced for hundreds of kilometers. South of the Crimea, these layers are disrupted by small, nearly vertical, normal faults. This fault zone may be coincident with a northeast-southwest trend in gravity anomalies that divide the eastern and western parts of the basin. A subsurface anticline near the central part of the sea may be related to the Caucasus orogeny.