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Subduction to Strike-Slip Transitions on Plate Boundaries

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Penrose Conference SUBDUCTION TO STRIKE-SLIP TRANSITIONS ON PLATE BOUNDARIES Puerto Plata, Dominican Republic January 18-24, 1999 ABSTRACT VOLUME Conveners: Paul Mann, Nancy Grindlay and James Dolan Sponsored by the Geological Society of America, Petroleum Research Fund, NSF and NASA Penrose Conference: Subduction to Strike-Slip Transitions on Plate Boundaries, Jan. 18-24, 1999 STRUCTURAL GEOLOGY AND SEDIMENTOLOGY OF A PLIOCENE INNER-TRENCH SLOPE SUCCESSION, NORTHWESTERN ECUADOR K. R. AALTO Dept. of Geology, Humboldt State Univ., Arcata, CA, USA 95521, [email protected] The Pliocene Upper Onzole Formation exposed in the vicinity of Punta Gorda, near Esmeraldas, Ecuador, is composed mainly of fine-grained mud turbidites, having regular vertical sequences of sedimentary structures associated with a positive grading, and bioturbation restricted mostly to the tops of beds. The remainder of beds measured consist of volcanic ash, mud pelagite, and glauconitic silt-sand turbidites. Vertical sequential analysis of stratigraphic sections for the most part show no pronounced trends in bed thickness or grain size. Volcanic ashes are crystal-vitric tuffs occurring in four bedding styles: A) normally- graded ashes with burrowed gradational tops and sharp wavy bases; B) ashes that form part of a complex microstratigraphy consisting of thinly-bedded mudstone, silt-sand turbidites, tuffaceous mudstone, and ash beds; C) less conspicuous ash laminae and ash-filled burrows; and D) a tuffaceous bed with ash and mud swirled together in convolute layers. Ash chemistry suggests an Andean high-K calc-alkaline provenance. Facies relations, paleontologic data and regional geologic setting suggest sediment accumulation on an inner trench slope in a basin situated oceanward of the Pliocene trench-slope break. Faulting is extensive and reflects two deformational episodes, the youngest involving Holocene marine and fluvial terraces containing Pre-Columbian artifacts. Faults group into an older, northerly-trending, listric set, and a younger, WNW-trending high-angle set. Fault striations of both sets suggest dominantly dip-slip motion. Associated with the older set of faults is a spaced fracture cleavage defined by dark, curvilinear to sinuous S-surfaces that are less than 1 mm wide but tens-of-cm long, oriented subparallel to listric faults. Cleavage surfaces are spaced over distances as great as several meters from larger faults. Individual surfaces are defined in thin section by cataclastic deformation resulting in grain diminution, commonly characterized by small amounts of offset. The faults reflect an episode of Quaternary trench-normal extension in the forearc, followed by trench-parallel extension. Both sets of faults crosscut large NE-trending regional folds. Daly (1989) documented east-west extensional deformation among late Oligocene-middle Miocene rocks in northwest Ecuador, but did not refer to similar strain recorded among Pliocene rocks. This deformation resulted in uplift of the southern and eastern margins of the Borbón Basin in the middle Miocene, which resulted in influx of coarse clastics into the basin from the east. Evans and Whittaker (1982) recognize successive episodes of uplift to the south that resulted in the influx of coarse turbidite clastics into the basin during the early Pliocene. Following deposition of these turbidites, mudstone deposition and basin deepening resumed, and the portion of the Upper Onzole exposed at Punta Gorda accumulated. It is possible that the episode of extensional deformation that produced the older, north-trending faults in the study area reflects reactivation of faults in the accretionary wedge that were responsible for the earlier episodes of Neogene basin inversion proposed by Evans and Whittaker (1982). Daly (1989) relates trench-normal extension in the forearc to decreasing plate-convergence rates, which resulted in collapse of the forearc wedge. Although he does not specifically relate a Neogene rate decrease to an episode of extensional deformation, this decrease may help to explain the genesis of the older, north-trending faults observed in the study. During the Neogene the landward margin of the Ecuador trench may have also suffered net tectonic erosion, which is an alternative hypothesis to explain Neogene east-west extension recorded by the first generation of faults in the study area. If Neogene basin evolution in the Ecuadorian forearc reflects trench-normal extension, the resulting basins may be structurally 1 Penrose Conference: Subduction to Strike-Slip Transitions on Plate Boundaries, Jan. 18-24, 1999 and morphologically similar to those of the sediment-rich Central American forearc, a region also characterized by block-faulting reflecting extension orthogonal to the Middle America Trench. Another analog for the depositional setting of the Upper Onzole is the outer deep- marine basin portion of the terraced compound forearc basin of the Alaskan-Aleutian Arc, although these outer basins formed in response to transpressional and transtensional deformation associated with strike-slip faulting in the forearc (Ryan and Scholl, 1989). No concrete evidence of strike-slip faulting exists in either the study area. Daly (1989) invokes oblique convergence-induced strike-slip-related deformation in the Ecuadorian forearc during the late Miocene to Recent to explain a variety of regional structures. However, the stress vectors associated with his deformational fields (resulting in NE-SW crustal shortening and NW-SE extension) are incompatible with what is recorded in the Esmeraldas region, unless the structures reflect a complex partitioning of strain within a discrete block. The younger WNW-trending high-angle normal faults reflect NNE-SSW extension, with faults oriented generally orthogonal to the modern Ecuadorian trench, a deformational field reflected in the orientation of some of the large faults on the regional map. Both these faults and the north-trending set of faults shown on the map truncate the large regional folds. Following Upper Onzole deposition, changes in stress regime, from generally NW-SE compression, to E-W tension, to NNE-SSW tension are difficult to explain. Perhaps this progressive deformation reflects stress rotation that resulted from the encroachment and collision of the leading edge of the continent with the thick crust of the Carnegie Ridge (Daly, 1989). This change in stress fields would have produced a final episode of “basin inversion” in the Esmeraldas region. Daly, M. C., 1989. Correlations between Nazca/Farallon plate kinematics and forearc basin evolution in Ecuador. Tectonics 8, 769-790. Evans, C. D. R., and Whittaker, J. E., 1982. The geology of the western Part of the Borbón Basin, northwest Ecuador. In: Trench-Forearc Geology (edited by J. K. Leggett). Geological Society of London Special Publication 10, 191-198. Ryan, H. F., and Scholl, D. W., 1989. The evolution of forearc structures along an oblique convergent margin, central Aleutian arc. Tectonics 8, 497-516. 2 Penrose Conference: Subduction to Strike-Slip Transitions on Plate Boundaries, Jan. 18-24, 1999 Overview of structures associated with subduction to strike-slip transitions in New Zealand P.M. Barnes1, J-C. Audru2,5, J-Y. Collot3, B. Davy4, J. Delteil2, R. Herzer4, G. Lamarche3,1, J-F. Lebrun3, B. Mercier de Lépinay2, R. Sutherland4, R. Wood4. 1National Institute of Water and Atmospheric Research (NIWA), PO Box 14901, Kilbirnie, Wellington, New Zealand. 2UMR Géosciences Azur, UNSA-CNRS, Valbonne, 06050, Sophia Antipolis, France. 3UMR Géosciences Azur, ORSTOM, 06235, Villefranche sur Mer, France. 4Institute of Geological and Nuclear Sciences, PO Box 30368, New Zealand. 5Presently at Institut de Physique du Globe, 67084 Strasbourg cedex, France The structure of the Pacific- Australian plate boundary in the New Zealand region varies along its length in response to changes in crustal structure, interplate coupling, and obliquity of relative motion between the plates. Two opposite-facing subduction to strike-slip transitions occur on the plate boundary, one at each end of the South Island of New Zealand, where oblique subduction of oceanic crust in the north and south merges with an oblique continental collision zone in central South Island (Fig. 1). The northern transition occurs from the west-dipping Hikurangi subduction zone, through the continental strike-slip Marlborough Fault System, to the oblique dextral Alpine Fault collision zone. The Fig. 1. Two subduction to strike-slip transitions in southern transition occurs from the the New Zealand region. alpine collision zone to the east- dipping Puysegur – Fiordland subduction system south of New Zealand. Both regions of tectonic transition have existed as subduction to strike-slip transition plate boundaries since the Miocene. However, the geometry of the boundaries, the activity of geological structures, and the kinematics of faulting have changed through time as relative plate motion changed and as the northern part of the plate boundary rotated clockwise. The transition regions are presently undergoing oblique convergence at a relative plate motion rate of 3.5-4.0 cm per year. Penrose Conference: Subduction to Strike-Slip Transitions on Plate Boundaries, Jan. 18-24, 1999 During the last five years several complementary research teams have utilised deep-crustal and shallow-penetration seismic reflection data, shipborne gravity and magnetic data, EM12Dual multibeam swath imagery, MR1 side-scan sonar, 3.5 kHz profiles, seabed samples,
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