Plate-Tectonic Controls on Diagenesis

Abstract

Plate tectonics can be used as a framework for relating diagenesis to geological and geophysical parameters: regional tectonics, including rates of subsidence, uplift and deformation; volcanism and plutonism; heat flow; and transport of formation waters. These parameters affect diagenetic variables: the primary detrital-chemical assemblage; composition of early interstitial waters; rates and depth of burial; length of time buried; geothermal and pressure gradients; and oscillatory vertical movements. The consequences of changes in the values of these variables are illustrated for the diagenesis of the silica polymorphs, Ca- and Mg-carbonates, zeolites, and clay minerals. The unstable nature of biogenic opaline silica components of the primary assemblage leads to successive polymorphic inversions, ultimately to microcrystalline quartz chert. Pressure and temperature control the solubility and rate of inversion of all silica polymorphs. Carbonate systems also are controlled both by the presence of unstable phases (Mg-calcite, aragonite) in the primary assemblage and the effects of pressure and temperature on solubility. In later diagenesis oscillatory movements lead to reversals or replacements of carbonates. Zeolites are derived from the marine and nonmarine alteration of volcaniclastics. Burial, accompanied by pressure and temperature increases, produces the diagenetic zeolite facies. Clay minerals of both terrigenous and marine volcanic derivation are responsive to ion-exchange and silicate reconstitution reactions in early interstitial waters. Pressure and temperature increases lead to the stable chlorite-illite assemblage under deep-burial conditions. Plate-tectonic environments are for simplicity classified as mid-ocean ridge, trailing edge (passive), subduction, continent-continent collision, rift valley, and intraplate (cratonic). Mid-ocean ridges are sites of thin pelagic sediment accumulation affected by high heat flow, hydrothermal activity, and volcanic activity, followed by decreasing heat flow and slow burial as the sediments spread from the ridge. Geothermal gradients are higher in these marine sediments than they are on the continents. Muddy and sandy sediments remain largely unlithified except for layers of chert and limestone. Trailing edges are characterized by lowered heat-flow rates and reduced geothermal gradients, as well as rapid subsidence and burial of terrigenous detrital and chemical sediment after the initial opening phase in which evaporites and heavy metal deposits are formed. The resulting rocks are moderately cemented and lithified by carbonate and quartz and, in deeper zones, have the stable chlorite-illite mineral pair. Volcaniclastics are rare, as are zeolites. Subduction zones, a diverse group of subenvironments, are regions where low heat flow, rapid subsidence, and burial, accompanied by crumpling, operate on turbidites and pelagics, together with large amounts of volcaniclastic debris from nearby island arcs. Zeolites, smectites, and low-temperature-high-pressure mélanges, including ophiolites, are produced. Continent-continent collisions produce enormous quantities of terrigenous detritus deposited in both terrestrial and marine environments, from intermontane basins with subaerial exposure and meteoric water infiltration to submarine fans and plains. Geothermal gradients are low, sedimentation and burial are rapid, and diagenesis is mild, except in sections that have remained deeply buried for 100 m.y. or more. Diagenesis of rift-valley sediment is related to the abundance of volcaniclastics, mixed with alluvial and lacustrine deposits, all buried rapidly in regimes of high heat flow and hydrothermal activity. Intraplate diagenesis is the result of low subsidence and sedimentation rates in low continental heat-flow regimes. But oscillatory movements are frequent, and complex parageneses of authigenic mineral replacements indicate multiple changes in formation-water composition. A uniformitarian approach to this subject is valid only for the Phanerozoic, for present-ay patterns of plate tectonics may not have existed during earlier times when heat flow from the interior was differently distributed.