Jonas Kley

TECTONIC SHORTENING AND CRUSTAL THICKNESS IN THE CENTRAL ANDES : HOW GOOD IS THE CORRELATION?

Exceptional thickening of continental crust beneath the Central Andes is believed to be mainly a result of tectonic shortening of the South American plate in Neogene time. This shortening has been estimated to have contributed as much as 70%–80% of the present crustal volume. A compilation of published shortening values and our own estimates based on balanced cross sections from the Central Andes between lat 3°S and 40°S suggest that 70%–80% is a maximum rather than an average value for this part of the Andes. Tectonic shortening and the crustal cross-section area are only loosely correlated. Variations in tectonic shortening are more abrupt than those of crustal areas, particularly near the northern and southern ends of the Altiplano-Puna high plateau, where thick crust is associated with relatively small amounts of shortening. Shortening there may account for no more than about 30% of the present crustal cross-section area. The processes that created the remaining crustal area are not clear, but are likely to involve poorly constrained pre-Neogene tectonic shortening, moderate magmatic additions to the crust, tectonic underplating of material derived from the forearc, and possible flow of ductile lower crust along strike.

Deformation of the Central Andean Upper Plate System — Facts, Fiction, and Constraints for Plateau Models

We quantitatively analyse the spatial pattern of deformation partitioning and of temporal accumulation of deformation in the Central Andes (15–26° S) with the aim of identifying those mechanisms responsible for initiating and controlling Cenozoic plateau evolution in this region. Our results show that the differential velocity between upper plate velocity and oceanic plate slab rollback velocity is crucial for determining the amount and rate of shortening, as well as their lateral variability at the leading edge of the upper plate. This primary control is modulated by factors affecting the strength balance between the upper plate lithosphere and the Nazca/South American Plate interface. These factors particularly include a stage of reduced slab dip (33 to 20 Ma) that accelerated shortening and an earlier phase (45 to 33 Ma) of higher trenchward sediment flux that reduced coupling at the plate interface, resulting in slowed shortening and enhanced slab rollback. Because high sediment flux and transfer of convergence into upper plate shortening constitute a negative feedback, we suggest that interruption of this feedback is critical for sustaining high shortening transfer, as observed for the Andes. Although we show that climate trends have no influence on the evolution of the Central Andes, the position of this region in the global arid belt in a low erosion regime is the key that provides this interruption; it inhibits high sediment flux into the trench despite the formation of relief from ongoing shortening. Along-strike variations in Andean shortening are clearly related to changes of the above factors. The spatial pattern of distribution of deformation in the Central Andes, as well as the synchronization of fault systems and the total magnitude of shortening, was mainly controlled by large-scale, inherited upper plate features that constitute zones of weakness in the upper plate leading edge. In summary, only a very particular combination of parameters appears to be able to trigger plateau-style deformation at a convergent continental margin. The combination of these parameters (in particular, differential trench-upper plate velocity evolution, high plate interface coupling from low trench infill, and the lateral distribution of weak zones in the upper plate leading edge) was highly uncommon during the Phanerozoic. This led to very few plateau-style orogens at convergent margins like the Cenozoic Central Andes in South America or, possibly, the Laramide North American Cordillera.

Transition from basement‐involved to thin‐skinned thrusting in the Cordillera Oriental of southern Bolivia

The eastern flank of the Central Andes in southern Bolivia and northern Argentina is underlain by a fold-and-thrust belt of Neogene age. It comprises an external thin-skinned segment with basal detachment in Silurian shales (Subandean Ranges) and an internal segment where units as deep as Precambrian metasedimentary and igneous rocks are involved in thrusting (eastern Cordillera Oriental). Between the Cordillera Oriental and the Subandean Ranges an intermediate segment (Interandean Zone) intervenes north of the Argentinian-Bolivian border (22° S). The structure of the Interandean Zone is characterized by folded and thrust-faulted Cambrian to Triassic strata. Balanced cross sections across the western Subandean Ranges, the Interandean Zone, and the eastern Cordillera Oriental are based on previously available data and recent field work. They suggest that the geological structure can be interpreted in terms of two major basement-involved thrusts, here referred to as Interandean and Subandean thrusts. The more internal Interandean thrust caused the development of broad basement anticlines in the eastern Cordillera Oriental. However, at shallower levels, its displacement was accommodated by thin-skinned folding and thrusting in the Interandean Zone. In-sequence activation of the Subandean thrust uplifted the Interandean Zone on top of an imbricate basement sheet and caused the development of the thinskinned Subandean Ranges farther east. Published radiometric data and subsidence history of the Subandean foreland basin suggest that the Interandean thrust was active between about 10 Ma and 5 Ma. The Subandean thrust is still active. Southward displacement transfer from the Subandean thrust onto the Interandean thrust and a southward dipping lateral ramp of the Interandean thrust result in the gradual transition from the Interandean Zone north of 22° S to the Cordillera Oriental of northern Argentina. A similar but larger-scale transfer may be involved in the southern termination of the thin-skinned Subandean Ranges of Argentina at 23° S. Shortening in the western part of the thrust belt is of the order of 80–90 km. Total shortening in the Eastern Andean thrust belt from the undeformed foreland to the eastern margin of the Cordillera Oriental is 140–150 km.

Geologic and geometric constraints on a kinematic model of the Bolivian orocline

Abstract The “Bolivian orocline” is the change in trend of the Andes from NW to N near 18°S. Paleomagnetic data have been used to infer that this bend was in part produced by wholesale opposed rotations (15–20° counterclockwise north of the bend and 15–20° clockwise south of it) of the two limbs of the orocline. Besides paleomagnetic data, shortening estimates from balanced cross-sections and variations in crustal cross-sectional area provide quantitative information on rotations and translations. The three sets of data do not agree closely, and therefore only loosely constrain the kinematics of the orocline. A map view kinematic model incorporating the major faults gives a more detailed picture. The resulting displacement field suggests that movement perpendicular to the orogen increases toward the bend, whereas the component of orogen-parallel movement increases away from it. Only weak rotations are indicated for the bend region. It is speculated that large-scale regional rotations of the limbs of the orocline are markedly lower than previously suggested, probably of the order of 5–10°. Stronger rotations are associated with strong lateral shortening gradients or may result from superimposed local phenomena.

Late Cretaceous intraplate thrusting in central Europe: Effect of Africa-Iberia-Europe convergence, not Alpine collision

A well-established event of intraplate basin inversion and basement thrusting affected central Europe in Late Cretaceous time. It is widely accepted to have resulted from the collision of the Alpine orogen with Europes margin. At that time an early Alpine orogen, located on the leading edge of the Adria microplate, still lay far southeast of its present-day position and had entered a phase of extension after a first orogenic event characterized by W- to NW-directed thrusting. This configuration is not likely to have induced SSW-NNE–directed thrusting and folding in the future European foreland that was still separated from the Alpine wedge by a strip of oceanic lithosphere. By contrast, the onset of intraplate contraction coincides with an important change in relative motion between the European and African plates. At ca. 90 Ma, Africas SSE-directed sinistral transform motion relative to Europe changed to NE-directed convergence. This agrees well with the timing and kinematics of intraplate thrusting in central Europe. Structures of similar age and kinematics occurring in southern France, Spain, and North Africa suggest that the Late Cretaceous pulse of contraction was caused by pinching west-central Europes thin lithosphere between Baltica and Africa. Only since the onset of N-directed thrusting in the Alps in Paleocene or Eocene time are the kinematics of the Alps and their European foreland compatible, indicating that mechanical coupling between Africa-Europe and the Adria microplate had been achieved.

Pre-Andean and Andean-age deformation in the Eastern Cordillera of southern Bolivia

Abstract The Eastern Cordillera of southern Bolivia is situated between the Altiplano high plateau in the west and the thin-skinned Subandean thrust belt in the east. It is mainly built of Ordovician anchimetamorphic sedimentary rocks with a discontinuous, unconformable cover of Cretaceous to Neogene, mostly continental sediments. The eastern margin of the Eastern Cordillera exhibits E-verging basement-involved thrusts linked to the Subandean belt. Its central and western parts, discussed in this paper, represent a province of dominantly W-verging, basement-involved thrusts of Tertiary age superimposed on a deeply eroded pre-Cretaceous fold belt. Several well-preserved remnants of the pre-Cretaceous fold belt show kilometer-scale, W-verging folds with axial plane slaty cleavage. The main activity of Andean (post-Cretaceous) thrusts is bracketed between about 25 and 12 Ma. However, some Andean contractional deformation occurred earlier. The exposed major Andean thrusts dip steep to intermediately at the surface. Cross-section balancing suggests that they merge into a detachment at maximum depths of 9 to 17 km. Neogene shortening is concentrated in an imbricated thrust system at the border of the Eastern Cordillera with the Altiplano in the west, but is relatively moderate (about 10–15%) within the Eastern Cordillera itself. W-verging thrusts in the Eastern Cordillera predate the E-verging thrust belt which underlies the eastern flank of the Andes. Strain rates are comparatively low during these first stages of deformation, but increase markedly at the onset of thin-skinned thrusting farther east. Balancing of a regional cross-section of 150 km present length indicates that the western and central Eastern Cordillera has shortened by about 55–80 km since Cretaceous time. Overall Tertiary tectonic shortening between the deformation front and the magmatic arc is about 215–250 km. This is insufficient to build the entire crustal root of the Andes by thrust thickening alone.

Consistency of geologic and geodetic displacements during Andean orogenesis

Displacement and convergence rates inferred from GPS and marine magnetic data suggest that this trend may be continuing at present. Hence an increasing fraction of convergence is being absorbed by mountain building. This change may reflect increased plate coupling due to decreased sediment supply, younger subducting lithosphere, or increased normal stress at the interface from the effects of uplift. INDEX TERMS: 8102 Tectonophysics: Continental contract orogenic belts; 8157 Evolution of the Earth: Plate motions—past (3040); 8158 Evolution of the Earth: Plate motions—present and recent (3040); 8099 Structural Geology: General or miscellaneous

Basement-involved blind thrusting in the eastern Cordillera Oriental, southern Bolivia: evidence from cross-sectional balancing, gravimetric and magnetotelluric data

Abstract A Neogene to Recent thrust belt forms the Subandean Ranges and Cordillera Oriental along the eastern slope of the Central Andes in Bolivia. Over much of its length, there is an area of intermediate topographic height and structural level between the Subandean Zone and the Cordillera Oriental proper. Cross-sectional balancing suggests that in southern Bolivia the basal detachment of this tightly folded zone links up with a blind thrust that dips westward beneath the high standing eastern margin of the Cordillera Oriental and rides piggy-back on a large buried thrust sheet which forms the backstop for the thin-skinned Subandean belt. Magnetotelluric and gravimetric data indicate that this thrust sheet is made up of basement rocks, probably including intrusives and higher-grade metamorphics. Such blind basement thrusts in a relatively external position may characterize much of the eastern Andean thrust belt and their geometry is possibly related to preexisting structural patterns linked to the configuration of early Palaeozoic basins and their margins.

The geometry of the central Andean backarc crust: Joint interpretation of cross-section balancing and seismic refraction data

Abstract Detailed cross section balancing and the interpretation of seismic refraction data in the Andean backarc at 21°S latitude allow the development of a crustal balancing model for the backarc region. The thrust belt developed on the eastern flank of the Andean orogen can be divided into three major units: the Subandean fold and thrust belt incorporating Paleozoic to Cenozoic sediments, the Interandean with tightly folded Silurian to Triassic strata, and the eastern margin of the Eastern Cordillera with predominantly Ordovician anchimetamorphic sediments, structured by large open folds. The structure of the thrust belt is dominated by two large “blind” thrusts. High seismic velocities in the upper to middle crust of the Eastern Cordillera with an underlying thick low velocity zone, derived from seismic refraction investigations, are interpreted as lower crustal material overthrust to the east over the crust of the Brazilian shield. The minimum shortening between the Eastern Cordillera and the foreland since Upper Cretaceous is about 145 km, as derived from cross section balancing. The crustal thickening, derived from the seismic refraction data, can be explained only from the foreland to the Eastern Cordillera by tectonic shortening. At least two main thrusts, cutting the whole backarc crust, are necessary to fit the high velocity bodies in the Eastern Cordillera to the geological observations. Further to the west, the thick continental crust, as derived from seismic refraction and gravimetric data, cannot be explained by tectonic shortening alone.

Geothermal and Tectonic Evolution of the Eastern Cordillera and the Subandean Ranges of Southern Bolivia

Combined tectonic and geothermal investigations along a profile from Villamontes to San Vicente provide new ideas about the evolution of the Eastern Cordillera and Subandean Ranges of southern Bolivia. Sediment maturity was determined using reflectance and infrared spectroscopy measurements on organic matter as well as the crystallinity of illites and ranges from diagenetic in the Subandean Ranges to weak metamorphic in the Eastern Cordillera. The tectonic evolution is characterized by Hercynian movements producing a “Protocordillera” and the Andean orogeny with substantial crustal shortening which is compensated at a shallower level mainly in the Subandean Ranges while the Eastern Cordillera remains relatively stable.