T42A-01
On The Significance Of Tectonic Underplating Beneath Continental Margins
More than half of the modern subduction margins are eroding - that is the preserved amount of terrigenous and pelagic sediment accreting to the trench is less than predicted for the lifetime of the trench. In many cases in fact, long-lived trenches are entirely sediment starved. Lateral continental disassembly and/or subduction erosion (sediment subduction and tectonic underplating) are the possible suspects at eroding margins. Tectonic underplating is the process of burial of accretionary wedge sediments and forearc sequences by transferring them from the upper plate to the lower plate at the subduction interface followed by reattachment of these materials to the deep crust of the upper plate. I will give examples for both and establish some general rules for differentiating among the two in the geologic record. As a specific and more detailed example, I will then use the Acapulco trench of southern Mexico to demonstrate that at least about 90% of the sediment mass predicted for this trench wedge is missing and was most likely removed by subduction erosion. This calculation integrates marine seismic, drilling and forearc thermochronology for the past 25 M.y. of the life of the Acapulco trench. Most conceptual models that take sediment recycling into consideration assume that the sediment is almost entirely recycled into the mantle, with perhaps the exception of a small fraction of material returned to the upper plate via arc magmatism. In contrast, I suggest that tectonic underplating is a major process at many convergent margins. The modern lithospheric architecture of southern Mexico from the Acapulco trench to the Mexican Volcanic Belt is a world-class example of tectonic underplating, and I will present evidence for that. Tectonic underplating is fundamentally a mechanism that: (1) provides a way to deliver upper crustal materials to the lower crust in arc settings, and significantly weakens the upper plate, (2) temporarily localizes some subduction megathrusts entirely within the crust, (3) leads to a hotter-than-normal subduction system that can influence the crustal melting function in arc regions.
T42A-02
Pre-Cenozoic History of the Jalisco Block (Guerrero Terrane of West-Central Mexico) From U- Pb Geochronology
The Jalisco Block is a fault-bounded crustal fragment, located along the western edge of the Trans-Mexican Volcanic Belt of west-central Mexico, and it forms part of the Triassic-Jurassic Guerrero terrane. Because of its extensive Miocene-Quaternary volcanic cover and Early Cretaceous-Tertiary intrusions, little is known about its Pre-Tertiary history. U-Pb (LA-MC-ICPMS) zircon geochronology of metasedimentary (Quartz Feldespathic Schist), marine sedimentary and plutonic rocks from several localities of the Jalisco Block (southern Mexico) were determined to constrain the geological evolution of this region. Results for the metasedimentary rocks, which are present as roof pendants, indicate a depositional age between the Late Triassic and Middle Jurassic. The older depositional sequence (Late Triassic) shows an important continental influence with subordinate clusters from the Carboniferous (297 Ma), Ordovician (446 Ma), Late and Middle Proterozoic (760, 1041, 1200, 1500 Ma) and Archean (~~2700 Ma), whereas the youngest depositional sequence (Middle Jurassic) have mainly Jurassic zircons, indicating sedimentation in a restricted basin. Ages of turbiditic marine sediments reveal a maximum depositional age of ~~130 Ma (Early Cretaceous), comparable to zircon populations from other areas of the Guerrero Terrane, and with minor age peaks at 320, 480-560, 743, 997-1260, and ~~2750 Ma. Magmatic granitic rocks from Atenguillo, Puerto Vallarta, Mascota and Tomatlan have ages of ~~60 Ma, ~~74 Ma and 80-90 Ma. These ages overlap with previously determined ages of granites and rhyolitic ash flow tuffs from the Jalisco Block, Baja California, Sonora and Sinaloa states. On the other hand, a granitic sample of Cuale region yielded a middle Jurassic age. Metasedimentary and sedimentary rocks of the Jalisco Block have a depositional evolution similar to coeval formations in the Guerrero and Alisitos terranes. We conclude that this region has been under a modest to large- scale arc plutonism from the Middle Jurassic to the Tertiary.
T42A-03
Exploring the Notion That Subduction Erosion Has Removed or Submerged Costa Rica's Early Tertiary Arc Massifs
Arc igneous rocks of Paleocene, Eocene, and Oligocene age are widely exposed in the southern, coastal region of Panama (Lissinna et al., EGU abstract, 2006). These rocks intrude or overlie mafic basement rock of the Caribbean Large Igneous Province (CLIP) of Late Cretaceous age that extends to the east to underlie the Caribbean Basin and form the Caribbean plate. Immediately west of Panama, in coastal Costa Rica, exposures of CLIP basement are not intruded or overlain by arc magmatic rocks of early Tertiary age. EXPLANATIONS: Potentially, the early Tertiary subduction zone that dipped beneath the Pacific margin of Panama did not extend to the west, thus no arc magmatism occurred where Costa Rica presently exists. Alternatively, the subduction zone bordering the Pacific edge of the CLIP extended below Costa Rica but former exposures of early Tertiary arc magmatic rocks piled there have been erosionally removed or buried beneath Miocene and younger arc massifs of interior mountain belts. EXPLORING A SUBDUCTION EROSION EXPLANATION Onshore and offshore evidence documents that subduction erosion thins and truncates the submerged rock framework of the Middle and South America forearc. The eroded (removed) material is transported toward and into the mantle within the subduction channel separating the upper plate of the forearc and lower plate of the subducting oceanic crust. The long-term (greater than 10 Myr) rate of truncation (i. e., migration of the trench toward a fixed, onshore reference) averages 2 to 3 km/Myr. Because of the subduction of the aseismic Cocos Ridge beneath Costa Rica, during at least the past 4 to 5 Myr the rate of truncation at this margin has been much higher. It is proposed that during the past 50 Myr subduction erosion has truncated the Costa Rica forearc by at least 100 km and either obliterated or deeply submerged arc massifs of early Tertiary age. Their exposed presence to the east in neighboring Panama reflects the circumstance that since the early Miocene the Panama sector of the Middle America margin has been a transform margin separating the Cocos and Caribbean plates, thus subduction erosion here has been minimal. During the past 25 Myr the virtual lack of subduction erosion along the Panama margin has thus preserved arc massif evidence of the early Tertiary subduction of Pacific ocean crust beneath the western edge of the CLIP, but similar evidence has been destroyed or submerged along the neighboring Costa Rica margin.
T42A-04
Missing Forearcs: A Model for Oligocene Antithetic Thrusting and Subduction Erosion in the Xolapa Complex, Southern Mexico
Two distinct E-striking S-dipping zones of ductile mylonization were identified on four transects from Mexico's Pacific coastline roughly 25km north into the Sierra Madre del Sur centered on the town of Puerto Angel, Oaxaca. One of these shear zones appears to be coincident with the previously reported sinistral Chacalapa Fault. However, stretching lineations with significant down dip components predominate along three of these transects, together with smaller populations of horizontal stretching lineations on discrete planes within the shear zones. This two-group population of lineations suggests either two distinct episodes of shearing or, more likely, one episode of obliquely driven shearing with strain partitioning. Regardless, the down dip lineations coupled with an analysis of rotated porphyroblasts, strain shadow zones and mica fish all indicate a major component of thrusting antithetic to subduction on the Acapulco Trench. Since the mylonitization affects otherwise undeformed, probably Oligocene, granitoids, it is assumed here that the previously published upper (29 Ma) and lower (23 Ma) bounds for Chacalapa shearing constrain the time of thrusting. Previous studies suggest that a large sliver of fore-arc (>150 km wide perpendicular to the trench) was removed from southern Mexico during the Oligocene. Attempts to locate the suspected missing forearc at Earth's surface suffer from a lack of tectonic and geological support for the required motion. I propose that tectonic motion of the Farallon(-Cocos) plate(s) relative to North America brought the Farallon images of the Molokai Fracture Zone and the Clarion Fracture Zone into play as significant topographic elements capable of generating subduction erosion, especially when combined with increased convergence rates at the Middle America Trench associated with fragmentation of the Farallon plate into the Cocos and Nazca plates. I argue that the antithetic thrusting observed in the field is directly related to subduction erosion. Geochronological data tied to the structures observed in the field are required to compare the geometry and kinematics of arc deformation with times of arrival of oceanic fracture zones in Farallon(-Cocos)-North America reconstructions. Confirmation of this model will justify the creation of a predictive space-time map for similar cases of subduction erosion in the geological record of North America's Pacific margin.
T42A-05
Frontal Arc Migration, Forearc Subduction Erosion and Adakitic-like Magmas: Examples From the Andean Margin and Implications for Other Arcs
Geochemical evidence for forearc subduction erosion linked to crustal modification of the mantle wedge is most evident in arc magmas erupted at times of frontal arc migration. The most convincing signals are from changes in isotopic ratios in mafic magmas erupted in the same place before and after arc migration, and from transient steep REE patterns and strong HFSE depletions in magmas erupted near times of arc migration. Such patterns are well seen on the margins of the Chilean flat-slab around 27S and 34S lat. where the arc front migrated about 50 km to the east in the last 8 Ma. The best evidence comes from increasing 87Sr/86Sr in mafic lavas that cannot be explained by enriched mantle, subducted sediments or in situ crust. Transient steep adakitic-like REE patterns at times of arc migration fit with a model in which forearc crust is transported down the subduction channel, enters the tip of the asthenospheric wedge and is incorporated into the arc mantle source. Sharp increases in 87Sr/86Sr ratios and transient steep REE patterns in Andean arc rocks between 21S and 26S lat. described by Haschke et al. (2002) and Mathews and Cornejo (2004) also occur in the final stages of magmatic cycles near times of frontal arc migration at about 130 (?), 85, 65-60 and 40-35 Ma. These patterns are interpreted by the authors as reflecting contractional crustal thickening at the end of the magmatic cycles. The sharp changes near the end of these cycles could additionally reflect crust introduced into the mantle source by subduction erosion at the time of arc migration. Transient adakitic-like arc magmas with steep REE patterns erupted elsewhere at times of arc instability and front migration could also reflect crustal contamination of the mantle source by forearc subduction erosion. Forearc subduction erosion often provides a better explanation for adakitic magmas in arcs with thin crust than slab melting as has been argued for Aleutian and Central America arc rocks.