The Central Andean subduction system is one of the most active geological structures on Earth. Although there have been a few previous studies, the structure and dynamics of the system are still not ...well understood. In the present study, we determine a combined regional gravity model of the Andean convergent subduction region for constraining lithospheric models. After a thorough validation and cleaning of the terrestrial gravity and height databases, the method of Least Squares Collocation was applied to consistently combine terrestrial and satellite gravity data, putting much emphasis on the stochastic modelling of the individual data components. As a result, we computed the first high-resolution regional gravity model of the study region that includes GOCE satellite gravity information. The inclusion of GOCE is an essential distinction from the independent global gravity model EGM2008. Validation against EGM2008 reveals that our regional solution is very consistent in regions where terrestrial gravity data are available, but shows systematic differences in areas with terrestrial data gaps. Artefacts in the EGM2008 of up to 150 mGal could be identified. The new combined regional model benefits from the very homogeneous error characteristics and accuracy of GOCE gravity data in the long-to-medium wavelengths down to 80–100 km. Reliable density modelling became possible also in the region of Central Andes, which lacks terrestrial gravity data. Finally, density models were adapted to fit the new regional gravity field solution. The results clearly demonstrate the capabilities of GOCE to better constrain lithospheric models.
With the advent of global positioning system (GPS) technology and its applications to high precision geodesy, earth scientists have gained an unprecedented opportunity to study the kinematics and ...dynamics of present-day deformation processes along plate boundary zones. However, until now our knowledge of the deformation processes associated with the Andean subduction along the western coast of South America has been limited. Here we present first estimates of present-day crustal deformation rates in the central and southern sections of the Andes, between latitudes 22°S and 42°S. We find that the deformation in the central section of our study area is dominated by the interseismic phase of an earthquake deformation cycle, caused by 100% coupling of the thrust interface between the subducting Nazca and the overriding South America plates. The estimated depth of coupling is not uniform along strike: north of 30°S it is ∼33 km deep, while south of 35°S it reaches 50 km depth. In addition, we observed postseismic relaxation effects in the northern part of our network in the area of the 1995
M
w8.0 Antofagasta earthquake. South of 38°S, we detected a similar deformation pattern as in the Antofagasta area which we attribute to postseismic relaxation effects of the 1960
M
w9.5 Valdivia earthquake.
New U–Pb SHRIMP zircon ages together with a geochemical and isotopic (Sr and Nd) study have been carried out on a hornblende gabbro, a biotite granodiorite, and host andesitic rocks of the North ...Patagonian batholith at Paso Puyehue, close to the Argentinean-Chilean border. The results yield ages of 18 ± 1 Ma for the gabbros and 12.4 ± 0.3 Ma for the granodiorites. An age of 125 ± 2 Ma was obtained for the host volcanic rock, and an age of 22 ± 2 Ma for the basaltic trachy-andesite. The intrusive suite is composed of dominant hornblende-biotite granodiorites and subordinate hornblende gabbros and diorites. These form a typical metaluminous calc-alkaline series. The time-isotopic magmatic evolution of plutonic and volcanic rocks shows an early Eocene beginning of disparate isotopic ratios as a consequence of coeval primitive and more crustal compositions with no spatial constraints. This disparate isotopic assemblage is also coeval with the presence of the Aluk-Farallon-SAM triple junction at this latitude and the shift of the calc-alkaline magmatism from the batholith axis to the back-arc. Considerations are made for the possibility of a Paleogene slab window by slab detachment, and the development of a new subduction front since the beginning of Nazca Plate development at the 23 Ma major ocean plate rearrangement.
► First Tertiary U-Pb SHRIMP zircon ages of the North Patagonian batholith. ► Disparate isotopic asemblage of coeval primitive and more crustal compositions. ► A new subduction front at the 23 Ma major ocean plate rearrangement.
New mid Miocene to present plate tectonic reconstructions of the southern Central American Volcanic Arc (CAVA) reveal that the inception of Cocos Ridge subduction began no earlier than 3 Ma, and ...possibly as late as 2 Ma. The Cocos Ridge has been displaced from the Malpelo Ridge to the southeast since ∼9 Ma along the Panama Fracture Zone (PFZ) system. Ambiguous PFZ and Coiba Fracture Zone (CFZ) interaction since ∼9 Ma precludes conclusively establishing the age of initial Cocos Ridge subduction. Detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern Central America that preceded subduction of the Cocos Ridge, including southeastward migration of the Nazca–Cocos–Caribbean triple junction along the Middle America Trench (MAT) from 12 Ma to present, and subduction of ≤2 km high scarps both parallel and perpendicular to the trench from 6 to ∼1 Ma.
The timing of changes in subduction processes has commonly been determined by (and correlated with) geologic changes in the upper plate. However, reliable
40Ar/
39Ar dating of these events has become available only recently Abstr. Programs-Geol. Soc. Am. (2002). These new dates better constrain the magmatic and structural history of southern Costa Rica. Observations from this data set include: a gap in the volcanic record from 11 to 6 Ma, which coincides temporally with emplacement of most plutons in southern Costa Rica, normal arc volcanism ceased after 3.5 Ma in southern Costa Rica, and Pliocene (mostly ∼1.5 Ma) adakite volcanism was widely distributed from central Panama to southern Costa Rica (though volumetrically insignificant).
This new data reveals that many geologic phenomena, commonly attributed to subduction and underplating of the buoyant Cocos Ridge, in fact precede inception of Cocos Ridge subduction and seem to correlate more favorably in time with earlier tectonic events. Adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern Panama. Regional unconformities and an 11–6 Ma gap in arc volcanism match temporally with oblique subduction of the Nazca plate beneath central and southern Costa Rica. Cessation of volcanic activity, low-temperature cooling of plutons in the Cordillera de Talamanca (CT), and rapid increases in sedimentation in the fore-arc and back-arc basins coincide with passage of the Nazca–Cocos–Caribbean triple junction and initiation of subduction of “rough” crust associated with Cocos–Nazca rifting 3.5 Ma, closely followed by initial subduction of the Cocos Ridge 2–3 Ma. None of the aforementioned geologic events occurred at a time that would allow for underplating by the Cocos Ridge. Rather they are probably related to complex interactions with subduction of complicated plates offshore. All of the aforementioned events indicate that the southern Central American subduction system has been in flux since at least ∼12 Ma.
The influence of convergent plate boundary curvature on the stress distribution in an overriding plate is explored using analytical and numerical modeling techniques. Trench‐parallel compression can ...be produced near the symmetry axis of a seaward‐concave plate boundary if the interplate friction is high and/or if the subducting lithosphere has a low flexural rigidity, which produces little nonhydrostatic normal stress on the plate boundary. This situation favors the formation of trench‐parallel thrusts with minor trench‐parallel component of slip. Trench‐parallel compression is reduced along the most oblique parts of the plate boundary, which favors the formation of strike slip faults with major trench‐parallel slip. Both the stress conditions on the interplate zone and the 3‐D geometry of this zone control whether the trench‐parallel stress in the center of a seaward‐concave curvature is a tension or compression. Low dip angle and high convergence obliquity angle favor trench‐parallel compression. In the central Andes, N‐S minor shortening in the center of the Arica bend and strike slip systems north and south of the symmetry axis suggest that the effect of shear traction dominated during Cenozoic time when the curvature of the plate boundary was forming. This result suggests that the processes responsible for the formation of the plate boundary curvature were assisted by enhanced interplate friction and/or reduced compressive nonhydrostatic normal stress. For a geometry resembling the present‐day South American plate margin, estimations of normal and shear stresses on the plate boundary suggest that the trench‐parallel stress in the center of the curvature is compressive.
We conducted a preliminary structural and tectonic analysis of the region north and south of the Lake General Carrera–Buenos Aires (LGCBA, Southern Patagonian Andes, Chile and Argentina) in order to ...study the effects of the subduction of the active South Chile spreading Ridge (SCR) on the development of the tectonic front of the Patagonian Andes at the latitude of the Chile Triple Junction (CTJ). Here, one segment of the SCR entered the trench at 6 Ma and the corresponding slab window is now located below the study area. We emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments (A to D) trending mainly N170 and right laterally offset. This geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction. The four segments of the tectonic front are separated and locally deformed by N170 oriented, more recent sinistral strike-slip fault zones. Kinematic analysis of minor fault and striaes confirms the abundance of strike-slip faults and suggests rotation of the stress tensor after at least 9 Ma. A consequence of the subduction of the SCR beneath the South American plate at the latitude of the LGCBA is a two-step history: (1) Pre-ridge subduction, oblique to the western margin of the South America plate showing contractional regime with its main axis trending NE–SW responsible for dextral transpressive deformation and (2) Post-ridge subduction more orthogonal convergent regime. Recent uplift occurred later, starting at 3.5 Ma, and could still be active. Neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial–lacustrine terraces. Anomalies in stream development can be found. Although evidences for recent to active faulting can be shown, effects of syn-glacial and post-glacial-related processes cannot be ruled out. The evolution of this region during the Late Cenozoic can be divided into three distinct periods: (1) 20–15 Ma, deposition of foreland continental detrital sediments (Rio Zeballos Group), followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10–9 Ma. (2) 9–4 Ma, erosion, peneplanation and emplacement of voluminous plateau basalts coeval with fluvial–glacial sedimentation and, (3) 3–0 Ma, renewed orogenic activity, uplift and disruption of former erosional surfaces in relation to activation of strike-slip fault zones and north trending thrusts. Onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the Chile Ridge at 14–15 Ma in southernmost Patagonia, inducing a different stress regime in the frontal arc region.
The 1.5‐km‐high, obliquely subducting Nazca Ridge and its collision zone with the Peruvian margin have been imaged by wide‐angle and reflection seismic profiles, swath bathymetry, and gravity ...surveying. These data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust. The lowermost layer contributes 10–12 km to the total crustal thickness of the ridge. The sedimentary cover is 300–400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges. At the collision zone of ridge and margin, the following observations indicate intense tectonic erosion related to the passage of the ridge. The thin sediment layer on the ridge is completely subducted. The lower continental slope is steep, dipping at ∼9°, and the continental wedge has a high taper of 18°. Tentative correlation of model layers with stratigraphy derived from Ocean Drilling Program Leg 112 cores suggests the presence of Eocene shelf deposits near the trench. Continental basement is located <15 km landward of the trench. Normal faults on the upper slope and shelf indicate extension. A comparison with the Peruvian and northern Chilean forearc systems, currently not affected by ridge subduction, suggests that the passage of the Nazca Ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long‐term erosive regime.
Several trench-outer rise settings in subduction zones worldwide are characterized by a high degree of alteration, fracturing and hydration. These processes are induced by bending-related faulting in ...the upper part of the oceanic plate prior to its subduction. Mapping of P- and S-wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere, and serves as a baseline for comparison with seismic measurements elsewhere. Active source seismic investigations at the outer rise off Southern Central Chile (∼43°S) were carried out in order to study the seismic structure of the oceanic Nazca Plate. Seismic wide-angle data were used to derive 2-D velocity models of two seismic profiles located seaward of the trench axis on 14.5 Ma old crust; P01a approximately parallel to the direction of spreading and P03 approximately parallel to the spreading ridge and trench axes. We determined P- and S-velocity models using 2-D traveltime tomography. We found that the Poisson's ratio in the upper crust (layer 2) ranges between ∼0.33 at the top of the crust to ∼0.28 at the layer 2/3 interface, while in the lowermost crust and uppermost mantle it reaches values of ∼0.26 and ∼0.29, respectively. These features can be explained by an oceanic crust significantly weathered, altered and fractured. Relative high Poisson's ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization. Thus, the seismic structure of the oceanic lithosphere at the Southern Central Chile outer rise exhibits notable differences from the classic ophiolite seismic model (‘normal’ oceanic crust). These differences are primarily attributed to fracturing and hydration of the entire ocean crust, which are direct consequences of strong bending-related faulting at the outer rise. On the other hand, the comparison of the uppermost mantle P-wave velocities at the crossing point between the perpendicular profiles (∼90 km oceanward from the trench axis) reveals a low degree of Pn anisotropy (<2 per cent).
The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the ...subducting Nazca plate transitions into the Chilean “flatslab” to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3–1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive “caldera-forming” event at 0.51 Ma that produced the 14 km
3
Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO
2
= 67–72 wt%; La/Yb = 37–56; Ba/La = 16–28; La/Ta = 30–50;
87
Sr/
86
Sr = 0.70638–0.70669;
ε
Nd
= −4.2 to −4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6–8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano–Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated “adakite-like” dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.
The paper presents proxies from an interdisciplinary geoarchaeological working group. Sediment analyses and geomorphological studies, radiocarbon ages of snail shells and luminescence dating of loess ...allow a preliminary chronology of the environmental evolution of the eastern Atacama desert, Nazca–Palpa region (southern Peru). Until now, typical desert loess was unknown from the arid western flank of the Andes (southern Peru). The loess points to periods of more humid conditions with open grasslands at the eastern Atacama desert margin in the early and middle Holocene. In the footzone of the Andes, aridification set in before the Paracas Culture (c. 800–200 bc) evolved, but the Cordillera Occidental remained semi‐arid. A second push of increasing aridity started at the latest in the Middle Nazca Period (after ad 250). During this time, the Nazca settlement centres moved upstream through the river oasis, following the eastward‐shifting desert margin. It is possible that culminating aridity after ad 600 caused the collapse of the Nazca civilization. During the Late Intermediate Period (ad 1000–1400), more humid conditions favoured the massive reoccupation of the eastern Atacama up to a distance of about 40 km from the Pacific coast. Since the 14th and 15th centuries, the Palpa region has again been part of the hyper‐arid Atacama. The study shows that in the Nazca–Ica region, the deep cultural changes of Pre‐Columbian civilizations were not caused by catastrophic run‐off of El Niño events, but by a shifting eastern desert margin due to the changing monsoonal influence.
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