The thermal structure of convergent margins provides information related to the tectonics, geodynamics, metamorphism, and fluid flow of active plate boundaries. We report 176 heat flow measurements made with a violin bow style probe across the Costa Rican margin at the Middle America Trench. The probe measurements are collocated with seismic reflection lines. These seismic reflection lines show widespread distribution of bottom‐simulating reflectors (BSRs). To extend the spatial coverage of heat flow measurements we estimate heat flow from the depth of BSRs. Comparisons between probe measurements and BSR‐derived estimates of heat flow are generally within 10% and improve with distance landward of the deformation front. Together, these determinations provide new information on the thermal regime of this margin. Consistent with previous studies, the margin associated with the northern Nicoya Peninsula is remarkably cool. We define better the southern boundary of the cool region. The northern extent of the cool region remains poorly determined. A regional trend of decreasing heat flow landward of the deformation front is apparent, consistent with the downward advection of heat by the subducting Cocos Plate. High wave number variability at a scale of 5–10 km is significantly greater than the measurement uncertainty and is greater south of the northern Nicoya Peninsula. These heat flow anomalies vary between approximately 20 and 60 mW m −2 and are most likely due to localized fluid flow through mounds and faults on the margin. Simple one‐dimensional models show that these anomalies are consistent with flow rates of 7–15 mm yr −1 . Across the margin toe variability is significant and likely due to fluid flow through deformation structures associated with the frontal sedimentary prism.
Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Since
some 185 Ma the rifting in our research area caused the dispersal of Gondwana and Eastern Africa. The timing
and geometry of the break-up as well as the amount of volcanism connected to the Jurassic rifting are still
controversial. In the southern part of the Mozambique channel a prominent basement high, the Beira High, forms
a specific crustal anomaly along the margin. It is still controversial if this high is a continental fragment or was
formed during a period of enhanced magmatism.
Therefore a deep seismic profile with 37 OBS/H was acquired from the deep Mozambique Channel, across
the Beira High and terminating on the shelf. The main objectives are to provide constraints on the crustal
composition and origin of the Beira High as well as the amount of volcanism and the continent-ocean transition
below the Zambezi Delta. To obtain a P-wave velocity model of this area the data was forward modelled by means
of 2D-Raytracing. Furthermore, potential field data acquired in parallel to the seismic data were used to calculate
a 2D gravity model.
Preliminary results indicate a 20-24 km thick crust for the Beira High. In good agreement to the adjacent
oceanic crust in the Mozambique Channel the upper crust has velocities between 5.5-5.9 km/s. The middle crust
is characterised by velocities between 6.2-6.7 km/s and the lower crust higher than 6.7 km/s and a density of 3.0
g/cm3. However, these velocities are only constrained by Moho reflections, since no diving waves are observed for
the lower crust. In the area of the Zambezi Delta Depression the top of the acoustic basement is at 11.5 km depth
and the crust thickness thins to 7 km. The basement here is overlain by a 2 km thick layer of 4.9-5.1 km/s, which
we interpret as pre-rift sediments (Karoo-Belo-Group, including Lava Flows on top). Furthermore, evidence for
the presence of a high velocity body (HVB) at below the western part of Beira High with a velocity of 7.2-7.4
km/s and 3 km thickness is found.
Below the shelf our results indicate evidences for an increased volcanism during the initial break-up. The
location of the continent-ocean boundary as well as the geometry of the break-up depend strongly on the tectonic
classification of Beira High. Future work will provide further constraints by amplitude modelling, a 3D gravity
model of Beira High and by means of interpretation of the magnetic anomalies.
Main objective of the project is the investigation of the crustal structure of the margin of Mozambique. This will improve our understanding of the driving forces and processes leading to the initial Gondwana break-up.
Some 185 Ma the onset of rifting caused of the opening of the Mozambique and Somali Basin and the dispersal of this vast continent into several minor plates. The timing and geometry of the initial break-up between Africa and Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still controversial. However, the conjugated margin in the Riiser-Larsen Sea is covered by an up to 400 m thick ice cap, precluding the set-up of a deep seismic experiment in this area. Consequently, the investigations focus on the continental margin of central Mozambique. Here, a prominent basement high, the Beira High, forms a critical geological feature of uncertain crustal fabric. It is still controversial if this area of shallow basement is a continental fragment or was formed during a period of enhanced magmatism and is of oceanic origin.
Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi River (Fig. 1). The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the position of the continent-ocean transition along the margin of central Mozambique. To obtain a P-wave velocity model of this area the data were forward modelled by means of the 2D-Raytracing method, supported by an amplitude and gravity modelling.
In the Mozambique Basin mainly normal oceanic crust of 5.5–7 km thickness with velocities of 6.5–7.0 km/s in the lower crust is present (Fig. 2). A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23 km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0 km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsic material in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 11 km of sediments cover the underlying 7 km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4 km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage. The acquired shipborne magnetic data complement our dataset in the Mozambique Basin and reveal clear evidence for the presence of lava flows and intrusions, pointing to an increased break-up related magmatism.
Combined structural and biostratigraphic analyses and seismic interpretation help them to balance cross sections through the southern Apennines from the Adriatic to the Tyrrhenian Sea and to propose an overall model for the evolution of the belt. Three lithostratigraphic units have been distinguished according to their Mesozoic facies and style of deformation: the western platform (upper unit), the Lagonegro-Molise basin, and the eastern platform. Foreland deformation migrated from west to east, and external domains were reached progressively by synorogenic flysch deposits (foredeep) and later incorporated into the thrust sheets. Presently, only the most external part of the eastern platform is still unaffected by thrusting, while internal parts are building the overthrust belt at depth, which is masked on the surface by allochthonous basinal nappes. The evolutive geometry of thrust and piggy-back basins results from the continuous understacking of new material at the bottom of the tectonic prism. The deeper basement is also progressively involved in the deformation, giving rise to large nappe anticlines. Despite the early subsidence and deformation of the western platform and basinal domains in Langhian to Tortonian time, all the deformation of the eastern platform has occurred since Messinian time. These compressive structures are thus contemporaneous withmore » the opening of the Tyrrhenian Sea. To the west, the upper tectonic units of the Apennines are indeed affected by listric normal faulting, with previous thrust planes having been locally reactivated during the distension. Post-Messinian shortening in the sedimentary cover is accompanied by a crustal thickening outlined by the Moho's geometry. The authors interpret it as a result of the subduction of the Apulian continental lithosphere. Recent uplift of the Apennines is indeed directly related to this crustal root.« less
Up to Jurassic times the Antarctic and African continents were part of the supercontinent
Gondwana. Some 185 Ma the onset of rifting caused the dispersal of this vast continent into
several minor plates. The timing and geometry of the initial break-up between Africa and
Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still
controversial. In the southern part of the Mozambique Channel a prominent basement high,
the Beira High, forms a distinct crustal anomaly along the Mozambican margin. It is still
controversial if this area of shallow basement is a continental fragment or was formed during
a period of enhanced magmatism and is of oceanic origin.
Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep
Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi
River. The main objectives are to provide constraints on the crustal composition and origin of
the Beira High as well as the amount of volcanism and the position of the continent-ocean
transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data
were forward modeled by means of the 2D-Raytracing method.
Preliminary results indicate a clear thickening of the crust below the Beira High up to 20-24
km. Evidences for a high velocity body are found in the area below the Zambezi shelf with
velocities of 7.2-7.4 km/s and up to 5 km thickness. Oceanic basement velocities at the very
eastern part of the line start with values of 5.5 km/s, and increase to 6.9 km/s at lower crustal
levels, that are typical for Jurassic oceanic crust. Across the Beira High the starting velocity
and its gradient slightly change, presenting typical values for continental fragments. However,
due to a sparse ray coverage of diving waves for the Beira High lower crust, these velocities
still have to be proved. Thus, we will introduce the final results of a Finite Difference
amplitude modeling, which will constrain the lowermost velocity gradients to allow a sound
interpretation of the Beira High origin. The acquired shipborne, magnetic data show a
complex magnetic pattern and strong influences by the presence of lava flows and intrusions
and require further investigations.
We will introduce the latest results of the joint interpretation of seismic and potential field
data sets.
