Analysis of the discrepancies in extension measurements in the western Iberia margin

Abstract

Western Iberia is considered an archetypal example of a hyper-extended Atlantic margin. Throughout the last few years, the western Iberia margin has been studied in detail by multiple authors, who have defined its regional-scale structure and mapped out the different tectonic domains. Several authors have remarked on the discrepancy in extension measurements between the upper crust and whole crust in passive margins. Likewise, western Iberia seems to have a marked deficit of syn-rift subsidence based on the estimates of crustal extension. The reasons for this discrepancy are not clear but can be due to poor fault interpretation due to limitations in seismic resolution (leading to erroneous estimates of extension), depth dependent stretching, thermal and dynamic uplift, igneous addition or mantle serpentinization during rifting. Our interpretation of the western Iberian margin indicates that estimates of beta factor (crustal extension) based on fault heaves can be a factor of two less than estimates from crustal thickness derived from refraction seismic or isostatic equilibrium methods. Part of the discrepancy in this case arises from the difficulty in accurately quantifying all of the fault displacements; some faults are smaller than the resolution of seismic data, and erosion of the tilted fault blocks may give rise to flawed estimates of the pre-rift’s sequence original length. Nonetheless, even an aggressive model-driven interpretation, constrained by gravity and magnetic modeling, fails to account for the extension discrepancy. The present study describes a methodology to resolve the origins of the extension discrepancy in three different sectors of the western Iberian margin; Galicia, Peniche-Alentejo and Algarve-Gulf of Cadiz. To better understand the origin of the extension discrepancy in western Iberia, we present a detailed study where kinematically-constrained seismic interpretation has been performed, in integration with gravimetric and magnetic models to constrain the depth to basement and Moho geometry. The resulting interpretation has been sequentially restored to identify the structural elements that give rise to the extension discrepancy. These observations have in turn been used to iteratively improve the seismic-based interpretation. The methodology presented in this study has allowed us to reach a number of conclusions that help to reduce exploratory risk in Atlantic-type margins, where the lack of well control in the deeper parts of the basin makes it difficult to constrain elements of the petroleum system. We believe that this approach is relevant to address the deficit of extension of other hyperextended Atlantic margins.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018