Introduction
A ridgelet transform method for constraining tectonic models
Introduction
2002] were able to map the location of the Pacific–Phoenix–Farallon triple junction track by examining changes in abyssal hill trends. The advantage of this technique is that averaging after shape estimation makes it easier to identify the location of systematic changes in abyssal hill shape.
The ridgelet transform
The local RMS amplitude is easily calculated during our implementation of the ridgelet transform (see §3.1 below). This ridgelet domain analysis gives us considerable leeway in the design and implementation of the ridgelet transform algorithm.
Application of the ridgelet transform to abyssal-hill morphology
Conversely, the Radon transform concentrates linear features in the spatial domain at single points in the Radon domain. In this case, however, the size of the radon opening is limited to the width of the multibeam slit.
Discussion and conclusions
The magnetic profiles are aligned with the axis of the Osbourn Trough (the trough axis coincides with the central solid vertical grid line). Note that much of the anomalous force associated with the Congo Basin has been removed.
History of the Cretaceous Osbourn spreading center
Data
Profiles of the magnetic field strength recorded during our cruises, along with those of NBP9806A [Billen and Stock, 2000] are plotted in Figure 3a. Single channel seismic (SCS) data were collected during NBP0207 between latitudes 27°S and 25°S; during NBP0304B, a multichannel seismic (MCS) survey was conducted at the locations shown in Figure 2.
Magnetic anomalies
Gravity models
The gravitational field of the Osbourn Trough is similar to that observed in the Labrador Sea, Aegir Ridge and other extinct range centers. However, they do show that the gravitational field of the Osbourn Trough is typical of that of other extinct dispersal axes.
Abyssal-hill fabric
Menard [1967] has observed that the root-mean-square (RMS) amplitude of abyssal mounds negatively correlates with the spreading rate of the maternal ridge during abyssal mound formation. The abyssal mound analysis provides some important constraints on the tectonic history of Osbourn's spreading center.
Tectonic model
After this rotation, another ∼900 km of crust was built up at the Osbourn Ridge before the rate of spreading of the ridge slowed. The configuration of the Osbourn region immediately before this slowing event is shown in Figure 11c. The fracture zones observed in Figure 9 may indicate the location of the easternmost extent of the Hikurangi plate at the time of Figure 11c.
It may have been possible that when the Osbourn spreading center died, the eastern arm of the Wishbone Scarp began to form. The magnetic anomaly profiles in Figure 3 suggest that the extinction of the Osbourn Spreading Center occurred sometime during Chron C34. This waning event is expressed as a change in the mean abyssal-trough amplitude near 25°S north of the Osbourn Trough (Figure 2b).
Previous work estimated a minimum age of 100 Ma for the crust at this site [Menard et al., 1987].
Conclusions
The admittance estimates obtained with these data imply that the topography of the Congo Basin is dynamically supported. The observations detailed in section three only limit the current state of the Congo basin. The gravity and topographic anomalies associated with the Congo Basin also provide constraints on the density structure of the lithosphere.
We are unable to precisely constrain the viscosity of the lithosphere beneath the Congo Basin. The location of the thickest sediment fill coincides with the location of the Congo gravity anomaly (Figure 2). We present data constraining the development of the Congo Basin since the Early Cretaceous.
We present two possible models for the history of the Congo Basin since the early Cretaceous.
Instantaneous dynamics of the cratonic Congo basin
Previous studies of the cratonic Congo basin
The Congo Basin lies almost entirely within the Democratic Republic of the Congo (formerly Zaire and the colonial Belgian Congo), with its northern and northwestern extent extending into the Republic of the Congo and the Central African Republic. Since 1960, most geological studies of the Congo Basin have been carried out by oil exploration companies and are proprietary. Much of our current knowledge of the tectonic history of the Congo Basin depends on the interpretation of these seismic and well data.
The lithosphere underlying the Congo Basin is compressed between the East African Rift Zone and the Mid-Atlantic Ridge, as evidenced by earthquake focal mechanisms [ Atalay , 2002 ]. This estimate, although similar to estimates of EET in other cratonic regions, may only be an upper bound on the true elastic thickness of the Congo lithosphere [McKenzie and Fairhead, 1997]. Nevertheless, the ∼2000 km wavelength of the Congo Basin suggests that flexural support is not the primary mode of compensation for its topographic expression.
Hartley and Allen [1994] suggest that the anomalous gravity and topography of the Congo Basin is the result of the action of a downward dynamic force on the base of the lithosphere caused by convective downwelling in the mantle.
Observations and data
We again use the spectral localization method to calculate the RMS amplitude spectrum of the topography localized near the Congo basin (Figure 3b). These analyzes also relied on admittance spectra calculated by Bouguer gravity anomalies in their estimates of the EET of the Congo lithosphere. It is therefore prudent to re-estimate the admittance spectrum of the Congo using our new datasets to confirm the conclusions of Hartley and Allen [1994] and Hartley et al.
For the subtle topography of the Congo region, the gravity coefficients to the right of (8) can be approximated by:. The mean size of Congo entrance at long wavelengths (Figure 5a) is consistent with the Te value of 101 km estimate by Hartley and Allen [1994] and Hartley et al. We agree with Hartley and Allen's conclusion (1994) that there is likely a downward dynamic force, due to mantle convection, acting on the base of the Congo lithosphere.
The area of significant sediment accumulation (> 50 m) measures ~1200 km from east to west and 900 km from north to south and coincides with the location of the Congo free air anomaly.
Instantaneous dynamics of the cratonic Congo basin
The thickness and depth of the density anomalies control their coupling to the surface as illustrated by the trends in the size of the topographic depression (Figure 11). For group 3 (Figure 11a), as the density anomalies are placed deeper, the resulting deflection from the surface decreases. At the same time, the magnitude of the gravity due to the density anomaly is also reduced because it is deeper in the mantle.
The topographic deflection is related to the anomalous density via the viscosity structure and determines the topographic component of gravity. The total anomalous mass of the best-fitting models in Table 3 varies only from 8–10×1018 kg. This misfit is due to the upward continuation of gravity due to the density anomaly.
This inability to constrain the viscosity structure within the lithosphere is likely the result of the shallow location of the density anomaly.
Discussion and conclusions
By fitting both gravity and topography at the same time, we can determine the magnitude of the total mass anomaly associated with the Congo anomaly. Such a large temperature anomaly implies that the lithosphere under the Congo Basin has a temperature similar to that of the crust. Our final constraint on the density and viscosity structure of the lithosphere stems from the analysis of the shear wave velocity anomaly observed under the Congo Basin.
In conclusion, the observations of topography, shear wave velocity anomalies, and gravity at the Congo Basin suggest a dynamic origin for the depression of the basin's surface. Note that there are two wavebands over which the topography of the Congo is anomalous. The color of each symbol shows the maximum topographic depression observed in the middle of the Congo basin.
For example, the late Jurassic to Quaternary sedimentary infill of the Congo Basin forms a disconformity-bounded sedimentary succession typical of intracratonic basins. The Late Jurassic to Quaternary stratigraphy of the Congo Basin is characterized by thick Mesozoic sections overlaid with a thin Cenozoic envelope. Sahagian (1993) attempted to discern the tectonic subsidence of the Congo Basin by withdrawing the sediments from the Samba pit.
Subsidence history of the cratonic Congo basin
Time-dependent properties of intracratonic basins
Regarding the mechanical properties of their substrate, intracratonic basins overlie ancient cratonic lithosphere, which is thought to be stable over long periods (>1 Ga). Despite this superposition, subsidence within intracratonic basins is generally not associated with rift activity because rift formation generally precedes basin subsidence. The isopachs of intracratonic basins bear no resemblance to the linear structures of their underlying rifts.
Intracratonic basins typically undergo several cycles of renewed subsidence, resulting in the formation of overlying unconformity-bound sediment packages. Additionally, based on chronostratigraphic correlation, these cycles are observed to occur somewhat simultaneously in intracratonic basins on a global scale [ Leighton and Kolata , 1990 ]. However, it is not possible to identify this synchronicity on the time scale of the shortest observed depositional events using chronostratigraphic data.
Similarly, it is not known whether initiation and termination of sedimentation within intracratonic basins occur simultaneously, or whether these basins subside at rates determined by local tectonics in response to global tectonic events.
Cretaceous-Quaternary history of the Congo basin
The basal Stanleyville Formation, observed in the Samba and Gilson wells but not in the Dekese well (Figure 3), is a conglomerate unit that conglomerates upward and lies unconformably over Paleozoic and Triassic units [ Giresse , 2005 ; Daly et al., 1992]. Sahagian [1993] interpreted this curve to show that the Congo Basin is a passive reservoir of sediments eroded from the surrounding uplifted regions. A reexamination of the paleogeographic data described above indicates that a correction for sea level change in the Congo Basin subsidence analysis is not warranted given the continental nature of the sediment fill.
We present a 1-D backstripping analysis of the Gilson, Dekese and Samba wells where we have not applied a correction for sea level change (Figure 3). The ages of the units in the wells also differ slightly from the ages of Sahagian [1993]. We based our age on the review by Giresse [2005] (Table 1); however, the age differences are not significant given the large uncertainties associated with dating the strata.
The resulting subsidence history is consistent between wells and indicates that the Congo Basin subsided at a rate of approximately 15-20 m/Myr throughout the Early Cretaceous, relative to a local base level unique to each well. observed total subsidence rate, the estimated tectonic subsidence rate for the same period is 6-8 m/Myr).
Dynamic models of Congo basin subsidence
