Global Tectonics and the Sediments of Modern and Ancient Trenches: Some Different Interpretations1
David W. Scholl, Michael S. Marlow, 1974. "Global Tectonics and the Sediments of Modern and Ancient Trenches: Some Different Interpretations", Plate Tectonics—Assessments and Reassessments, Charles F. Kahle
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The general mechanism of global tectonics implies that deep-sea deposits should be either scraped off and folded against the inner walls of trenches, or subducted along with oceanic plates. However, evidence that these tectonic processes are taking place in modern trenches is extremely difficult to recognize. For example, seismic reflection records have shown structures in only a remarkably few areas which suggest that an underthrusting oceanic plate is steadily folding the sedimentary fills of either the Peru-Chile or Aleutian Trenches. The lack of widespread evidence for compressive deformation may mean that the folding in the vicinity of the inner wall is so intense that individual folds are acoustically unresolvable. However, because the trench fill is undisturbed except in the immediate vicinity of the inner wall (i.e., a gradual buildup of folding intensity is typically not observable), it seems questionable that the semiconsolidated deposits of the fill have sufficient strength to fold abruptly to form the steeply sloping surface of this commonly 500–1,500-m-high escarpment, which in many areas is steeper than 15–20° and locally may exceed 40°.
It is also reasonable to believe that in some manner the sedimentary sequences filling the Peru-Chile and Aleutian Trenches are subducted from them (i.e., inserted below the crust rather than folded against it) without producing much acoustically “visible” evidence of deformation. This supposition means that the existing volume of terrigenous debris in the trench can be only a fraction of the volume actually supplied. However, conservative figures show that the volume of Pleistocene detritus forming the bulk of the turbidite sequence in the Peru-Chile Trench and flooding the adjacent seafloor is in good balance with the amount that could have been eroded from the nearby continents. The conclusion therefore can be drawn that no significant volume of the turbidife fill of this trench has been removed tectonically. The figures are less convincing for the Aleutian Trench, but they are permissive of such a conclusion.
The sedimentary and volcanic fillings of "ancient" trenches generally are presumed to be represented in part by the deformed and intruded (ultramafic) Mesozoic rocks of the Circum-Pacific eugeosyncline. The amount of underthrusting involved in the formation of segments of this fold belt is shown by field mapping to be on the order of a few hundred kilometers. Internal tectonic churning (i.e., formation of mélange units) possibly can account for a few hundred kilometers more. However, modern rates of convergence and plate-reconstruction schemes for the Mesozoic suggest that thousands of kilometers (5,000–7,000) should have been involved. Large distances of relative plate motion also imply that enormous volumes of oceanic pelagic debris were swept into the Mesozoic trenches fringing the Pacific. If these deposits accumulated in the trenches as tectonic shavings, then approximately 30 percent of the rocks of the folded Circum-Pacific eugeosyncline should be pelagic offscrapings. Although the terrigenous masses of these belts of rocks are present in geosynclinal proportions, field mapping reveals that the volume of pelagic deposits in them is very low (e.g., less than 1 percent in the Franciscan Formation of California). If many thousands of kilometers of underthrusting took place, then the disproportionately small amount of pelagic deposits can mean that selective subduction of pelagic deposits took place. Selective subduction of pelagic deposits is consistent with a model allowing for subduction of the bulk of the deposits reaching the trench (whether pelagic or not) and for entrapment of the greater part of the terrigenous debris shed to the offshore areas in marginal basins or structural terraces upslope from the trench. Therefore, the bulk of the highly deformed terrigenous masses of the Circum-Pacific belt is not trench deposits.
Our observations can mean that little underthrusting (no more than several hundred kilometers) has taken place in either modern or ancient trenches peripheral to the Pacific Basin. However, because geological and geophysical data are convincing that new oceanic crust has been added to the earth since at least early Mesozoic time, our observations can be cited as evidence that the earth is expanding. Alternatively, in view of the strong circumstantial evidence that thousands of kilometers of oceanic crust have been subducted beneath marginal trenches, we can make the following conclusions: (1) for geologically long periods of time (at least several million years), tectonic removal of deposits from trenches may take place only periodically or at an average pace that is slower than the geophysically inferred rate, which is typically 5 cm/year or more; (2) if terrigenous debris has been swept from modern trenches at the inferred rate, then continental erosion during late Cenozoic time has been at least twice as high as other data would indicate; (3) when tectonic removal does occur, subduction rather than off-scraping may be the dominant process; and (4) ancient trench deposits may not be represented by the preserved or nonsubducted terrigenous masses of the circum-Pacific.