T44A-01 INVITED
Crustal Recylcing at Ocean Margin and Continental Subduction Zones and the Net Accumulation of Continental Crust
CRUSTAL RECYCLING PROCESSES AND VOLUMES: At convergent ocean margins large volumes of rock and sediment are missing from the global length of submerged forearcs. Material is removed by the kindred tectonic process of sediment subduction and subduction erosion, both of which insert sediment and eroded crustal debris into the subduction channel separating the upper and lower plates. The channel transports entrained debris toward the mantle where it is ultimately recycled. Large tracks of exposed high P/T rocks are exposed remnants of subduction channels. Over the past 100-200 my, the average solid-rock volume of recycled crust is estimated to have averaged globally 2.5-3.0 km3/yr--or 2.5 to 3 Armstrong Units (AU). Exposed tracts of UHP rocks at collisional orogens document that crustal material is subducted deep into the mantle at continental subduction zones. Based on missing terranes of extended lower plate, a volume of recycled continental crust detached by slab failure can be estimated at ~5000 km3 for each km of the early Proterozoic Wopmay orogen of the NW Canadian Shield (Hildrenbrand and Bowring, 1999, Geology, v. 27, p.11-14). Averaged over an orogenic episode of ~40 my, the corresponding rate is ~125 km3/my/km of margin. Using the Wopmay- rate as a guide, and assuming that similar to the Cenozoic, collisional orogenic margins averaged 10-12,000 km in global length, then since the early Proterozoic crustal recycling at collisional subduction zones has averaged close to 1.5 AU (i.e., 1.5 km3/yr). Crustal losses from the upper plate have also been recognized for sectors of the Variscan orogen (Oncken, 1998, Geology, v. 26, p. 1975-1078). The missing crust is roughly 40 km3/my for each km of upper plate, thus globally tallying an additional ~0.5 AU. CRUSTAL GROWTH: New information implies that at intra-oceanic subduction zones the long-term (~50 my), global rate of arc magmatic productivity has averaged close to 5 AU, a much higher rate than formerly estimated (~1 AU). It is not clear that this rate, which is based on the growth of the Aleutian and Izu-Bonin-Mariana arc massifs corrected for subduction erosion losses, can be applied to continental or Andean arcs. But allowing that it can, then the combined global rate of additions of juvenile igneous rock to build continents ( 5 AU) is similar to that recycled at ocean margin (2-3 AU) and continental subduction zones (2 AU). Additional losses can arise from delamination of magmatically or tectonically thickened convergent-margin crust. The implication of these estimates and linked assumptions support the Armstrong posit that since the early Archean the yang of magmatic additions to the continents has been matched by the yin of recycling losses.
T44A-02
Rapid Exhumation of the Rand Schist: Constraints From Natural Garnet Diffusion Couples
Intensive study of the Rand schist (hereafter referred to as the "schist") of southern California demonstrate convincingly that it formed in the Late Cretaceous by subduction of Cordilleran eogeoclinal detritus along a shallow dipping segment of the Farallon plate. Comparatively little is known about the exhumation history of the schist. Recent thermochronologic and thermobarometric analyses suggest that, in some localities, the schist was deposited, underplated beneath western Sierra Nevada batholithic assemblages (the "upper plate"), and exhumed with the upper plate from 30 - 40 km depths in less than 3 million years. Such a rapid cycling interval implies exhumation rates in excess of 5 mm/yr. To corroborate these existing data on the exhumation rate of the schist, we exploit diffusional annealing of garnet zonation. Some garnets from the schist of the San Emigdio Mountains were broken during late stages of their growth. Overgrowth along broken margins resulted in natural diffusion couples ca. 25 μm inward from the rims, where peak metamorphic temperatures reached 610 °C. A maximum time interval, representing exhumation prior to diffusion cessation, of 1 Ma was estimated from the degree of annealing of the originally sharp compositional step. Similar time intervals were calculated from narrow (10 μm) retrograde zones at the rims of compositionally homogenized garnets belonging to the upper plate. Garnets from the schist conspicuously lack retrograde zonation patterns, likely due to exhumation rates exceeding those of retrograde diffusion. Both garnet-based geospeedometry and thermochronometric studies indicate that the schist was exhumed at rates comparable to lateral tectonic plate motions (> 5 mm/yr). This conclusion, in the context of field-based analyses suggestive of west-directed ductile transport of the schist relative to the upper plate during exhumation, reveals a profound Late Cretaceous extensional event. These results yield important insight into the timing of Late Cretaceous schist ascent, in addition to the mechanisms necessary for such a rapid process.
T44A-03
Earthquake-induced submarine sliding on the trench-slope basin within the Late Pliocene to Pleistocene Chikura Group, Central Japan
Fossil example of large-scale sediment mass wasting occurred on the trench-slope-basin was identified. A large-scale chaotic sedimentary body within Late Pliocene to Pleistocene trench-slope cover sediments of the Chikura Group, Central Japan, can be traced E|W over a distance of 5 km. A recently excavated road cutting has exposed a cross-section through the entire chaotic sedimentary body, including its upper and lower contacts with coherent sediments. Evidence of sand intrusions and block rotation within the body indicates that the chaotic nature of the sediments arose from earthquake-induced liquefaction and subsequent submarine sliding (liquefied sediment flow). It is possible that the formation of such a large-scale submarine slide body generated a large tsunami. We will report a detailed description of the occurrence and structural characteristics of this recently exposed outcrop of chaotically mixed sediments.
T44A-04
Constraints on the Timing and Kinematics of Deformation Within a Thick-skinned, Inner Forearc Thrust System, Northeastern Japan Margin
New structural and chronologic data from the inner forearc of the northeastern Japan erosive subduction margin provide evidence for Neogene to Quaternary thick-skinned deformation accommodated largely by a high-angle, near-coastal, margin-parallel reverse fault. This structure, the Futaba fault, bounds the exhumed Abukuma massif on the east and places Cretaceous granites in fault contact with Miocene to Pliocene near-shore to terrestrial, tephra-rich sands and silts. Recent exhumation of the massif by slip along the Futaba fault is implied by a regionally extensive footwall syncline, the absence of Neogene sediments in the hanging wall, and high relief on the eastern margin of the massif. New structural data from three transects across deformed footwall units in the central and southern segments of the Futaba fault are used as inputs for forward trishear kinematic modeling. The footwall of the central segment of the Futaba fault is characterized by an open syncline with a steeply dipping to overturned western limb. Toward the south, displacement on the Futaba fault diminishes and dissipates into a gently south-plunging fold within the cover sequence where steeply dipping Miocene units are unconformably overlain by Pliocene growth strata. Dated tephra beds within the pre-growth and growth section at this location bracket the onset of deformation between middle Miocene and early Pliocene. Our results show that these structures are best modeled as the result of deformation associated with the propagation of a steep, west- dipping reverse fault. This interpretation is supported by the orientation of Riedel shears within the fault zone, which are consistent with stress orientations that favor reverse fault motion. The kinematics and time of onset of deformation associated with the exhumation of the Abukuma massif imply a transition to compressive stresses in the inner forearc during the middle Miocene to early Piocene, which may be linked to changes in plate boundary tractions related to Neogene subduction erosion. Thus subduction erosion processes can result in uplift of the inner forearc as a consequence of out-of-sequence thrusting near the margin of the extinct arc.
T44A-05
Subduction Tectonic Erosion, Sediment Accretion and Arc Collisions in maintaining the Continental Crust
Estimates of modern continental crustal recycling in subduction zones can be made from plate convergence velocities, the thicknesses of trench sediments, volumes and ages of accretionary complexes together with rates of trench retreat. Plate convergence rates appear to be the primary control on crustal subduction, with convergence >7.5 cm/yr associated with tectonic erosion. Collision of aseismic ridges with trenches drives around two thirds of forearc tectonic erosion over periods >10 m.y.. Globally material subduction at least as deep as the magmatic roots of arc systems is around 3.0 Armstrong Units (1 AU = 1 km3/yr), of which 1.65 AU comprises subducted sediments, with 1.33 AU of eroded forearc crust. Recycling rates along a single margin may show strong temporal variation over 1 m.y. periods. Isotopic variations in Costa Rican tephra suggest that sediment accretion is the most common mode of tectonism, but this is separated by short periods of dramatic erosion that cause net crustal loss. Even where erosion is continuous this can operate in a fast steady-state mode or a slower temporary style. On the Central Andean margin tectonic erosion since 20 Ma has caused trench retreat, but slow subsidence under the coastal zone implies steepening of the forearc taper rather than large scale retreat. The Neogene mass loss rate of 13 km3/m.y./km is 5-10 times lower than the long-term average. Since 2 Ma this rate has slowed further due to underplating under the coastal zone. A climatic role in driving continental erosion and moving the margin into a more accretionary state has been suggested but is hard to demonstrate. Average global mass loss requires that Cenozoic arc productivity lies close to 75 km3/m.y./km if the volume of the continental crust is to be maintained. Efficient accretion of oceanic arc crust is essential in maintaining the total crustal volume. In the classic Taiwan-Luzon example local crustal mass balancing implies that ~90% of the igneous arc crust is accreted.
T44A-06
Modeling Flat Subduction Initiation and Accretion of the Pelona and Related Schists of Southern California
The Pelona and related schists of Southern California represent continental detritus deposited, subducted to depths of ~30 km, and accreted during a major Late Cretaceous-Early Tertiary collisional event. Emplacement of the schists coincided with the termination of magmatism along a ~500 km segment of the California continental arc, the disappearance and probable subduction of the forearc and western portions of the arc, and the loss of the mafic lower crust and root of eastern portions of the arc. Conceptual models of schist emplacement differ in detail, but generally involve a dramatic flattening of subduction. In order to better understand the stresses, accretion rates, and emplacement histories of the schists, we constructed a two- dimensional, finite element, kinematic-thermal model of flat subduction initiation using the program Pecube (Braun, 2003). The model is able to satisfy geologic constraints available from the literature on cooling rates (35- 100 °/m.y.), peak temperatures in upper portions of the schists (550-720 °C), and steep inverted metamorphic gradients (>70-240 °/km). Preliminary models indicate shear stresses were less than ~20 MPa on the decollement and accretion rates were ~1 mm/yr. With constant accretion and a convergence rate of 100 km/m.y., the thickest exposures of schist were built over ~4 m.y. as material was scraped off ~400 km of lower plate. In two less widely accepted alternative tectonic models, the schists represent either transpressional basin or forearc Great Valley deposits. Both alternative basin types are too narrow (<100 km) to produce the observed inverted gradients.
T44A-07
Geochemistry of Cretaceous Magmatism in Eastern Cuba: Recycling of North American Continental Sediments and Implications for the Subduction Polarity in the Greater Antilles Paleo-arc
We present whole rock major and trace element data and Nd-Sr-Pb radiogenic isotope ratios of Cretaceous igneous suites from eastern Cuba. These rocks are related to the Greater Antilles paleo-island arc magmatism and have three different igneous styles. Group 1 consists of tholeiitic basalts and rare basaltic andesites that have N-MORB-like compositions similar to those found in back-arc basin basalts (TiO2 = 1.2-2.9 wt%; La/Yb(N) = 0.7-0.9, Th/Nb = 0.06-0.08, and initial 208Pb/204Pb = 37.65-37.74). Group 2 comprises basaltic and rare basaltic andesitic subvolcanic dykes with major-, trace-element and isotopic compositions similar to island arc tholeiites (TiO2 = 0.7-1.4 wt%; La/Yb(N) = 0.6-0.9, Th/Nb = 0.06-0.68, and initial 208Pb/204Pb = 37.74-38.25). Group 3 is composed of low-Ti (TiO2 = 0.3-0.9 wt%) calcalkaline igneous rocks that have an unambiguous subduction- related character (La/Yb(N) = 1.1-5.0, Th/Nb = 0.35-1.55, and initial 208Pb/204Pb = 37.94-38.39). The parental magmas of the three groups formed by variable melting degrees (< 5-25%) of spinel lherzolite, with the more depleted mantle sources for Groups 2 and 3 than Group 1. The trace element and radiogenic isotope compositions of primitive Group 3 samples are strongly bimodal. One subgroup of samples is characterized by low Ta/Yb (0.02-0.03) and Th/La (0.10-0.13), slightly subchondritic Nb/Ta (13.3-17.3), and relatively high initial 206Pb/204Pb (18.57-18.62) and Nd (7.6-9.4). The remaining primitive Group 3 samples have higher Ta/Yb (0.06-0.11) and Th/La (0.24-0.32), highly subchondritic Nb/Ta (7.6-9.1), coupled with lower initial 206Pb/204Pb (18.24-18.29) and Nd (3.4-5.5). These signatures were induced by two distinct slab components that mainly reflect the contributions of Cretaceous Atlantic marine and North American continental sediments, respectively. Nb/Ta in the first subgroup was induced by melting of rutile-bearing subducted crust, whereas in the second it was inherited from recycled continental material. The involvement of Atlantic and North American sediments in Cuban Cretaceous magmatism indicates that the Proto-Caribbean (North American-Proto Atlantic) lithosphere subducted beneath the Greater Antilles arc during Late Cretaceous (pre-Campanian) time, consistently with geotectonic models involving onset of south-westward-dipping subduction beneath the Greater Antilles paleo-arc during the Aptian. The variable source depletion and magnitude of subduction component probably reflect different settings across arc, from the arc front to a back-arc spreading ridge.
T44A-08 INVITED
Changes in Seismogenic Zone Coupling Along the Middle America Margin?
Subduction of seamounts, aseismic ridges and bathymetric highs leads to subduction erosion, deformation of the forearc and increased coupling within the seismogenic zone. The latter can result in large magnitude earthquakes, deformation of the forearc and potentially collision tectonics. Variability in bathymetry of the Cocos plate offshore the Middle America Trench allows for the investigation of the effects of bathymetric highs on plate coupling. Here, relatively smooth crust formed at the East Pacific Rise subducts along the MAT in Nicaragua and northern Costa Rica, whereas, rough crust formed at the Cocos Nazca spreading center (CNS-2) and modified by Galapagos hotspot interacts with the margin in central and southern Costa Rica. We present a regional surface velocity field for Central America showing the crustal response to interaction of the Cocos and Caribbean plates, and investigate the degree of plate coupling and long-term implications of seamount and aseismic ridge subduction. The velocity field indicates significant trench-parallel motion for most of the region, including central Costa Rica, where plate convergence is perpendicular to the trench. Interseismic strain accumulation is observed in the outer forearc Nicoya and Osa Peninsulas, but not in the forearc of Nicaragua or central Costa Rica. Large subduction zone earthquakes occur in Nicaragua (e.g., September 2, 1992, Mw 7.6), however there have been no >Mw 7 earthquakes recorded in central Costa Rica. The velocity field is not well fit by a simple model of interseismic elastic strain accumulation. Inboard of Cocos Ridge, southern Costa Rica, velocity vectors are parallel to convergence and up to 44 mm yr- 1. We present a collision (rather than subduction) model involving CNS-2 - Cocos Ridge crust, and compare our model results and geodetic observations to geological and geophysical data for the region. CNS-2 Cocos Ridge crust resists normal subduction, instead acting as an indenter to the Caribbean plate, driving crustal shortening in central and southern Costa Rica at maximum rates of 35 mm yr-1 across the fore arc and back arc. The indenter, rather than oblique convergence, drives trench-parallel forearc motion in Costa Rica and Nicaragua at rates up to 14 mm yr-1. In total, our results indicate the broad effects of seamount and aseismic ridge interaction with the MAT.