Tectonic Synthesis, Leg 66: Transect and Vicinity

Abstract

Cores recovered from eight sites drilled along a transect across the Middle America Trench off southwestern Mexico show that accretion and offscraping of trench turbidites and the pelagic-hemipelagic blanket covering the ocean crust has been active for approximately 10 m.y. Active offscraping and imbricate underthrusting have been limited to the lower slope, whereas middle and upper slopes show uplift due to underplating by subducted sediments and younging of ocean crust. Offscraped trench sands of Pleistocene to Miocene age have been uplifted as much as 2 to 3 km above the present-day trench floor by combined accretion and underplating.Sediment input into the subduction zone is partitioned, with roughly equal amounts being offscraped, underplated, and subducted. The offscraped component appears to consist primarily of trench turbidites, whereas the subducted component is thought to consist primarily of pelagic-hemipelagic sediments. The trench turbidite component probably dominates the underplated component.Partitioning values based on mass balance budget and on theoretical lithospheric bending were comparable. This comparability lends confidence to our theory of flux partitioning.Although we find no direct evidence of tectonic erosion, there is indirect evidence to support the feasibility of the mechanism, and although there is no present forearc basin, differential uplift rates suggest incipient development.Thus we infer the following history of the region: 23-20 Ma-reorganization of plate motion: relative movement of Pacific and North American plate changes from oblique normal (subduction) to parallel (sinistral strike-slip) with margin subsidence and marine transgression.19-17 Ma-margin sinks rapidly to, or just below, CCD, then begins slow rise. Sinistral strike-slip motion continues.10 m.y.-plate motion changes from strike-slip to normal-oblique; accretion begins and continues to present.

Rates of sediment recycling beneath the Acapulco trench: Constraints from (U‐Th)/He thermochronology

“…The history of growth of this accretionary wedge can also be quantitatively modeled using the methods of DeCelles and DeCelles [2001], in which the key input parameters are as follows: velocity of subduction = 5.2 cm/yr [ Pardo and Suárez , 1995]; taper of the accretionary wedge = 8° to 10° [ Moore and Watkins , 1982]; average thickness of sediment in the trench = 190 m [ Watkins et al , 1982]; zero erosional loss of material from the subaqueous portion of the wedge; and zero change in taper through time (i.e., self‐similar growth). If the wedge has been growing since 25 Ma, its expected width would be ∼53–61 km if all of the material entering the trench were accreted into the wedge (Figure 6).…”

“…for the Acapulco trench in southern Mexico (Figure 2). The trench was active for the past 25 Ma [ Watkins et al , 1982], and marks the plate boundary where the Cocos plate subducts beneath mainland Mexico, currently a part of the North American continent. In this case, the sediment preservation rates are well known for the past 23 Ma from DSDP drilling and seismic images offshore Acapulco and Puerto Escondido.…”

A geophysical study of the Manila Trench, Luzon, Philippines: 1. Crustal structure, gravity, and regional tectonic evolution

“…Splays of the Philippine Fault system on land strike across the eastern shoreline of Lingayen Gulf into the fore arc ( Figure 19; Philippine Bureau of Mines [1963]) and form a third offshore fault system. We have assumed that trench sediment is reduced 25% in volume during accretion in accord with the drilling results from the Middle America Trench [Watkins et al, 1981]. The Vigan High is a broad, flat-topped basement high that shoals to 60 m at its crest [Shepard, 1981] A simple mass balance calculation for sediment in the Manila Trench that subsequently becomes accreted to the upper plate can also provide a possible constraint on the age of initiation of subduction.…”

“…The canyon is fault controlled and ents part of the A simple mass balance calculation for sediment in the Manila Trench that subsequently becomes accreted to the upper plate can also provide a possible constraint on the age of initiation of subduction. We have assumed that trench sediment is reduced 25% in volume during accretion in accord with the drilling results from the Middle America Trench [Watkins et al, 1981]. Only the trench turbidite section (onlapping fill seismic facies) was included in calculating the mass budget.…”

Offscraping and Underthrusting of Sediment at the Deformation Front of the Barbados Ridge: An Introduction to the Drilling Results of Leg 78A and Explanatory Notes

“…"Offscraping" describes accretion at the base of the trench slope (Scholl et al, 1980). "Underplating" designates a process of mass addition at depth beneath a rind of offscraped deposits (Watkins, McMillen, et al, 1982). "Subduction" describes the subcrustal descent of sediment or crust of the lower plate beneath the bedrock framework of a convergent margin (Scholl et al, 1980).…”