Thermotectonic
Evolution of the Cap St. André Arch and its Influence on Deepwater Depositional
Systems in Northwest Madagascar
Kraus, Jeffrey U.1, William A.
Matthews1, Lanirina Ronosrosoa2 (1) ExxonMobil
Exploration Company, Houston, TX (2) OMNIS, Antananarivo 101, Madagascar
The Cap St. André Arch is a diffuse
tectonic boundary between the Majunga and Morondava basins in western Madagascar. We investigated the
thermotectonic evolution of the Arch to determine its uplift history and role
as a source of sediment to the offshore Majunga Basin. We examined
depositional patterns in outcrops along the northeastern flank of the Arch and
fission tracks from basement rocks near the center of the Arch to determine the
Arch's uplift history. Apatite fission track analyses indicate periods of
uplift in the Carboniferous and Late Cretaceous-Tertiary.
Basaltic magmatism affected the Arch
during the Cenomanian and Turonian in close spatial association to the
Bongolava and Bemaraha shear zones. That coincidence, along with
contemporaneous uplift and volcanism, suggests that magmatism occurred
preferentially along deep-seated faults that were reactivated in the Late
Cretaceous. Seismic data in the deep water Majunga Basin record thickening of
the Upper Cretaceous section toward the Arch. Changes in lithic content of
sands deposited in fluvial environments along the northeastern flank of the
Arch provide additional evidence for uplift. These observations support our
interpretation of the Cap St. André Arch as a long-lived, deep-seated tectonic
feature that has undergone episodic reactivation, most recently in the Late
Cretaceous-Tertiary. During this event, the Arch was uplifted more than 1 km
and is inferred to have been the principal provenance for the thickened Upper Cretaceous
deep water sedimentary deposits in the southwestern part of the offshore Majunga Basin.