Helmut Echtler

The Southern Andes between 36° and 40°S latitude: seismicity and average seismic velocities

Abstract The project ISSA 2000 (Integrated Seismological experiment in the Southern Andes) consists of a temporary seismological network and a seismic refraction profile. A network of 62 seismological stations was deployed across the Southern Andes at ∼38°S. Three hundred thirty-three local seismic events were observed in a 3-month period. P and S arrival times of a subset of high quality data were inverted simultaneously for 1-D velocity structure, hypocentral coordinates and station delays. Seismic refraction data along a transect at 39°S provide further constraints on the crustal structure. Low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere. The continental Moho is not clearly observed in this region. Average velocities of the crust beneath the arc are higher than those beneath the forearc. Crustal thickness is about 40 km. Crustal seismicity concentrates in the forearc region along the Bio-Bio and Gastre fault zones. The area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region. Benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the Benioff zone in the Southern Andes. A maximum of seismicity at 60 km depth may be caused by dehydration embrittlement.

Segmentation of megathrust rupture zones from fore-arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile

(1) This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore-arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepcion and Valdivia megathrust segments. This boundary, however, is only based on � 500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10 2 -10 6 years. Rapid exhumation of Nahuelbutas dome-like core started at 4 ± 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an Mw 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.

Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S)

Isla Santa Maria at the active margin of south-central Chile is the result of earthquake-related uplift and deformation in the forearc since at least late Pleistocene time. Field mapping, dating of key depositional horizons, and analysis of seismic-refl ection profi les reveal ongoing deformation in this sector of the Chilean forearc. The 30 km 2 island is located ~12 km above the interplate seismogenic zone and 75 km landward of the trench. It is situated near the southern termination of the Concepcion earthquake rupture segment, where Charles Darwin measured 3 m of coseismic uplift during a M > 8 megathrust earthquake in 1835. Permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged, landward-tilted abrasion surfaces. Uplift at ~2 m/k.y. and tilting at ~0.025°/k.y. of the island have been fairly constant throughout the late Quaternary and have resulted in emergence of the island above sea level ~31 k.y. ago. The island is composed of a late Pleistocene upper, tilted surface with two asymmetric tilt domains, and Holocene lowlands characterized by uplifted and tilted strandlines. Industry offshore seismic-refl ection profi les covering an area of ~1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding Isla Santa Maria; the principal fault apparently roots in the plate-interface thrust. These reverse faults in the upper plate result from inversion of late Cretaceous to early Pliocene normal faults and rift structure of the Arauco forearc basin. Positive inversion of these inherited structures started between 3.6 and 2.5 Ma and resulted in continuous shortening rates of ~0.8 mm/yr. The seismic-refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines, whereas their backlimbs have been removed by cliff retreat. The 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the Arauco-Concepcion region. The close relation among the asymmetric uplift and tilt of the island, modern deformation patterns, and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences, localizes, and segments surface deformation during large interplate earthquakes. Furthermore, the link between positive inversion of pre-existing structures, uplift, and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate-boundary deformation.

Inversion of forearc basins in south-central Chile caused by rapid glacial age trench fill

This study examines the response of a forearc to the increase in sediment flux to the trench caused by the onset of glacial denudation in the Patagonian Andes. We investigated shelf-coastal basins in south-central Chile, which generally comprise Eocene–early Miocene nearshore facies overlain by late Miocene–early Pliocene deep-water siltstones and by late Pliocene–Quaternary nearshore deposits. Seismic profiles and coastal exposures reveal Eocene–early Pliocene extension followed by ongoing late Pliocene compression evidenced from growth strata adjacent to seismically active reverse faults. The onset of major global cooling ca. 6 Ma triggered glacial denudation in the uplifted high Andes. Exhumed material transported along the steep and humid Andean western slope increased trench sedimentation rates and caused continuous accretion and subduction of terrigenous material. We interpret forearc basin inversion as a response to a decrease in slope and basal friction of the wedge caused by frontal accretion and subduction of water-rich material, respectively, in order to reach a critical taper. This process lifted the shelf ∼1.5 km during the middle Pliocene. The Juan Fernandez Ridge and Chile Rise confined >2 km trench fill between 45 and 34°S, limiting accretion and basin inversion. Glacial age trench fill and the steady decrease in plate convergence rate shifted this segment of the margin from erosive to accretionary during the Pliocene.

Deep structure of the continental lithosphere in an unextended orogen: An explosive-source seismic reflection profile in the Urals (Urals Seismic Experiment and Integrated Studies (URSEIS 1995))

Explosive-source near-vertical seismic reflection data from the Urals Seismic Experiment and Integrated Studies (URSEIS) profile displays an image of orogenic lithosphere to depths of 225 km. The reflective crustal section is characterized by dipping, crustal-scale shear zones, and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late Paleozoic collision. Toward the east, the Uralian crustal root is composed of a partially subducted volcanic arc, while to the west it has affinity with the East European platform. The reflection character of the Moho varies across tectonic strike and implies that the Moho (1) acted as a structural detachment beneath the Trans-Uralian Zone, (2) forms a tectonic boundary between terranes beneath the East Uralian Zone, (3) consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root, (4) is a 3–9 km thick reflective zone beneath the fold and thrust belt, and (5) is a boundary <200 m thick in the west. Mantle reflections at 80–100 km depth (22–24 s) may represent a continuation of the Main Uralian fault into the subcrustal lithosphere. Alternatively, these reflectors may signify Paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle. Deeper mantle reflections at 140–160 km (35–45 s) and 225 km (55 s) may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle.

Central and Southern Andean Tectonic Evolution Inferred from Arc Magmatism

Patterns of spatial distribution, and geochemical and isotopic evolution from subduction-related igneous rocks provide tools for scaling, balancing and predicting orogenic processes and mechanisms. We discuss patterns from two Andean key arc segments, which developed into fundamentally different types of orogens: (1) A plateau-type orogen with thick crust in the central Andes, and (2) a non-plateau orogen with normal crust in the southern Andes.

Geophysical Signatures and Active Tectonics at the South-Central Chilean Margin

The ISSA 2000 (Integrated Seismological experiment in the Southern Andes) and SPOC 2001 (Subduction Processes Off Chile) onshore and offshore projects surveyed the Chilean margin between 36 and 40° S. This area includes the location of the 1960 earthquake (M w = 9.5) that ruptured the margin from ∼38° S southwards for ~1000 km. Together with gravity and magnetotelluric components, the active-passive seismic experiments between 36 and 40° S provide the first, complete, high-resolution coverage of the entire seismogenic plate interface.

Using uplifted Holocene beach berms for paleoseismic analysis on the Santa Maria Island, south-central Chile

Major earthquakes ( M > 8) have repeatedly ruptured the Nazca-South America plate interface of south-central Chile involving meter scale land-level changes. Earthquake recurrence intervals, however, extending beyond limited historical records are virtually unknown, but would provide crucial data on the tectonic behavior of forearcs. We analyzed the spatiotemporal pattern of Holocene earthquakes on Santa Maria Island (SMI; 37 degrees S), located 20 km off the Chilean coast and approximately 70 km east of the trench. SMI hosts a minimum of 21 uplifted beach berms, of which a subset were dated to calculate a mean uplift rate of 2.3 +/- 0.2 m/ky and a tilting rate of 0.022 +/- 0.002 degrees/ky. The inferred recurrence interval of strandline-forming earthquakes is similar to 180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is similar to 0.4 m per event

Subduction- and exhumation-related fabrics in the Paleozoic high-pressure–low-temperature Maksyutov Complex, Antingan area, southern Urals, Russia

We use structural and petrologic data from a cross section through the high-pressure‐lowtemperature Maksyutov Complex to develop a new model for its tectonometamorphic evolution. The Maksyutov Complex is located within the southern Urals, the only Paleozoic orogen that apparently preserved its collisional architecture without overprinting by late orogenic extensional deformation. The high-pressure complex constitutes a large antiform in the footwall of the east-dipping Main Uralian fault and is composed of two tectonometamorphic units. The core of the antiform exposes wellpreserved eclogites and blueschists in the structurally lower unit 1 that underwent peak metamorphic conditions of ~17 kbar and ~570 °C. In contrast, the structurally overlying unit 2 contains lawsonite-bearing assemblages indicating both lower peak pressure (<8 kbar) and temperature (<450 °C). Both units exhibit a composite foliation S 1 affected by northwestvergent F 2 folds. F 2 fold axes and S 1 /S 2 intersection lineations trend northeast-southwest, oblique to the present north-south trend of the Maksyutov antiform. The D 1 /D 2 fabrics record a progressive northwest-directed shearing under prograde metamorphic conditions and are interpreted as the result of eastward subduction beneath the Irendyk island arc during oblique northwest-southeast‐directed plate convergence at 370‐380 Ma. After their subduction to different depths, the structurally lower unit 1 was tectonically juxtaposed against the upper unit 2 by a ductile, top-to-the-northeast extensional D 3 shear zone associated with the retrograde metamorphic evolution. The exhumation of unit 1 occurred in Late Devonian‐Early Carboniferous time, during continuous plate convergence that was accommodated by a thrust that imbricates the basement of the East European platform and is situated below the high-pressure rocks. Further exhumation of the Maksyutov Complex to a shallow crustal level was accomplished by ductile D 4 shear zones exhibiting east-west‐trending stretching lineations present at the margins of the complex. Large-scale folding of the Maksyutov antiform and minor top-to-the-east backthrusting on the western limb took place during a late stage of the Uralian orogeny, coeval with formation of the foreland thrust-and-fold belt in Permian time.

Long-Term Geological Evolution and Mass-Flow Balance of the South-Central Andes

In south-central Chile (36–42° S), the western edge of South America has evolved as an active margin since the Pennsylvanian (∼305 Ma). Active margins are considered as sites of both potential continental growth and continental destruction. Continental growth in a margin setting can proceed by accretionary offscraping of juvenile material from the oceanic plate and by magmatic additions, whereas net mass loss can be achieved by subducting continental material, delamination, and chemical weathering. In southcentral Chile, margin evolution was never interrupted by island-arc accretion or continental collision. Thus, the area provides an excellent field laboratory for studying mass flux through a long-term, persistent, convergent-margin system.