Intraplate Foreland Deformation in the Neuquén Embayment*

By

Alfonso Mosquera ¹ and Victor A. Ramos ²

Search and Discovery Article #30035 (2005)

Posted August 31, 2005

*Oral presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005

Click to view presentation in PDF format.

       Right mouse and select "Save Target as..." to download higher resolution PDF file.

¹Tecpetrol S.A, Buenos Aires, Argentina ([email protected])

²University of Buenos Aires, Buenos Aires, Argentina

Abstract 

The Neuquén embayment, developed along the eastern foothills of the southern Central Andes and one of the most prolific hydrocarbon basins of southern South America (EUR: 9700 MMBOE), records a complex history of Mesozoic and Cenozoic retroarc-foreland intraplate deformation. Intraplate deformation was the result of three main elements:

·        Paleozoic basement fabrics

·        Early Jurassic hemigrabens

·        Convergence vectors between the Aluk, Farallon, and Nazca oceanic plates and the Gondwana or South American continental plates

Three main basement fabrics, E-W (Huincul system - southern margin), NW-SE (Entre Lomas system - eastern margin) and N-S (Chihuidos system - western depocenter) resulted form the accretion of three terranes to the Gondwana margin during the Paleozoic.

Three stages of intraplate deformation were identified that postdated a brief Early Jurassic rift stage that resulted in a dense mosaic of hemigrabens:

·        The Aluk stage (Early Jurassic-Valanginian) is the earliest contraction episode, with the main stress oriented in the N-NW sector. It resulted in the tectonic inversion by contraction and right-lateral strike-slip deformation of the Huincul system and subtle deformation in the Chihuidos and Entre Lomas systems.

·        The Farallon stage (Valanginian-Miocene), with a more orthogonal orientation, resulted in a westward retreat of the deformation along the southern margin and an increase of the inversion along the Entre Lomas system during the Cenomanian, together with the westward inception of the Agrio fold belt.

·        Nazca stage, with orthogonal stress, resulted in the development of the fold belt and broken foreland system. Stress changes were correlated with the Pacific and Gondwana - South America plate convergence vectors.

          Location

          Location of Neuquén Basin

Present Tectonic Setting 

          Tectonic setting

Complex Cenozoic deformation history

Miocene main contractional episode

·     Main development: Agrio fold and thrust belt development

·     Deformation of Late Cretaceous foreland basin

·     Development of an intraplate deformation front oblique to the orogenic front

Three main structure orientations in specific basin locations

          N-S

          NW-SE

          -W

Delimitation of an outstanding E-W intraplate deformation belt ( Huincul system )

Strong indications of basement control and pre-Miocene intraplate deformation

Basement Fabrics 

Basement fabrics (1)

Basement fabrics (2)

Penetrative deformation and sutures associated with the accretion of three terranes to the Gondwana during the Paleozoic.

They are responsible for the three main orientation in Mesozoic and Cenozoic structures.

The Patagonia suture and related fabrics are the most important elements that influenced the deformation history of the Neuquén embayment.

Seismic information in selected location allowed to image the subsurface extension of syn-collisional deformation into the basin.

These fabrics were reactivated as weakness zones that controlled the extensional faults of the early rift phase.

Early Jurassic Rift System 

            Rift system

Rift phase enhanced the pre-existing basement anisotropies.

Hemigraben geometry and polarity could be controlled by low angle basement fabrics.

Weakest zone corresponds to the Patagonia suture.

Early inversion by contraction and transpression of many hemigrabens  started during the deposition of the Pre-Cuyo Group.

This early inversion is the beginning of a larger multi-episodic process that continued mainly during the Jurassic-Cretaceous

Mesozoic Deformation

Click to view Jurassic – Early Cretaceous deformational features.

            Click to view maps during Aluk and Farallon deformational stages.

Definition of two stages of deformation

Rotation of regional stress field

Advance and retreat of Mesozoic deformation fronts.

Correlation with Paleopacific convergence vector??

Conclusions 

1. The Neuquén embayment is truncated by a major fault system known as the Huincul high.

• Its origin relates to the collision of the Patagonia terrane against the Gondwana margin in Permian times.

• This major structure controlled the intraplate deformation during most of the Mesozoic and Cenozoic times.

2. Changing pattern of the main stress.

• It is proposed that since Early Jurassic times the main stress vector was first located in the north-northwest to northwest quadrant.

• This stress rotated through time to a more orthogonal orientation in the Early to Late Cretaceous.

• Final uplift, related to almost orthogonal compression, took place in the Miocene, followed by Pliocene extensional structures in the foreland.

3. There is a striking match between the main stress orientation deduced from the structural pattern and the convergence vector inferred from different oceanic plate kinematics.

• This implies that strain partitioning at this time was low or inexistent and that the intraplate deformation was mainly controlled by the convergence vector.

• The crustal erosion by subduction was minimal at these latitudes, and therefore the orientation of the continental margin has not changed since Paleozoic times.

4. The structural patterns and the timing of deformation consequently can be explained by four different stages:

• Aluk from the Early Jurassic to Valanginian times;

• Early Farallon from Valanginian to Cenomanian;

• Late Farallon from Cenomanian to Paleogene,

• Nazca stage from the Miocene onwards.