Paleomagnetism and Thermochronology in tertiary syntectonic sediments of the South-Central Pyrenees: Chronostratigraphy, Kinematic and Exhumation constraints
Resumen Abstract Índice Conclusiones
Elisabet Beamud Amorós
This PhD Thesis has been prepared as a research papers compilation. These research papers have been published in journals belonging to the Journal Citation Report of the Institute for Scientific Information.
The thesis is organized in 4 chapters:
Chapter 1 is an introductory chapter that describes the geological setting of the study area, the stratigraphic framework, the methodologies used to accomplish the objectives of this thesis (Paleomagnetism and detrital fission track thermochronology) and the European Paleogene continental mammal biochronology.
Chapter 2 represents the core of the thesis and includes the main results of the study. It is composed by threepapers:
Chapter 2.1 is composed by the paper: Beamud, E., Garcés, M., Cabrera, L., Muñoz, J.A., Almar, Y. (2003) A new Middle to Late Eocene chronostratigraphy for NE Spain. Earth and Planetary Science Letters 216, 501-514. In this paper a new chronostratigraphy for the Eocene continental units of the South-Central Pyrenees is presented. It is based on the integration of the magnetostratigraphic dating of the La Pobla de Segur and Sis conglomerates to the previously established chronology of the continental units within the Ainsa Basin (Escanilla Fm.). The age of La Pobla de Segur conglomerates was losely constrained based on fossil localities within the succession that were assigned to the MP17 reference level of the European Paleogene mammal reference levels, suggesting a Priabonian age for the middle part of the succession. The magnetostratigraphic results of this work challenge the previous dating of these materials and implications for the calibration of the European Paleogene mammal reference levels (MP bichronology) are also derived from the new chronology.
Paper Beamud, E., Muñoz, J.A., Fitzgerald, P.G., Baldwin, S.L., Garcés, M., Cabrera, L., Metcalf, J.R. (2011) Magnetostratigraphy and detrital apatite fission track thermochronology in syntectonic conglomerates: constraints on the exhumation of the South-Central Pyrenees. Basin Research 23, 309-331 forms Chapter 2.2. In this paper the magnetostratigraphic dating of the continental syntectonic conglomerates is combined with detrital apatite fission track thermochronology applied on granite clasts within these syntectonic materials. The magnetostratigraphy already introduced in chapter 2.1 is extended up with the magnetostratigraphic dating of the Senterada sub-basin conglomerates. The distribution of the fission-track ages and lengths through the studied sections combined with their depositional ages has yield information about the time and amount of exhumation in the Pyrenean orogen.
Chapter 2.3. is formed by the paper Muñoz, J.A., Beamud, E., Fernández, O., Arbués, P., Dinarès-Turell, J., Poblet, J. The Ainsa Fold and Thrust Oblique Zone of the Central Pyrenees: kinematics of a curved contractional system from paleomagnetic and structural data. Tectonics (accepted). In this paper a new kinematic model for the N-S trending structures of the Ainsa Oblique Zone and the origin of the curved shape of the South-Central Pyrenees are presented. Although many previous studies had already identified clockwise vertical axis rotations within the Eocene materials of the Ainsa Basin, a consistent kinematic model for the Ainsa Oblique Zone was still not available. The new model presented in this paper integrates new and previous paleomagnetic data with structural and sedimentological data yielding a coherent model with all the available information.
Chapter 3 includes a synthesis and discussion of the obtained results, presented as three sub-sections:
3.1. Chronostratigraphy of the Tertiary syntectonic materials of the South-Central Pyrenees.
3.2. Biochronological implications
3.3. Kinematic and exhumation model for the South-Central Pyrenees.
Finally, chapter 4 summarizes the main conclusions derived from this PhD Thesis.
This PhD Thesis presents the Tertiary kinematic evolution of the South-Central Pyrenees from the integration of magnetostratigraphic, magnetotectonic and thermochronological analyses on its synorogenic sedimentary record. The integration of these datasets permits unravelling the interplay between thrust emplacement, deposition in the adjacent basins and exhumation and denudation in the interior of the orogen.
One of the main contributions of this PhD thesis is the continuous absolute dating of the syntectonic conglomerates of La Pobla de Segur, Senterada and Sis. Magnetostratigraphy of these synorogenic materials establishes their deposition during Middle Eocene-Late Oligocene times (from chron 19r up to chron 9n). Integration of these results with previous magnetostratigraphic works within the Ainsa Basin has allowed the establishment of a new chronostratigraphy for the Eocene-Oligocene materials of the South-Central Pyrenees.
The obtained chronostratigraphy has biochronological implications as it substantially changes the traditionally accepted ages of the European reference levels MP14 to MP17. The South Pyrenean and Ebro basin fossil localities attributed to MP14 to MP17 reference levels would be significantly older than previously considered. MP14 and MP15 reference levels are proposed to correlate to Lutetian whereas MP16 and MP17 would be Bartonian in age. These results challenge the currently accepted Eocene marine-continental correlations in Europe but are not in conflict with the available biostratigraphic data of other western European reference basins. These results also reveal that the correlation between the continental and marine Paleogene record needs further refinement and therefore, that the chronostratigraphic age attributions based on MP reference levels should be taken with caution.
The magnetostratigraphic ages have been combined with detrital thermochronology on 13 granitic cobbles contained within the syntectonic conglomerates. The detrital apatite fission track ages obtained vary from 63 to 27 Ma. When these ages are combined with the stratigraphic ages, samples define 5 groups with thermochronological ages generally increasing down-section except in the most deeply buried ones due to post-depositional partial annealing. Thermal models reveal three periods of rapid-cooling within the Axial Zone: two well-defined periods around 50-40 Ma (exhumation rates around 0.2-0-3 km/My), and 30-25 Ma (exhumation rates higher than 1 km/My); and a poorly defined period around 70-60 Ma with exhumation rates around 0.2 km/My. These results, together with the available information of in-situ thermochronological data of the source region, reveal that these materials represent the stratigraphic record of the most intense period of exhumation in the interior of the orogen due to movement on large-thrust sheets. The dramatic increase in exhumation rate occurred during the latest Eocene-Early Oligocene would be related to the onset of movement in the Rialp thrust sheet and the increase in structural relief of the Axial Zone by underthrusting. The magnetostratigraphic and thermochronological ages obtained permit the link between the sedimentation rates in the surrounding basins and the exhumation rates in the hinterland, and reveal that accommodation space exerted the main control on sedimentation rates within the piggy-back basins. Post-depositional annealing of the stratigraphically lowest samples suggests about 2 km of burial by the younger synorogenic materials during progressive burial of the South Pyrenean fold and thrust belt. Thermal models also suggest a rapid exhumation event during the Late Neogene, linked to re-excavation caused by the base level drop during the Ebro River capture to the Mediterranean Sea.
Sedimentation of the studied synorogenic materials during Middle Eocene-Oligocene times occurred coeval to the development of thrust salients in the southern Pyrenees. Among them the most significant one is defined by the N-S trending structures of the Ainsa Oblique Zone. The magnetotectonic study of 36 sites carried out in the Ainsa Oblique Zone reveal clockwise vertical-axis rotations varying from ~80° in the lower Lutetian materials of the Mediano anticline to ~20° in middle Ilerdian materials cropping out at the northern edge of the Añisclo anticline. Sites in the central part of the Montsec and Bóixols thrust sheets don’t record any significant rotation as neither do the syntectonic materials of La Pobla, Sis and Senterada. An axial surface of rotation can be traced coinciding with the eastern edge of the circular bend connecting the N-S trending structures of the Ainsa Oblique Zone and the western edge of the Montsec thrust sheet with the E-W to WNW-ESE structural trend further east. The amount of CW vertical-axis rotation depends also on the age of the sampled sediments. The highest values of rotation have been recorded by the Lower Eocene sediments and these values generally decrease with age. The age of the main rotation event within the Gavarnie thrust sheet is constrained to Lutetitan to Bartonian times, when all the structures of the Ainsa Oblique Zone were active. This vertical-axis rotation stage obeys to a difference of about 50 km in the amount of displacement on the Gavarnie thrust sheet controlled by the NE-SW pinch out of the Triassic salts at its basal detachment. A second rotation event of at least 10° took place since Priabonian times, as a result of the differential displacement of about 22 km of the Serres Marginals thrust sheet, respect the Gavarnie one, on top of the upper Eocene-Oligocene evaporites. The synchronicity between thrusting and vertical-axis rotations suggests that the curved fold and thrust belt formed by progressive curvature with divergent thrust transport.
The results exposed in this Thesis reveal a strong relationship between the stratigraphic record of the synorogenic materials, thrusting and exhumation in the Axial Zone and the structural evolution of the South Pyrenean thrust system. Tectonic forces controlled the observed patterns of exhumation, the evolution of the synorogenic topography of the piggy-back and foreland basins and the depositional features of the synorogenic sediments.
INDEX – ix
FIGURES and TABLES INDEX – xi
ABSTRACT – 1
RESUM EXTENS EN CATALÀ – 6
MOTIVATION, OBJECTIVES and STRUCTURE of the THESIS – 23
CHAPTER 1: INTRODUCTION – 33
1.1. GEOLOGICAL SETTING – 35
1.2. STRATIGRAPHIC FRAMEWORK – 39
1.2.1. Stratigraphy of the syntectonic conglomerates – 42
1.3. METHODS – 52
1.3.1. PALEOMAGNETISM – 52
1.3.2. FISSION TRACK THERMOCHRONOLOGY – 59
1.4. EUROPEAN PALEOGENE MAMMAL BIOCHRONOLOGY – 66
1.4.1. Regional significance and correlation of the MP Reference Levels to the Geological Time Scale – 68
CHAPTER 2: RESULTS – 73
2.1. A new Middle to Late Eocene chronostratigraphy for NE Spain. – 75
2.2. Magnetostratigraphy and detrital apatite fission track thermochronology in syntectonic conglomerates: constraints on the exhumation of the South- Central Pyrenees. – 91
2.3. The Ainsa Fold and Thrust Oblique Zone of the Central Pyrenees: Kinematics of a curved contractional system from paleomagnetic and structural data. – 117
CHAPTER 3: SYNTHESIS and DISCUSSION – 167
3.1. CHRONOSTRATIGRAPHY of the TERTIARY SYNTECTONIC MATERIALS – 169
3.2. BIOCHRONOLOGICAL IMPLICATIONS – 172
3.3. KINEMATICS, EXHUMATION and TECTONOSEDIMENTARY EVOLUTION OF THE SOUTH-CENTRAL PYRENEES – 174
CHAPTER 4: CONCLUSIONS – 197
SUPPLEMENTARY MATERIAL 1: MAGNETOSTRATIGRAPHIC SECTIONS – 203
SUPPLEMENTARY MATERIAL 2: MAGNETOSTRATIGRAPHY of the PONTILS SECTION – 241
The results presented in this Thesis are derived from the integration of magnetostratigraphic, magnetotectonic and thermochronological data, together with structural and sedimentological information from the south-central Pyrenees. The integration of these datasets permits quantification and unraveling the timing of differential crustal movements during orogen evolution, including thrusting events, uplift and exhumation, subsidence or vertical axis rotations. This is important to establish coherent kinematic models and to understand the relationships between surface processes and tectonic forces.
The new magnetostratigraphic ages presented challenge previous dating of the synorogenic continental units preserved in the inner parts of the South Pyrenean fold and thrust belt and enable a precise chronostratigraphic framework for the Eocene-Oligocene syntectonic materials of the South-Central Pyrenees and the recorded tectonic and sedimentological events. In addition, this new chronostratigraphy substantially changes the traditionally accepted ages of the European reference levels MP14 to MP17. The South Pyrenean and Ebro basin fossil localities representing from MP14 to MP17 reference levels would be significantly older than previously considered. MP14 and MP15 are correlated to Lutetian whereas MP16 and MP17 would be Bartonian in age. These results reveal that chronostratigraphic age attributions based on MP reference levels should be taken with caution and revised in the light of all the available regional litho-, bio- and magnetostratigraphic information.
This new chronology establishes the deposition of the synorogenic materials of La Pobla/Senterada and Sis between Lutetian and Oligocene times. Therefore, these materials were deposited in piggy-back basins coinciding with the most intense periods of exhumation in the core of the orogen related to underthrusting and growth of the Axial Zone antiformal stack. Three main periods of exhumation linked to the movement on the Nogueres, Orri and Rialp thrust-sheets have been deduced from detrital thermochronology combined with in-situ thermochronology of the source region. Since sediment supply was granted by the uplifting Pyrenees, accommodation space must have exerted a significant control in changes in the sedimentation rates within the piggy-back and foreland basins. From Lutetian to early Bartonian times sedimentation rates in the piggy-back basins were similar to the exhumation rates in the inner zones caused by the underthrusting of the Orri thrust sheet. Sedimentation in the adjacent basins was characterized by aggradational stacking patterns and development of lacustrine environments indicative of high accommodation space in the South Pyrenean basins during this period. A dramatic decrease in sedimentation rates took place in the South Pyrenean basins during Bartonian times, whereas the exhumation rates remained almost constant. Progradational stacking patterns, sediment condensation zones and abandonment surfaces in the syntectonic conglomerates point to a reduction in accommodation space during this time. The reduction in accommodation space was more pronounced in the eastern sector, in La Pobla area, than to the west, in the Ainsa and Jaca areas, as a consequence of the migration of the deformation to the SW. The studied materials also reveal a strong interplay between base level changes in the Ebro foreland basin and the tectonosedimentary evolution of the south-central Pyrenees. Sedimentation rates recovered after the closure of the Ebro basin during Priabonian times, as most of the material was preserved due to the ceasing of the by-pass of sediments to the west. The closure of the south Pyrenean Ebro foreland basin and the related backfilling favored a migration of the deformation to inner parts of the chain and a dramatic increase in exhumation rate in the Axial Zone due to the onset of underthrusting of the Rialp thrust sheet. Base level drop by the posterior capture of the Ebro River by the Mediterranean Sea during Miocene times induced re-excavation of the piggy-back/foreland basins and forced post-orogenic exhumation within the Axial Zone.
Deposition of these materials also coincides with gradients of thrust displacement of the thrust sheets that account for the oblique structures and thrust salients in the South Central Pyrenees. The precise ages of the sedimentary successions of the south-central Pyrenees constrain the 60° to 45° clockwise vertical axis rotations on its western tip to Lutetian to Bartonian times when the structures of the Ainsa Oblique Zone developed. This rotation resulted from about 50 km of differential displacement on the Gavarnie thrust sheet caused by the distribution of the Triassic evaporites at its basal detachment. The synchronicity between thrusting and vertical-axis rotations points to a model of progressive arc due to divergent thrust emplacement to explain the origin of the curvature in the Ainsa Oblique Zone. Transverse extensional faults and diapirs formed at the outer arcs in response to rotation space problems. Translation of the Serres Marginals thrust sheet on top of the foreland since Priabonian times added a secondary rotation of about 10° in the western sector and produced anticlockwise rotations at its eastern part. Therefore, the accurate dating of these materials reveals a diachrony in the formation of both ends of the thrust salient of the south-central Pyrenees.
The results exposed in this Thesis highlight a strong relationship between the structural evolution of the South Pyrenean thrust system, thrusting and exhumation in the Axial Zone and the stratigraphic record of the synorogenic materials. Tectonic forces controlled not only the observed patterns of exhumation but also the evolution of the synorogenic topography of the piggy-back and foreland basins and the depositional features of the synorogenic sediments.