Marine gas hydrate systems and vertical tectonics in trujillo basin and lima basin central peruvian convergent margin

Herbozo Jiménez, Gerardo Manuel GND

During the last decades, drilling evidence from marine gas hydrate investigations at convergent margins has revealed how complex gas hydrate accumulations can be at these margins. So far, the complexity of marine gas hydrate systems in these tectonically active environments has not been accurately represented by numerical models often used to predict gas hydrate formation and accumulation. The occurrence and distribution of marine gas hydrates at these margins, thus, have been poorly constrained. It is still un- known if regional processes that have influenced the evolution of these margins, such as vertical tectonics and subduction erosion, are useful factors that can add new ideas to improve these constraints. This dis- sertation has been focused to address this idea and to test the Tectonic BSR Hypothesis that links marine gas hydrates and vertical tectonics. In this dissertation, I aim to investigate the relation of marine gas hydrate systems situated along two forearc sedimentary basins of the central Peruvian convergent margin that have different vertical tectonic histories. The Trujillo Basin (at 9°S) and the Lima Basin (at 12°S) are two marine basins separated by ∼300 km along the central Peru Margin which have been affected by distinct post-Late Miocene vertical tectonic histories due to the subduction of Nazca Ridge. The structural framework of Trujillo Basin has evolved under a post-Miocene low-to-moderate vertical tectonic regime and slow subduction erosion, whereas the structural framework of Lima Basin has been controlled by a moderate-to-strong vertical tectonics regime and fast subduction erosion as a consequence of the subduction of Nazca Ridge. In Chapter 2, I quantified subduction erosion in the Trujillo Basin. The aim of this quantification is to compare the tectonic evolution of this basin with the tectonic history of Lima Basin previously quantified. The main outcome from this study is that subduction erosion has varied in space and time and has dom- inated the structural evolution of this part of the Peruvian convergent margin during the last 60 million of years (Myr). Subduction erosion has been slower during the Quaternary (1 km Myr–1 of trench retreat, equivalent to average frontal subduction erosion of 20 km3 Myr–1 km–1) compared to the moderate sub- duction erosion which had occurred during the Late Miocene (3 km Myr–1 of trench retreat, equivalent to average frontal subduction erosion of 60 km3 Myr–1 km–1). Post-Miocene tectonic subsidence has been on average 150 m Myr–1. In this study, I showed that shorter geological periods of faster subduction erosion coincide with phases of major plate reorganizations in the East Pacific (e.g., the break-up of the Farallon Plate into the Nazca Plate and Cocos Plate during the Oligocene, ∼23 million years ago (Ma)) and with well-known phases of Andean orogeny. The main conclusion of this study is that subduction erosion has been minor in pre-Eocene times, then it has been faster and variable between the Eocene and Neogene, and finally it decelerated during the Quaternary. Comparatively, post-Late Miocene vertical tectonics and subduction erosion along Trujillo Basin have been three times slower than that of Lima Basin. In Chapter 3, I identified the post-Pliocene geological processes affecting marine gas hydrate deposits in Trujillo Basin. The aim of this study is to conceptualize the influence of this recent activity, product of ver- tical tectonics, on the distribution of bottom-simulating reflectors (BSRs) that suggest marine gas hydrate accumulations. The main result from this study is that BSRs widely occur across this sedimentary basin characterized by post-Late Miocene slow vertical tectonics and subduction erosion. Recent depositional activity (slumping) is related to low-to-moderate near-seafloor heat flow (7–39 mW/m2) and continuous BSRs, whereas recent tectonic activity (extensional faulting) is related to moderate-to-high near-seafloor heat flow (52–110 mW/m2) and patchy BSRs. The main conclusion of this study is that recent depositional activity may restrict the transfer of heat toward the seafloor, whereas recent tectonic activity may allow it. This study shows that recent geological processes are key factors that control the shallow dynamics of warm, gas-rich fluids through marine gas hydrate systems at convergent margins, hence demonstrating their complexity. In Chapter 4, I focused to search for possible mechanisms linked to the dynamic evolution of uncommon features known as double BSRs in Lima Basin. In this study it is shown that few BSRs occur along this strongly subsiding sedimentary basin. Only very few BSRs (restricted to morphological highs) are related to double BSRs, seafloor erosion, slump masses. Double BSRs occur above (as much as ∼100 m) and below (as much as ∼80 m) the current BSR. The main conclusion from this study is that Latest Quaternary climate warming could have been the main responsible factor for the evolution of double BSRs. This could have been the result of the paleo-regime of the Peru Current System (PCS). The existence of multiple BSRs evidenced that fluid dynamics in this marine gas system was disturbed, and this could have been caused by instantaneous processes such as mass slumping or post-glacial warming. These findings show that environmental processes add more complexity to the already complexly distinctive behavior of marine gas hydrate systems at convergent margins imposed by vertical tectonics. Therefore, I conclude from this dissertation that tectonic subsidence of the continental margin (the most common outcome of the long-term subduction erosion) enforces the development of distinctive ma- rine gas hydrate systems. In this sense, each gas hydrate system has a fundamental dependency on to vertical tectonics. I interpret that this regional process is the primary responsible for the development of characteristic marine gas hydrate systems along the Peruvian margin. This suggests that the Tectonic BSR Hypothesis could be valid, and may be also valid for other convergent margins. I propose that short-term processes such as recent depositional and recent tectonic activity as well as environmental changes as result of climate change lend more unevenness to the evolution of marine gas hydrates, especially in a local extent compared to subduction erosion.

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Herbozo Jiménez, Gerardo Manuel: Marine gas hydrate systems and vertical tectonics in trujillo basin and lima basin central peruvian convergent margin. 2015.

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