The earth’s largest earthquakes and tsunamis occur in the seismogenic zone of subduction margins, i.e. the seismically active part of the subduction channel that occupies a limited depth range (ca 4-45 km) along the plate interface: nearly all of the large earthquakes (Mw>8) occur along this interface at shallow depths, between 10 and 15 km. The plate interface of subduction margins, referred to as décollement zone, is a complex thrust zone involving fluid-rich unconsolidated sediments and hard rocks, through an ever-increasing pressure-temperature-strain regime from the surface down to a depth of ca. 80 km. Mélanges have been classically described as a typical subduction product, representing ancient strands of paleo-décollement zones formed in consequence of high shear strain and dewatering of a sediment pile undergoing progressive underthrusting. Pseudotachylytes are so far the only unambiguously recognized ancient evidence of fault slip at seismic rates. While pseudotachylytes are commonly described in crystalline rocks, mélanges are typically dominated by “weak” pelitic lithotypes and are extensively infused by carbonate and quartz veins, thus suggesting a possible aseismic creeping mode of deformation. Why are pseudotachylytes so rare along subduction mega-thrusts? Is the paucity of pseudotachylytes in the subduction setting due to our failing to recognize them in lithotypes so different from those typically described in continental settings? The present contribution constitutes a first attempt to answer these questions, focusing on a study conducted on a décollement-system thrust fault preserved in an accreted mélange unit of the Kodiak accretionary complex (Alaska). The accretionary complex of Kodiak Island is an ancient analogue of the active Alaskan subduction, a similar setting to the locus of the 1964 Mw9.2 Alaskan earthquake. On Kodiak Island, decimeter-thick black fault rocks (BFR) are at the core of 10's meters-thick foliated cataclasites. The cataclasites belong to mélange zones interpreted as a paleo-décollement active at 12-14 km depth and 230-260oC. Each black layer is mappable for tens of meters along strike. The BFR have complex layering made at microscale by alternation of granular and crystalline microtextures, both composed of micron-scale sub-rounded quartz and plagioclase in an ultrafine, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned microlites of plagioclase make much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grain size variability, crushed microlites and textures typical of fluidization and granular flow deformation. Crosscutting relationships between granular and crystalline microlayers include flow and intrusion structures and mutual brittle truncation. This suggests that each 10’s centimeter-thick composite BFR record multiple pulses of seismic slip. In each pulse, ultracomminuted fluidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidification of the friction melt and the fluidized rock. XRPD and XRF analyses show that BFR have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fluid-infiltrated system with BFR later alteration is suggested. The geochemical results indicate that these subduction-related pseudotachylytes, differ from those typically described in crystalline rocks and other tectonic settings.
The seismic character of mélange zones: record of mega-earthquakes in the Kodiak Accretionary Complex, Pasagshak Point, Kodiak Is., Alaska.
MENEGHINI, FRANCESCA;
2010-01-01
Abstract
The earth’s largest earthquakes and tsunamis occur in the seismogenic zone of subduction margins, i.e. the seismically active part of the subduction channel that occupies a limited depth range (ca 4-45 km) along the plate interface: nearly all of the large earthquakes (Mw>8) occur along this interface at shallow depths, between 10 and 15 km. The plate interface of subduction margins, referred to as décollement zone, is a complex thrust zone involving fluid-rich unconsolidated sediments and hard rocks, through an ever-increasing pressure-temperature-strain regime from the surface down to a depth of ca. 80 km. Mélanges have been classically described as a typical subduction product, representing ancient strands of paleo-décollement zones formed in consequence of high shear strain and dewatering of a sediment pile undergoing progressive underthrusting. Pseudotachylytes are so far the only unambiguously recognized ancient evidence of fault slip at seismic rates. While pseudotachylytes are commonly described in crystalline rocks, mélanges are typically dominated by “weak” pelitic lithotypes and are extensively infused by carbonate and quartz veins, thus suggesting a possible aseismic creeping mode of deformation. Why are pseudotachylytes so rare along subduction mega-thrusts? Is the paucity of pseudotachylytes in the subduction setting due to our failing to recognize them in lithotypes so different from those typically described in continental settings? The present contribution constitutes a first attempt to answer these questions, focusing on a study conducted on a décollement-system thrust fault preserved in an accreted mélange unit of the Kodiak accretionary complex (Alaska). The accretionary complex of Kodiak Island is an ancient analogue of the active Alaskan subduction, a similar setting to the locus of the 1964 Mw9.2 Alaskan earthquake. On Kodiak Island, decimeter-thick black fault rocks (BFR) are at the core of 10's meters-thick foliated cataclasites. The cataclasites belong to mélange zones interpreted as a paleo-décollement active at 12-14 km depth and 230-260oC. Each black layer is mappable for tens of meters along strike. The BFR have complex layering made at microscale by alternation of granular and crystalline microtextures, both composed of micron-scale sub-rounded quartz and plagioclase in an ultrafine, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned microlites of plagioclase make much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grain size variability, crushed microlites and textures typical of fluidization and granular flow deformation. Crosscutting relationships between granular and crystalline microlayers include flow and intrusion structures and mutual brittle truncation. This suggests that each 10’s centimeter-thick composite BFR record multiple pulses of seismic slip. In each pulse, ultracomminuted fluidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidification of the friction melt and the fluidized rock. XRPD and XRF analyses show that BFR have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fluid-infiltrated system with BFR later alteration is suggested. The geochemical results indicate that these subduction-related pseudotachylytes, differ from those typically described in crystalline rocks and other tectonic settings.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
