WESTWARD EXPORT OF SEDIMENT AND WATER FROM THE YAKUTAT COLLISION ZONE—EXPLORING NOTIONS ABOUT FAR-FIELD ARC TECTONISM, SEDIMENT RECYCLING, AND PALEOCLIMATIC CONSEQUENCES IN THE ALEUTIAN-BERING SEA REGION

 

David W. Scholl, Department of Geophysics, Stanford University, Stanford CA 94305; dscholl@pangea.stanford.edu

 

Andrew J. Stevenson, U.S. Geological Survey, Menlo Park, CA 94025; astevenson@usgs.gov

 

OVERVIEW

Coastal mountain building in response to the collision of the Yakutat block with the easternmost sector of the Aleutian subduction zone (ASZ)shed large volumes of sediment and runoff water to the deep sea floor of the Gulf of Alaska (GOA). An important fraction of the sediment stripped from the continent by elevated and glaciated drainages entered the eastern sector of the Aleutian Trench and continued westward in turbid flows to pool up seaward of the Aleutian Ridge. In the eastern GOA, a separate large fraction formed the upper sedimentary sequence of the Surveyor fan (Fig. 1).

Runoff accompanying sediment discharge was virtually entirely exported to the west by the coastal Alaska Stream of the Subarctic Gyre. This voluminous, low salinity current enters the Bering Sea by pouring northward across the Aleutian Ridge through between-island passes (Fig. 2).

            Distal or far-field consequences of the westward export of synorogenic sediment and water from GOA drainages are conjectured to have contributed importantly to:

·         formation of a 20-40 km wide, ~2500-km-long accretionary prisms,

·         recycling, via sediment subduction, of a large volume of synorogenic sediment to the mantle,

·         the onset of rapid late Cenozoic tectonism and volcanism along the Aleutian Ridge,

·         chilling and isolation of the north Pacific’s Subarctic Gyre by the snuffing of a center of thermohaline circulation in the Bering Sea

 

SYNOROGENIC SEDIMENT MOVED AND REMOVED BY SUBDUCTION ZONE PROCESSES

Subduction accretion is the tectonic addition of rock and sediment from the underthrusting lower or ocean plate to the rock and sediment framework of the upper plate—i.e., the convergent margin. During the past 5-7 Myr sediment from GOA drainages maintained a landward-thickening wedge of turbidite deposits along about 2500 km of the Aleutian Trench. Subduction of the Pacific plate continuously inserted the wedge into the ASZ and, as a consequence, constructed a frontal prism of accreted trench-wedge sediment (Fig. 3). The prism, in front of a backstop of much older material, extends more or less continuously from about Kayak Island (144.5OW) westward to about Attu Island (173OE). The typical width of the accretionary mass is 20-30 km.

Sediment subduction is the tectonic by-passing of the frontal prism by sediment lying below the interplate decollement. Sediment that passes landward beneath the base of the prism’s backstop of older rock is defined as subducted sediment. For the Alaska, the backstop is the seaward terminus of early Tertiary and older continental crust. Eocene arc magmatic rock forms the backstop for the Aleutian accretionary prism.

Drilling and geophysical observations demonstrate that sediment subduction is astonishingly efficient for all convergent margins except those bordered by large accretionary prisms. For margins fronted by medium size frontal prisms 10-40 km wide (~50 % of all subduction zones), the efficiency of sediment subduction is ~80%. The Aleutian frontal prism falls into this category, meaning that it incorporates only about 20 % of the sedimentary column that entered the ASZ during the past 5-7 Myr. The entering column is more than just the trench wedge but also underlying pelagic, hemipelagic, and terrigenous units that accumulated seaward of the trench, for example the largely clastic beds of the Surveyor fan body in the eastern GOA (Fig. 1). Virtually all of these non-wedge units are subducted beneath the backstop.

            Sediment masses tectonically removed from the trench axis and adjacent floor of the north Pacific can be roughly estimated. Since the late Miocene the along-trench thickness of sediment entering the subduction zone is estimated to have averaged ~2 km (presently 1-4 km), the average orthogonal underthrusting rate, which decreases westward, is ~50 km/Myr, and the length of trench involved is ~2500 km. During the past 6 Myr the volume of sediment that entered the ASZ is accordingly estimated to be ~1.5 x 106 km3 (average porosity ~50%). About 20% of this volume of synorogenic sediment, or 0.3 x 106 km3, has been moved from the ocean basin to the front of the upper plate and stored there as the Aleutian accretionary prism (average porosity 20-30%). The volume of the present axial wedge of synorogenic sediment is ~0.05 x 106 km3.

South of the Aleutian Ridge the lower half of the trench wedge lies below the interplate decollement and is being subducted landward of the prism (Fig. 3). The mass of sediment that entered this sector of the ASZ during the past 6 Myr is calculated to be 0.7 x 106 km3 (1500 km long, 2 km thick, at 40 km/Myr for 6 Myr years), the great part of which was trench wedge deposits. The corresponding volume (@80%) of synorogenic sediment that sediment subduction has removed from the north Pacific Basin is ~0.6 x 106 km3 (@50 % porosity).

Along Alaskan sector of the Aleutian Trench the interplate decollement lies not within but at the base of the turbidite wedge. As a consequence only underlying, non-trench-axis deposits, for example those of the Surveyor and Zodiac fans, are subducted (Fig. 1). The bulk of the 1.2–km-thick Surveyor fan is synorogenic deposits. However, the voluminous early Tertiary deposits of the Zodiac fan, although being subducted, are not products of the late Cenozoic Yakutat collision. During the past 6 Myr subduction of the 1-km-thick terrigenous section of the Surveyor fan took place at a rate of 60 km/Myr along a ~500-km-long segment of the eastern Aleutian Trench. The corresponding volume of synorogenic sediment lost via sediment subduction is ~0.2 x 106 km3 (@ 50 % porosity). The standing volumetric mass of the fan is approximately the same as that subducted.

In summary, synorogenic sediment removed from the trench and flanking ocean basin is estimated as 0.3 x 106 km3 stored as a frontal accretionary prism and a much larger mass of 0.8 x 106 km3  lost by sediment subduction beneath Alaskan and Aleutian crust. The actual volume of subducted sediment is larger, by ~0.2 x 106 km3,, if deposits older than late Miocene are included.

Since the late Miocene the solid-volume mass of synorogenic sediment removed from the Pacific Basin is estimated at ~0.5 x 106 km3. This volume is representative of the minimum mass of terrigenous material that must be included in the budget of sediment offloaded from the continent as a consequence of Yakutat orogenesis.

 

ALEUTIAN TECTONISM

            It has been recognized that beginning about 5-7 Ma rapid along-arc extension of the Aleutian Ridge, CW rotation and westward motion of large sectors of the ridge, and, increasingly westward, right-lateral, strike-slip shearing of the arc massif got underway. The regional style of arc fragmentation is consistent with the westward increase in obliquity of convergence, a setting that has been in place for at least the past 40-50 Myr (Fig .4). Other manifestations of enhanced Aleutian tectonism beginning in the late Miocene include subsidence of the forearc leading to the formation of the Aleutian Terrace, a sediment filled deep water forearc basin, and the outbreak of vigorous arc volcanism that continues today along the ridge’s northern or Bering Sea rim (Figure 4).

Because Yakutat orogenesis and regional Aleutian Ridge tectonism occurred concurrently, it is tempting to suppose that rapid late Miocene arc dismemberment, volcanism, and forearc subsidence are far-field or distal consequences of collisional orogenesis at the eastern end of the ASZ. The linkage may be through the subduction zone injection of water-rich turbiditic sediment transported to the western Aleutian Trench or, as speculative, the collision-forced extrusion (escape) of western Alaska toward the Aleutian subduction zone.

 

DEMISE OF THERMOHALINE CIRCULATION

The existence of a large sediment drift, the Meiji drift, in the northwestern Pacific Basin has been cited as evidence for the former existence of a major cell of thermohaline circulation in the Bering Sea Basin (Fig. 2). Along the path of Meiji drift, this cell is thought to have exported cold and salty surface water from the Bering Sea to the north Pacific Basin.

Because the salinity of surface water circulating through the Bering Sea is low, thermohaline convection does not presently take place here. Freshening and related cooling of the surface water of the Subarctic Gyre, which circulates between the GOA and the Bering Sea Basin, got strongly underway in the latest Miocene.

An orogenic factor contributing to the freshening of surface water is likely to have been increasing runoff arriving from the elevating coastal mountains of southern Alaska that, via the Alaska Stream, is shunted directly into the Bering Sea. Stifling,, by salinity dilution, of a cell of thermohaline circulation operating here would have lead to the chilling and isolation of the Subarctic Gyre. Such a scenario explores the notion that a far-field effect of Yakutat orogenesis was a factor in the climatic deterioration of the high north Pacific that culminated in northern Hemisphere continental glaciation.