Abstract

Middle Ordovician mudstones of the Greenwich slice of the Hamburg klippe, an inferred early Paleozoic subduction complex in the central Appalachians, comprise three mesoscopically distinct fabric zones that reflect progressive development of scaly foliation from undeformed-zone mudstones through deposits of the transition zone into stratally disrupted deformed-zone mudstones. Scanning electron microscopic analysis of these deposits demonstrates an over-all decrease in porosity from undeformed mudstone into the deformed zone. Undeformed-zone mudstones illustrate a porous clay fabric that becomes more closely packed in the transition zone. Transition-zone mudstones also show (1) discrete, discontinuous shear zones characterized by compact domains of oriented platy grains and (2) fracture surfaces along which there has been no grain reorientation. The bulk of the mudstone, however, has apparently been compacted by distributed fabric collapse rather than shear-related compaction. Mudstones of the deformed zone display a complex array of continuous, well-defined, in some cases striated or grooved, scaly folia bound on one side by very compact domains of strongly oriented platy grains that pass outward into a more open or porous clay fabric.

Deformation of Greenwich slice mudstones can be explained by strain hardening of accreted trench-fill deposits at the toe of the early Paleozoic accretionary prism. Initial deformation was probably concentrated in weak, water-rich sediment horizons sandwiched between more-consolidated strata. Strain hardening of the early deformed sediment in response to shear-related fabric collapse and dewatering, as evidenced by the scaly fabric of deformed-zone mudstone, probably resulted in lateral propagation of deformation and progressive scaly fabric development. The extent to which stratal disruption propagated may have been controlled by the slope of the water-content gradient of the deforming sediment. That is, the deformation front migrated outward from the initially deformed, water-rich sediment to that position where the strength of the sediment, as determined by its relatively low water content, precluded further sediment yield. The microscopic shear zones and fractures within transition-zone mudstones suggest that these deposits were "dry" and, therefore, strong enough to arrest further stratal disruption. Thus, the final width of a deformation zone characterized by stratal disruption and scaly clays may be inversely proportional to the slope of the water-content gradient that existed between deforming water-rich, underconsolidated sediment and stronger, more-consolidated deposits.

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