Fault Reactivation in the Southern North Falkland Basin: Complex Fault History or Partitioned Transtension?

Abstract

Typical problems when interpreting rifting histories include determining whether the rift system evolved under an extensional or transtensional stress regime, as well as the added complexity of reactivation of inherited basement structures and multiple rifting phases with different stress directions. Added to this are complexities regarding basin inversion, timing of inversion and compressional orientations. These can lead to misinterpretation of timings, stress orientations and relationships between structural elements. Further uncertainty arises from the range of possible interpretations when dealing with incomplete data sets, such as partial 3D and 2D seismic coverage and sparse wells. Regional geological interpretations of the Mesozoic North Falkland Basin (NFB) encounter all of the above problems.

The NFB is composed of several sedimentary basins in the northern Falkland Plateau. The Falkland Plateau has experienced a long and complex geological evolution, which partially accounts for the range of complex structures observed in the NFB. The NFB is comprised of a series of offset depo-centres that are affected by two dominant structural trends: north-south oriented faulting in the Central and Northern NFB, whereas north-west to south-east oriented faults dominate the southern NFB.

The main phase of rifting is likely to have initiated in the very latest Jurassic or early Cretaceous, with the NFB forming as a failed-rift arm associated with the opening of the South Atlantic. This rifting phase was then followed by a subsequent thermal sag phase that began in the Berriasian-Valanginian and continued under predominantly continental-lacustrine deposition until Albian-Cenomanian times, when the basin became increasingly marine in nature. Previous authors have suggested that a widespread Albian unconformity could be associated with regional uplift and inversion.

A number of earlier studies have presented different evolutionary models for the NFB, but none seem to fully satisfy all of the observed structures. This contribution provides an alternative model for the structural evolution of NFB and accounts for inversion events in the SNFB and Central NFB without invoking a regional compressional event. By resolving stress orientations using fault slip analysis we demonstrate that structures interpreted as a result of successive phases of extension and compression, can instead be attributed to a single protracted phase of partitioned sinistral transtension.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018