STRUCTURE, DYNAMICS, AND GAS HYDRATE SYSTEMS OF THE PERUVIAN MARGIN: RESULTS FROM INTEGRATED MARINE ANALYSIS

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U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10 CONGRESO GEOLÓGICO CHILENO 2003 STRUCTURE, DYNAMICS, AND GAS HYDRATE SYSTEMS OF THE PERUVIAN MARGIN: RESULTS

1 U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10 CONGRESO GEOLÓGICO CHILENO 2003 STRUCTURE, DYNAMICS, AND GAS HYDRATE SYSTEMS OF THE PERUVIAN MARGIN: RESULTS FROM INTEGRATED MARINE ANALYSIS NINA KUKOWSKI 1, ANDREA HAMPEL 1 *, CHRISTAN HÜBSCHER 2, JÖRG BIALAS 3, ANNE KRABBENHÖFT 3, INGO PECHER 4 1 GeoForschungsZentrum Potsdam, Telegrafenberg, D Potsdam, nina@gfz-potsdam.de 2 Insitute for Geophysics, University of Hamburg, D Hamburg, Germany 3 GEOMAR, Wischhofstr. 1-3, D Kiel, Germany 4 Institute of Geological and Nuclear Sciences, 69 Gracefield Road, Lower Hutt, New Zealand *Institute of Geological Sciences, University of Berne, Baltzerstr. 1, CH-3012, Switzerland INTRODUCTION At the Peruvian convergent margin, the oceanic Nazca Plate subducts obliquely beneath the South American continent at a convergence rate of ~61 mm/a. The age of the subducting plate varies from 28 Ma to 38 Ma at the trench south and north of the Mendaña Fracture Zone (MFZ), respectively. A prominent feature off Peru, which plays an important role in the evolution of the margin, is the 1.5 km high submarine Nazca Ridge. This ridge migrates southward along the margin due to its oblique orientation to both the convergence direction and the trench line. Therefore, the Peruvian margins displays different stages of its evolution during and after ridge subduction from north to south, in particular a temporal sequence of uplift and subsidence of the forearc and regional changes in its erosive mass transfer regime which also affected the widely present gas hydrate occurrences (Pecher et al., 2001), e.g. in the forearc basins. To reconstruct when the ridge passed specific sites of the margin in the past, updated plate motion data (Somoza, 1998) were used to calculate its paleo-positions (Hampel, 2002). Due to the deceleration of the Nazca Plate motion and the variable orientation of the ridge with respect to the trench, the ridge crest moved at a decreasing velocity parallel to the margin. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields an onset of its subduction ~11 Ma ago at 11 S. Therefore, the Peruvian margin offers the possibility to compare regions that have not been affected by the ridge passage (9 S) with regions that have been (12 S) and are presently (15 S) influenced by the ridge, but otherwise have similar geodynamic boundary conditions. To address the tectonics and gas hydrate systems of the Peruvian margin, extensive geophysical data acquisition took place within the framework of RV Sonne GEOPECO cruises in 2000 including swath bathymetry, reflection and wide angle seismics and geological sampling (Bialas & Kukowski, 2000). These data now provide a better structural image of the margin, allow to quantify and mechanically understand tectonic erosion and to throw new light on the margin s gas hydrate systems. Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.

2 THE MARGIN S CRUSTAL STRUCTURE AND GEODYNAMICS MORPHOLOGY OF NAZCA PLATE AND THE LOWER CONTINENTAL SLOPE Multibeam bathymetry data acquired during the RV Sonne GEOPECO cruise at different latitudes reveal steep lower slopes, escarpments and basins but significantly differing roughness. To quantify the roughness of the sea floor, in general several factors contributing to the roughness have to be taken into account: the incremental mean surface slope, the distance coefficient, i.e. the ratio of surface length to base line length, the magnitude of the peaks and the void volume. The roughness of the oceanic crust decreases from north to south and is significantly higher than at the Nazca Ridge, however roughness is not related to the crustal age (Kukowski et al., 2002). The volumes of "pockets" between adjacent highs providing possible sediment traps decreases from north to south and is significantly less than the estimated tectonically eroded volume (Clift et al., in press). A comparison to rates of magmatic growth indicates a net loss of continental material. Between about 8 S and the Nazca Ridge, rough and nearly sediment-free Nazca plate crust enters the Peruvian trench. The oceanic crust is extraordinarily rough with a relief of up to 800 m between narrowly spaced highs. Locally, the highs and lows have slope angles steeper than 20. As the roughness does not increase towards the trench, the structures are inherited and not caused by the bending of the plate. The sediment input along the margin varies, but never exceeds a thickness of about 300 m. The great variety of structures, which are all oblique to the strike of the trench, have different directions and do not seem to be related to each other or the current convergence direction. Surface deformation found at the very steep lower slope occurs at a very small local scale inferring mass wasting and the slope being at failure everywhere. The transition to the upper slope is marked by numerous steep gullies. At 10 S, where the Mendaña Fracture Zone enters the trench, the oceanic crust is also characterized by high roughness but has a completely different pattern with parallel ridges that are crossed by trenchparallel normal faults. The lower slope shows ridge-shaped features that are not related to the directional structures on the plate. At 13 S and 12 S, the Nazca Plate has a rough topography with trench-parallel normal faults. The steep lower slope shows erosional features. The roughness of oceanic plate and lower slope is of small scale, however, compared to other margins, the latter is still extraordinarily steep and rough. A small wedge is revealed by the wide-angle seismic data. The sea floor of the adjacent mid slope terrace is cut by numerous narrow furrows and canyons which may result from mass wasting through surficial erosion. On the upper slope, the Lima Basin opens towards the lower slope and comprises a pronounced drainage system. CRUSTAL STRUCTURE NORTH OF NAZCA RIDGE The structure and the P-wave velocity of the obliquely subducting Nazca and overriding South-American plates from 8 S to 15 S were determined by forward modelling and tomographic inversion of the wide-angle seismic data, which were acquired using ocean bottom hydrophones (OBH) and seismometers (OBS) recording marine airgun shots (Krabbenhöft et al., subm.). These data then were combined with the analysis of reflection seismic data. North of MFZ the oceanic crust is influenced by the Trujillo Trough trending N15E and extensional stresses leading to crustal thinning as can be seen in the northernmost refraction seismic model. The oceanic crust south of MFZ is overall homogeneous with a thin pelagic sedimentary layer and normal oceanic crustal layers. The P-wave velocity of the mantle is km/s. The Yaquina and Lima forearc basins and continental basement could be mapped with forward modelling and tomographic inversion as well as the continental

3 backstop on each profile. A frontal prism is set up with a width of 20 to 30 km and 4 to 5 km thickness which does not further increase in size as revealed by the profiles recorded further north of Nazca Ridge. This and a taper of at the collision zone indicates that current subduction along the Peruvian Margin is non-accretive. NAZCA RIDGE COLLISION ZONE The bathymetric and wide-angle seismic data reveal that the southeastern flank of the ridge, which marks its leading edge entering the trench, has a smooth topography compared to the rough relief of the surrounding Nazca Plate and shows volcanic structures of different size and elevation (Hampel et al., subm.). An increasing number of trench-parallel normal faults can be identified towards the trench which is characterized by a rugged surface and little to no sediment fill. The lower continental slope shows a steep topography and indications for erosion. There is no evidence for the presence of a former or recent accretionary prism. Refraction seismic profiles across of the ridge and along the crest of the ridge continuing on the continental slope show that the Nazca Ridge is covered by several hundred meters of sediment and has a topographical high at the crest. The cross-sectional model of the ridge indicates a thickened crust, with seismic velocities typical of oceanic crust. The Moho is located at a depth of about 20km below sea level beneath the ridge crest. The profile perpendicular to the continental slope shows thickening of the incoming sediments towards the trench, but as there is no accretionary wedge, we assume that they are completely subducted. As the dip of the lower slope is larger than on other profiles of the GEOPECO cruise, erosion of the continental slope in consequence of ridge subduction is suspected. The Nazca plate, which can be traced to a depth of about 27km, subducts at an angle of 9 S. IMPLICATIONS FOR MASS TRANSFER AND MECHANICS In terms of the taper stability field, the margin can be classified as near or at failure everywhere. Slumping occurs on a local scale, only one major slide can be identified from new bathymetry data. Regarding the tectonic evolution of the Peruvian margin, the Nazca Ridge has caused short term erosion of the continental slope, however, this effect is superposed on long term tectonic erosion (Clift et al., in press). In the Yaquina Area (8 s to 9 S), which has not been affected by Nazca Ridge subduction, evidence for recent major subsidence was scarcely found, thus "subduction erosion" may predominate close to the toe of the margin. Although this region has the largest void volume off Peru, the amount is much smaller than the eroded volume. The Lima Area (12 S), which has been affected by ridge subduction 9.5 Ma ago, shows strong subsidence at high rates. This indicates that "subduction erosion" is also efficient below Lima Basin and thus, erosion is distributed over larger area compared to the Yaquina area. The "lense model" (Ranero & von Huene, 2000) might probably be applicable in this case. The void volume is only a quarter of eroded volume. All together, long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin has been lost at an average rate of up to 3.1 km Myr-1. Long term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust. The four main factors controlling the erosive mass transfer regime of the Peruvian margin include the roughness (and therefore friction) of the lower plate, low sediment input, the presence of possible detachments ("weak layers") and vertical tectonism.

4 FOREARC BASIN GAS HYDRATE SYSTEMS LIMA-BASIN: THE RELATION AMONG SUBSIDENCE, EROSION, AND BSR DISTRIBUTION In Lima basin, bottom simulation reflectors (BSRs) were detected at locations undergoing both tectonic subsidence as well as non-sedimentation or seafloor erosion (Pecher et al., 2001). Tectonic subsidence (and additionally perhaps seafloor erosion) causes the base of gas hydrate stability to migrate downward with respect to gas-hydrate-bearing sediments. This process rules out dissociation of gas hydrates as a source of free gas for BSRs at these locations. Instead, free gas at BSRs is predicted to be absorbed into the gas hydrate stability zone. BSRs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow (Kukowski & Pecher, 1999). Seafloor observations with a TV sled revealed rounded boulders and slab-like rocks, which we interpreted as authigenic carbonates typically found at cold vents with methane expulsion. All these observations suggest that BSRs in Lima Basin are maintained predominantly by gas that is supplied from below, demonstrating that this endmember process for BSR formation exists in nature. Results from Ocean Drilling Program Leg 112 showed that methane for gas hydrate formation on the Peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin. The δ 13 C composition of a recovered carbonate cement was consistent with biologic methane production below the seafloor (although possibly above the BSR). If gas supplied for BSR formation in Lima Basin is mainly biogenic methane, this would suggest biologic productivity beneath the gas hydrate zone in Lima Basin to be relatively high in order to supply enough methane to maintain BSRs. YAQUINA BASIN: GAS HYDRATE AND GAS WITHIN THE GAS HYDRATE STABILITY ZONE The distribution of gas and gas hydrate within the central Yaquina Basin was derived from the interpretation of high-resolution reflection seismic data (Hübscher & Kukowski, in press). Upward migration of gas occurs predominantly along near vertical faults. The strongest BSR is observed where the base of gas hydrate stability (BGHS) parallels strata. Where the angle between BGHS and strata changes, just little gas reaches the BGHS. Fluids and gas are bypassed when they are predominantly guided along layers due to anisotropic permeability. Chemoherms evolve where tectonic activity is active until Recent. Here, faults and consequently gas reach up to the seafloor. The warm fluids contort the BGHS and the BSR bulges upwards. Increased heat flux and/or sediment interval velocity in this region is likely. Bright spots align beneath the actual BGHS and mark Paleo-BGHS depth, which can be correlated with the sedimentation of a particular sequence. Phase inversed reflections, the velocity distribution derived from Ocean-Bottom-Seismometer data, and hydroacoustic observations give a clear line of evidences that free gas is present within the gas hydrate stability zone. REFERENCES Bialas J, Kukowski N (2000): Geophysical experiments at the Peruvian continental margin; investigations of tectonics, mechanics, gas hydrates and fluid transport, Geomar rept. 96, Geomar, Kiel, pp.490. Clift PD, Pecher IA, Kukowski N, Hampel A (2003): Tectonic erosion of the Peruvian forearc, Lima Basin, by steady-state subduction and Nazca Ridge collision, Tectonics, in press Hampel, A., N. Kukowski, J. Bialas, C. Huebscher, Heinbockel R: Ridge subduction at an erosive margin - the collision zone of the Nazca Ridge in southern Peru, submitted to J. Geophys. Res.

5 Hampel A(2002): The migration history of the Nazca Ridge along the Peruvian active margin: A re-evaluation and some geological implications, Earth Planet. Sci. Letts., 203, Hübscher C, Kukowski N: Complex BSR distribution in Yaquina basin. Marine Geophysical Research, in press. Krabbenhöft A, Bialas J, Kopp H, Kukowski N, Hübscher C: Crustal structure of the Peruvian continental margin from wide angle seismic studies. Subm. to J. Geophys. Res. Kukowski N, Pecher IA (1999): Thermo-hydraulic modelling of the accretionary complex off Peru at 12 S. J. Geodynamics, Vol. 27, Kukowski N, Hampel A, Bialas J, Broser A, Huebscher C, Barckhausen U, Bourgois J (2002) Long-term and short-term tectonic erosion at the Peruvian margin between 9 S and 15 S: evidence from bathymetry data and 3D sandbox analogue modelling. 5th ISAG, Toulouse, Pecher IA, Kukowski N, Hübscher C, Greinert J, Bialas J. (2001): The link between bottom simulating reflections and methane flux into the hydrate stability zone New evidence from Lima Basin, Peru Margin. Earth Planet. Sci. Lettr. 185, Pecher IA, Kukowski N, Ranero C, von Huene R. (2001) Gas hydrates along the Peru and Middle America Trench system. AGU Monograph Series 124, Ranero, C. R., and R. von Huene, Subduction erosion along the middle America convergent margin, Nature 404, , Somoza, R., (1998) Updated Nazca (Farallon)-South America relative motions during the last 40 My: implications for mountain building in the Central Andean region, J. South Am. Earth Sci. 11,

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