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Durham E-Theses Durham E-Theses Gravity Anomalies, Geodynamic Modelling and the Eastern Venezuela Basin Evolution. Rodriguez Millan, Inirida How to cite: Rodriguez Millan, Inirida (2014) Gravity Anomalies, Geodynamic Modelling and the Eastern Venezuela Basin Evolution., Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/9454/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 GRAVITY ANOMALIES, GEODYNAMIC MODELLING AND THE EASTERN VENEZUELA BASIN EVOLUTION by INÍRIDA RODRÍGUEZ MILLÁN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT THE UNIVERSITY OF DURHAM Department of Earth Sciences 2013 GRAVITY ANOMALIES, GEODYNAMIC MODELLING AND THE EASTERN VENEZUELA BASIN EVOLUTION by INÍRIDA RODRÍGUEZ MILLÁN Geophysical Engineer, Central University of Venezuela Magister Scientiarum, University of Leeds, UK. ABSTRACT This work examines the tectonic evolution of the Eastern Venezuela foreland basin by analysis of gravity anomalies and geodynamical modelling. Gravity data (8ºN-12ºN and 60ºW-66ºW) were processed to produce gravity anomaly maps, the most prominent feature being the minimum of -200 mGal isostatic, Bouguer and free-air anomalies associated with the basin. Positive gravity values characterize the northern terranes. Backstripping analysis of the sedimentary successions of four boreholes penetrating the Eastern Venezuela basin was applied to evaluate the history of subsidence. This demonstrated that an early passive margin phase (Cretaceous to early Oligocene) was followed by Oligocene to Recent tectonic subsidence of the foreland basin linked to the northern coastal compressional tectonic belt. Geohistorical analysis shows a major contribution to subsidence from early Oligocene, renewed during mid- Miocene times, in response to the loading of the South American plate on its northern margin. This tectonic loading pattern is younger from north to south. The observed gravity anomaly paired in eastern Venezuela is adequately reproduced by crustal models along profiles OO´ and II´ showing the isostatic negative anomaly over the foreland basin, primarily caused by the 10-13 km of sediments, the downwarping of dense lower crust and Moho down to 48 km depth. The positive gravity anomalies to the North are associated with southward thrusting of metamorphic and magmatic terranes, and dense Caribbean lithosphere. Dense subducting mantle may also be contributing to the northern positive anomaly belt as deep seismicity suggests. Two mechanisms were applied to explain the formation of the Eastern Venezuela foreland basin during a collisional regime. First, the “hidden load” approach explains the deflection but includes an intracrustal load whose magnitude is around a third of the supracrustal loads computed by an iterative process. This gives rise to a major misfit between the gravity anomaly computed for the structural model which takes into account all the geological and geophysical constraints when the contribution of the “hidden load” is included in the calculations. Second, since the hidden load hypothesis fails to explain the gravity profiles, a remaining viable explanation is release of compressional strain energy involving N-S crustal shortening involving faulting. I therefore explored a fault- based hypothesis which does not depend on hidden gravitational loads, but takes into account the clear relationship between the subsidence and the complementary uplift. In conclusion the prime cause of the evolution of the system depends on the forces on fault planes as they move, where the tractions on a thrust fault develop when a frictionless fault occurs in response to horizontal deviatoric compression of an elastic layer. As compression acts at the edges of the plate, the stresses are re-orientated producing the movement of the two plates along dipping fault planes (including the El Pilar fault). This led to the formation of the Eastern Venezuela foreland basin and linked uplift of the Eastern Serranía del Interior by spasmodic release of elastic strain energy. ii GRAVITY ANOMALIES, GEODYNAMIC MODELLING AND THE EASTERN VENEZUELA BASIN EVOLUTION CONTENTS Page Abstract ii Contents iii List of figures v List of tables xiv Acknowledgments xvi CHAPTER 1 INTRODUCTION 1.1. Introduction 2 1.1.1. The objectives of this research 2 1.2. The Caribbean plate. 3 1.2.1. Eastern Caribbean plate boundary zone 14 1.2.2. Evolution of the Eastern Caribbean plate 21 1.2.3. Southern Caribbean plate boundary zone. 24 CHAPTER 2 EASTERN VENEZUELA TECTONIC SETTING 2.1. Introduction 29 2.2. Geology prior to Jurassic tectonism. 30 2.2.1. Guayana Shield. 30 2.2.2. Post-Cambrian and pre-Jurassic 33 2.3. Jurassic rifting: Extensional domain 34 2.4. Passive margin phase: Early Cretaceous to Eocene. 36 2.5. The northern overthrusting domain 39 2.5.1. Araya-Tobago terrane 39 2.5.2. Eastern Venezuela foreland thrust and fold belt. 44 2.6. The Eastern Venezuela foreland basin. 46 2.6.1. Oligocene to Present time. 47 2.6.2. Regional Tertiary to Present structures. 51 2.7. The structure of the Eastern Venezuela basin. 55 2.8. Key elements for the evolution of the Eastern Venezuela basin. 58 2.9. Seismicity in eastern Venezuela. 62 CHAPTER 3 SUBSIDENCE IN THE EASTERN VENEZUELA BASIN 3.1. Sedimentary basins. 67 3.1.1. Sedimentary basins classification. 67 3.2. Passive-Margin basins. 68 3.3. Foreland basins. 69 3.4. Mechanisms of basin formation. 70 iii 3.4.1. An introductory review. 70 3.5. Sedimentation vs. Subsidence in the Eastern Venezuela foreland basin. 74 3.5.1. Tertiary subsidence of the Eastern Venezuela foreland basin. 75 3.5.2. Sediment loading effect. 83 CHAPTER 4 GRAVITY ANOMALIES 4.1. Gravity anomalies in Eastern Venezuela. 91 4.2. Gravity data reduction. 91 4.2.1. Complete Bouguer anomaly. 93 4.3. Gravity anomaly maps of Eastern Venezuela. 94 4.4. Isostatic anomalies. 99 4.1.1. An introductory review. 99 4.4.2. Isostatic anomalies Eastern Venezuela -Airy and Pratt-. 102 4.4.3. Bouguer and free-air anomalies vs. topography. 104 4.4.4. Gravity anomaly-topography admittance and coherence analysis techniques. 110 4.5. Two-dimensional gravity modelling. 111 4.5.1. Constraints from borehole data. 112 4.5.2. Constraints from seismic data. 113 4.5.3. Constraints from density measurements. 115 4.5.4. Spectral analysis from gravity data. 118 4.5.5. Crustal sections in eastern Venezuela. 119 4.5.6. Seismicity and gravity modeling. 127 CHAPTER 5 EASTERN VENEZUELA FORELAND BASIN FORMATION 5.1. Introduction. 132 5.2. Flexure of the continental lithosphere. 133 5.2.1. Some theoretical considerations. 133 5.2.2. A flexural model for the Eastern Venezuela foreland basin. 137 5.3. Flexural modelling in the Eastern Venezuela foreland basin. 146 5.3.1. Effect of topography-thrusts-sediments. 146 5.3.2. Estimation of hidden loads. 151 5.3.3. Hidden load effect. 153 5.4. Results from flexural modelling. 155 5.4.1. Results Profile 00’ 157 5.4.2. Results Profile II’ 160 5.5. The origin of the “hidden load”. 164 5.6. The stress release mechanism. 167 5.7. The nature of the hidden load subsurface load required to explaining the Eastern Venezuela basin subsidence and its relation to subduction. 172 5.8. Mantle dynamic topography and tectonic overview 174 iv CHAPTER 6 SUMMARY and CONCLUSIONS 179 BIBLIOGRAPHY AND REFERENCES 188 APPENDIXES APPENDIX A.1 SEDIMENTATION vs. SUBSIDENCE STRATIGRAPHIC ZONAL DISTRIBUTION 216 APPENDIX A.2 SEDIMENT LOADING EFFECT BACKSTRIPPING 229 APPENDIX B FINITE ELEMENT METHOD FOR FLEXURE 233 APPENDIX C GRAVITY DATA CATALOGUE EASTERN VENEZUELAN BASIN 241 APPENDIX D EARTHQUAKE DATA CATALOGUE EASTERN VENEZUELAN BASIN 267 LIST OF FIGURES Figure page 1.1 Regional tectonic setting of the Caribbean exhibiting the most striking structural features of the Caribbean region. 4 1.2 Diachronous eastward displacement of the Caribbean plate relative to the North and South American plate with numbered, solid black lines representing the inferred locations of the leading edge of the Caribbean plate at these times (modified from Lugo and Mann, 1995; taken from Escalona and Mann, 2011). 6 1.3 Map of seismic activity in the Caribbean region. Approximate epicentre’s location from International Seismology Center –ISC. 8 1.4 Satellite Free-Air anomaly map of the Caribbean region. Gravity data from Sandwell and Smith (2009). 10 1.5 Bouguer Anomaly map of the Caribbean region. Gravity data from Sandwell and Smith (2009). 11 v 1.6 Magnetic anomaly map of the Caribbean region. Magnetic data from EMAG2: Maus et al. (2009) 12 1.7. Heat flow measurements in the Caribbean region. Values are in heat-flow units (1 h.f.u. = 1 μcal cm-2 s). Compiled from Epp et. al. (1970), Erickson et al. (1972), Simmons and Horai (1968) and Lee and Clarck (1968). After Bowin (1976), generated by Arnaiz-Rodríguez (2012, in press). 13 1.8. Seismic line BOL30 (From Aitken et al., 2011, Fig. 5): A) Uninterpreted seismic reflection data. B) Interpreted seismic reflection data. C) Interpreted seismic refraction data from Christeson et al. (2008). 19 1.9. Seismicity in the Eastern Caribbean: Lesser Antilles Island Arc. Depth: Yellow=0-35 km; red=36-70 km; green=71-150 km; blue =151-300 km.
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