The Relationship Between the Development of Seaward Dipping Reflectors, Oceanic Crust and the Maldonado Triple Junction During Early Gondwana Break-Up

Abstract

Seaward dipping reflectors (SDRs) are ubiquitous features along the South Atlantic margin occurring in the transitional zone between continental and oceanic crust. In most margin settings SDRs are dissected by margin parallel, syn-emplacement faults. Offshore Uruguay, however, significant margin perpendicular faults are evident which dissect the SDRs and play a critical role in their distribution as they define an abrupt termination along an E/W trend. This trend is also co-indecent with intruded, normally faulted and highly thinned (<4km) crust with a substantially elevated Moho. We relate these observations to the development of a short-lived triple junction where the E/W striking Rio de la Plata rift interacted with the evolving N/S orientated South Atlantic rift.

Whilst the triple junction formed during early Gondwana break-up, the timing relative to the emplacement of SDRs, the evolution of oceanic crust and the development of the Rio de la Plata rift is more enigmatic. Regional plate models suggest Gondwana break-up commenced with N/S extension in the Upper Jurassic resulting in initiation of E/W trending rift systems. As break-up continued an east-west orientated extensional stress system became more prevalent resulting in the opening of the South Atlantic rift north of the Falkland Plateau. During this phase, extensive SDR packages were emplaced and oceanic crust along the South Atlantic margin formed. Whilst E/W orientated rifting became more dominant in the South Atlantic the propagation of the Rio de la Plata rift continued eastwards, splitting the previously deposited SDRs and oceanic crust, resulting in the generation of newly formed crust (the Maldonado triple junction).

This work presents an attempt at integrating observations of the Maldonaldo triple junction with regional tectonic models and suggests that there was still active N/S orientated rifting after oceanic crust developed in the southern South Atlantic. This implies the offshore Uruguayan margin segment is not part of a simple N/S propagating rift but is more analogous to a triple junction setting. Furthermore, the crustal thinning observed within the Maldonado triple junction has had a significant long term impact on the evolution of accommodation space and temperature field of the basin with heat flow measurements revealing anomalously high values in the confluence of the rift arms.

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