Gravity and Magnetic Investigations in the Guiana Basin, Western Equatorial Atlantic

A free-air gravity map of the Guiana Basin between 15° N. and 6° S. in the western equatorial Atlantic, using all available shipboard and pendulum data, is presented. The gravity field is interpreted in terms of short wave-length components directly related to topographic features and a long wave-length regional field which is independent of surface or basement relief. The regional field is negative throughout the survey area, varying from −15 to −40 mgal. The magnetic anomalies over the large equatorial fracture zones indicate that the fracture zone trough is an area of zero or greatly reduced magnetization within a zone in which the magnetization is induced rather than remanent. Only about half of the gravity anomaly over the fracture zone can be assigned to topographic relief implying the presence of excess mass under the fracture zones. The gravity and magnetic evidence together suggest that large fracture zones serve as the site of intrusion of ultrabasic rocks from depth. The deformation of the lithosphere due to the sediment load of the Amazon cone and the resulting gravity anomalies were computed for various flexural rigidities, using two-dimensional elastic beam theory. The value giving the best fit to the observed gravity anomalies in both wave length and amplitude is 2 × 1023 Newton meters (nt m) (2 × 1030 dyne cm). This implies an effective lithospheric thickness of 30 km. It is suggested that the lithosphere behaves somewhat as a Kelvin (viscoelastic solid) material in its response to imposed long-term loads, approaching a minimum apparent flexural rigidity of 2 × 1030 dyne cm asymptotically in a period of a few million years

Northern Red Sea: Nucleation of an oceanic spreading center within a continental rift

The northern Red Sea is an amagmatic continental rift in which an oceanic spreading center is beginning to develop. A new compilation of marine geophysical data permits delineation of the structure of the northern Red Sea and of the manner in which the transition from continental to oceanic extension is occurring in this rift. The margins of the northern Red Sea are formed by large, apparently active faults on the seaward edge of the narrow continental shelves. The morphology of the main trough is a series of terraces stepping down to an axial depression. The terraces are a subdued expression of the basement structure, which consists of a series of large rotated fault blocks. The axial depression is an often fault-bounded axis of deep water that extends south from the Suez triple junction. The rift is segmented along-strike by through-going accommodation zones spaced at 40–60 km intervals along the rift. In the main trough, accommodation zones truncate or offset rift-parallel bathymetric and gravity features. The axial depression consists of a series of discrete depressions offset from each other and separated by slightly shallower areas corresponding to accommodation zones. Within each segment, the axial depression deepens away from the accommodation zones toward a small deep, a few kilometers across and a few hundred meters deeper than the surrounding seafloor. A pair of small volcanoes is perched on top of the scarps bounding the axial depression on either side of the deep within each segment. The volcanoes are all normally magnetized and interpreted as very young. The crust is uniformly thin (5–8.5 km) throughout the main trough, implying extension evenly spread across the rift through much of its development. As extension recently became concentrated at the axis, melt began to be generated and is focused to a location within the segment where it ascends along faults bounding the axial depression to form the pair of volcanoes flanking the axis. A volcano is also found on the floor of the axial depression in one segment. This isolated volcano appears to be the first step in the development of the seafloor spreading cells that are observed in the central Red Sea. The individual cells then grow and coalesce to become a continuous spreading axis

Himalayan Uplift, Sea Level, and the Record of Bengal Fan Sedimentation at the ODP Leg 116 Sites

Three closely spaced sites located 800 km south of Sri Lanka on the distal Bengal Fan were drilled during ODP Leg 116. Two of these sites, 717 and 718, together penetrated over 1300 of the 2000 m of stratigraphic section present at that location and provide a complete record of sedimentation since the lower Miocene. The entire section consists of turbidites, primarily derived from the Ganges-Brahmaputra delta. The main controls on sedimentation appear to be the uplift and erosion history of the Himalayas, along with the position of sea level relative to the shelf edge. Although the base of the fan was not penetrated, a number of lines of evidence suggest that it was approached and that fan sedimentation began in the lower Miocene. This corresponds in time with the oldest known molasse sedimentation in India and Pakistan and probably represents the time of beginning of major uplift and erosion in the Himalayas. Sedimentation continued at a substantial rate from the lower Miocene to the upper Pliocene indicating continued erosion and thus presumably continuing uplift and significant relief in the Himalayas during that time. The unconformity in seismic sections corresponding to the beginning of intraplate deformation can be dated as occurring between 7.5 and 8.0 Ma. It does not correspond to any change in the nature of the sediments. A change from the gray, silt-mud turbidites that had predominated throughout the Miocene to finer muddy, black organic-rich turbidites occurred in the uppermost Miocene. However, this occurred at 5.6-6.7 Ma in the Messinian, 1-2 m.y. after the beginning of the intraplate deformation. The Messinian change in the character of the sediments appears to result from a rise in mean sea level which occurred at that time. The beginning of intraplate deformation is probably due to a change in the plate boundary forces, perhaps resulting from the beginning of east-west extension in southern Tibet. A return to the deposition of coarser, silty turbidites occurred in the upper Pleistocene at about 800 ka. These sediments accumulated at very high rates until the Holocene. The beginning of the deposition of this unit corresponds in time to the marked intensification of the Pliocene-Pleistocene glaciations. The greater sea level variations resulting from the more intense glacial cycles may have resulted in completely exposing the shelf during glacial maximum with the sediment load of the rivers delivered directly to the continental slope

Effects of finite rifting times on the development of sedimentary basins

Most thermo-mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post-rift development of the basin. This assumption greatly simplifies the mathematical treatment, but is not in accord with what is found in nature, where 10- to 50-m.y. rifting events commonly accompany the formation of sedimentary basins and continental margins. The effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer. This technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced. The effect of a finite-duration extension event is that heat is lost during rifting increasing the syn-rift subsidence at the expense of the post-rift. Lateral heat flow, which was not included in previous studies of the effect of finite rifting times, has a significant effect on the subsidence history, distribution of sediments and thermal history. In particular, the post-rift subsidence is decreased by more than 25% for a 20-m.y. rifting event and by more than 10-15% for a rifting event as short as 10 m.y. This will significantly decrease the subsidence rates in the post-rift stage and implies that inferences concerning the structure, development and thermal history of the basin derived from using "β-curves" to interpret backstripped subsidence can be greatly in error. Variations in syn-rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors, such as the flexural response of the lithosphere to sediment loading, to affect the final width of the basin, the total amount of sediments that accumulate and the basin stratigraphy

An analysis of isostasy in the world's oceans: 2. Midocean ridge crests

Cross-spectral techniques are used to analyze the relationship between gravity and bathymetry at the Mid-Atlantic Ridge and the East Pacific Rise crests. The resulting transfer functions were used to study the nature of the isostatic mechanism operative at these ridge crests. The most satisfactory results were obtained for models in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The best fitting elastic thickness to explain gravity and bathymetry at the fast spreading (v > 5 cm/ yr) East Pacific Rise is in the range of 2–6 km and at the relatively slow spreading (v 80 m.y.) parts of the oceanic lithosphere. This difference is consistent with the fact that ridge topography is formed near the ridge axis, where isotherms are shallower and the lithosphere is thus weaker than in older regions. The difference between the elastic thickness of the East Pacific Rise and Mid-Atlantic Ridge is significant and may represent differing temperature structures at these ridges. Simple models in which it is assumed that the elastic thickness represents the depth to the 450°C isotherm show that these variations can be explained by differences in the spreading rate at these ridges. Thus the lower effective thickness at the East Pacific Rise can be attributed to higher average temperatures at shallow depths in a region surrounding the ridge crest. This is due to the faster spreading rate which results in isotherms having a shallower dip away from the axis than at the slower spreading Mid-Atlantic Ridge. This model cannot, however, explain gravity and bathymetry data over the Rekyjanes Ridge. The best fitting elastic thickness for this slow spreading ridge is similar to the thickness determined for the East Pacific Rise, suggesting an anomalous thermal regime at this ridge crest

Somali Basin, Chain Ridge, and origin of the Northern Somali Basin gravity and geoid low

The Northern Somali Basin, located between Chain Ridge and the Horn of Africa north of 4°N, is characterized by a distinct 5-m geoid low and by large negative gravity anomalies. The boundaries of the basin are marked by steep gradients in both gravity and geoid. Basement in Northern Somali Basin is 1-2 km deeper than on the Carlsberg Ridge flank to the southeast or on the Sheba Ridge flank to the north with a sharp discontinuity across the boundary of the basin. The Western Somali Basin, to the south, was created in the Late Jurassic and Early Cretaceous by the movement of Madagascar away from Africa. Reinterpretation of the magnetic anomalies in the Western Somali Basin shows that they record both limbs of a mid-ocean ridge that was active by M22 time (Kimmeridgian) and died soon after MO (Aptian). Magnetic and gravity data allow the relict ridge crest to be traced from Davie Ridge near the African coast to the Dhow-VLCC-ARS fracture zone complex at 50øE. Davie Ridge is a transform fault connecting the Western Somali Basin spreading center with a similar age spreading center in the Mozambique Basin. The Dhow-VLCC-ARS complex can be shown to continue north of the 4°N bathymetric high separating the Northern and Western Somali Basins and to intersect the African margin near 7°N. The Northern Somali Basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between East and West Gondwanaland. The original Northern Somali Basin was split apart by the northward motion of India in the Late Cretaceous and Chain Ridge formed along the new boundary. Thermal and gravity modeling shows that the flexure resulting from differential subsidence across Chain Ridge combined with the difference in lithospheric thermal structure (Late Jurassic vs. Early Tertiary) on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across Chain Ridge. The geoid step up from the basin up to the African coast can be modeled as an edge effect between the juxtaposed Jurassic oceanic and African continental crust. Thus, the geoid and gravity low over the Northern Somali Basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

Seamount volcanism along the Gakkel Ridge, Arctic Ocean

The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global midocean ridge system. Total spreading rates vary from 12.8 mm yr−1 near Greenland to 6.5 mm yr−1 at the Siberian margin. Melting models predict a dramatic decrease in magma production and resulting crustal thickness at these low spreading rates. At slow spreading ridges, small volcanic seamounts are a dominant morphologic feature of the rift valley floor and an important mechanism in building the oceanic crust. This study quantitatively investigates the extent, nature and distribution of seamount volcanism at the ultraslow Gakkel Ridge, the manner in which it varies along the ridge axis and the relationship of the volcanoes to the larger scale rift morphology. A numerical algorithm is used to identify and characterize isolated volcanic edifices by searching gridded swath-bathymetry data for closed concentric contours protruding above the surrounding seafloor. A maximum likelihood model is used to estimate the total number of seamounts and the characteristic height within different seamount populations. Both the number and size of constructional volcanic features is greatly reduced at the Gakkel Ridge compared with the Mid-Atlantic Ridge (MAR). The density of seamounts (number/area) on the rift valley floor of the Western Volcanic Zone (WVZ) is ∼55% that of the MAR. The observed volcanoes are also much smaller, so, the amount of erupted material is greatly reduced compared with the MAR. However, the WVZ is still able to maintain a MAR-like morphology with axial volcanic ridges, volcanoes scattered on the valley floor and rift valley walls consisting of high-angle faults. Seamount density at the Eastern Volcanic Zone (EVZ) is ∼45% that of the WVZ (∼25% that of the MAR). Seamounts are clustered at the widely spaced magmatic centers characteristic of the EVZ, although some seamounts are found between magmatic centers. These seamounts tend to be located at the edge of the rift valley or on the valley walls rather than on the valley floor. Seamounts in the Sparsely Magmatic Zone (SMZ) are located almost entirely at the 19◦E magmatic center with none observed within a 185 km-long portion of the rift valley floor. The EVZ and SMZ appear to display a mode of crustal accretion, characterized by extreme focusing of melt to the magmatic centers. Magmas erupted between the magmatic centers appear to have ascended along faults. This is very different from what is observed at the WVZ (or the MAR), and there is a threshold transition between the two modes of crustal accretion. At the Gakkel Ridge, the location of the transition appears to be localized by a boundary in mantle composition

The Gulf of Aden: Structure and Evolution of a Young Ocean Basin and Continental Margin

New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies on Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45°E and to anomaly 2' (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45°E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphere of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid-ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 100 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading

Some Remarks on Isostasy and the Long-Term Behavior of the Continental Lithosphere

The interpretation of linear transfer function (admittance) studies of the relationship between gravity anomalies and topography depends on what assumptions are made in limiting the acceptable set of solutions. However, whatever assumptions are made, the value of the flexural rigidity determined from admittance studies of Australia and the continental United States is less than about 10**28 dyne-cm (5 km plate thickness). This is much less than values determined from other studies of the loading of continental lithosphere which consistently give values in the range of 4 X 10**30 to 2 X 10**31 dyne-cm (elastic plate thickness of 35-60 km). It is argued that the latter values represent the long-term rigidity of the continental lithosphere and that the rigidity does not change greatly with age of loading

The Assassin Bug \u3ci\u3eZelus Luridus\u3c/i\u3e (Heteroptera: Reduviidae) in Michigan\u27s Upper Peninsula

(excerpt) On 17 July 1992, an assassin bug (Zelus luridus Stal) was flushed from the stomach of a smallmouth bass (Micropterus dolomieu) collected in West Long Lake of the University of Notre Dame Environmental Research Center, Gogebic County, Michigan