Clay mineralogy and clay geochemistry in the vicinity of the décollement zone, northern Barbados Ridge

Abstract

Fine-grained sediments recovered during Leg 156 of the Ocean Drilling Program contain assemblages of clay minerals that change considerably with depth. Key structural and stratigraphic features within the toe of the northern Barbados accretionary prism include the basal décollement and a sharp boundary between lithologic Units II and III; this lithologic boundary occurs within the décollement zone. We analyzed three size fractions (4−8 µm, 1−4 µm, and <1 µm) by X-ray diffraction and obtained chemical data from the <1-µm size fractions by scanning electron microscopy coupled with energy dispersive spectroscopy. Relative proportions of expandable clay minerals increase significantly, but erratically, near the top of the décollement zone. Proportions of discrete smectite and smectite-rich illite/smectite (I/S) mixed-layer clays decrease steadily down the décolle-ment zone. The base of the décollement zone is characterized by unusually high concentrations of illite. These changes in clay mineralogy influence the shear strength of claystones at the deformation front and help control the position of the décollement.Relative abundances of illite within all lithofacies units increase in coarser size fractions, and this trend is consistent with a detrital source. Kaolinite does not display any systematic partitioning within Unit II, but its proportions increase in the finer size fractions within Unit III. The smectite content of Unit II increases with decreasing grain size; Unit III shows the opposite trend between particle size and % smectite. Bentonite-like samples probably originated through in situ alteration of disseminated volcanic ash, but we believe that the pyroclastic material was mixed into a background sediment that already contained abundant detrital smectite. Illitic mixed-layer clays within the underthrust domain (Unit III) also appear to be detrital in origin. Correlations among both % smectite and % kaolinite and most major-element oxides are statistically significant, but correlations for illite are not. Expandable clays vary considerably in their chemical make-up; the most common type appears to be a Mg-poor, K-rich, Fe-Al-beidellite. We rely upon actualistic provenance connections for late Quaternary clays to guide our interpretations of Miocene and Oligocene sediment dispersal. If these analogues are valid, then the proto-Orinoco and protoAmazon Rivers were the most important detrital sources for the older hemipelagic and turbidite deposits of Barbados Ridge.

Acoustic and mechanical properties of Nankai accretionary prism core samples

“…In contrast, V p (measured at atmospheric pressure) in sediments from ODP Site 808 (outer marginal trench wedge to USB and LSB, between 400 and 950 mbsf) is lower in the vertical (i.e., core parallel) direction than in the horizontal direction (irrespectively of azimuth) by up to ∼10% [ Brückmann et al , 1993]. In accreted sediments within the Barbados accretionary complex, Brückmann et al [1997] reported either positive anisotropy (i.e., horizontal V p faster than vertical one), mostly between 0% and +4%, in hole 948C or no anisotropy in hole 949B (both claystone with total clay content mostly around or above 60%) [ Underwood and Deng , 1997), without a clear explanation for the difference between the two sites.…”

Record of forearc devolatilization in low‐T, high‐P/T metasedimentary suites: Significance for models of convergent margin chemical cycling

“…Sedimentary clay mineral composition is more difficult to determine, as the clays are both finer‐grained and more reactive than the other clasts. However, clay‐mineral analysis of other accretionary wedge and trench sediments suggests that these rocks probably contained a variety of sedimentary clay minerals such as chlorite, illite, smectite, kaolinite and montmorilinite, and some mixed‐layer clays [i.e., Karnosov et al , 1977; Karpoff , 1992; Underwood and Deng , 1997] (ODP‐Leg 185). Clay minerals have variable, but high, water contents; ideal kaolinite (Al 2 Si 2 O 5 (OH) 4 ) contains 14 wt.% H 2 O and most illites contain ∼10 wt.% H 2 O (for comparison, white‐micas, stabilized at higher grades by the breakdown of clays, contain ∼5 wt.% H 2 O).…”

“…Sedimentary clay mineral composition is more difficult to determine, as the clays are both finer-grained and more reactive than the other clasts. However, clay-mineral analysis of other accretionary wedge and trench sediments suggests that these rocks probably contained a variety of sedimentary clay minerals such as chlorite, illite, smectite, kaolinite and montmorilinite, and some mixed-layer clays [i.e., Karnosov et al, 1977;Karpoff, 1992;Underwood and Deng, 1997] [25] As these sediments progressed downward in the subduction zone, they underwent mineralogical changes that must have resulted in significant chemical redistribution, at least at the local scale. Even the lowest-grade rocks of the Coastal Belt differ from likely sedimentary protoliths in several important ways.…”

The roles of quartz and water in controlling unstable slip in phyllosilicate-rich megathrust fault gouges

“…Because the composition of our illite/ quartz gouge is similar to that of the protolith sediment at many subduction margins (e.g. Steurer and Underwood 2003;Underwood and Deng 1997), the temperaturedependent frictional behaviour described here is directly relevant to understanding temperature-and depth-dependent transitions in slip stability along subduction megathrusts.…”