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.