Laura Giambiagi

Structural evolution of the Andes in a transitional zone between flat and normal subduction (33°30′–33°45′S), Argentina and Chile

Abstract The sector of the Andes studied in this paper (33°30′–33°45′S) presents a key region to study the relationship between tectonic setting and deformation history in a transitional zone between flat (north of 33°S) and normal (south of 33°45′S) subduction segments. The Andes at these latitudes are primarily composed of the Neogene Aconcagua fold-and-thrust belt and the basement-block uplift of the Cordillera Frontal. Detailed mapping has revealed that the structure within the inner part of the fold-and-thrust belt resulted from both thin- and thick-skinned tectonic interactions. In the outer part, displacement is transferred to Mesozoic decollement levels, accounting for a thin-skinned architecture. Geographically, the switch from thick- to -thin skinned tectonics occurs near the border between Chile and Argentina. Although the geometry of the subducted Nazca Plate may have influenced the timing and style of deformation in the foreland, plate geometry alone does not adequately explain the style of deformation within the fold-and-thrust belt. Here it is shown that the change in deformation style in the fold-and-thrust belt correlates with the location of pre-existing Mesozoic structure, and specifically that Neogene-age thick-skinned thrusting was controlled by the presence of Mesozoic margin-boundary normal faults. Deformation in this region of the Andean fold-and-thrust belt was thus controlled by a combination of tectonic setting and pre-existing extensional structure.

The control of pre-existing extensional structures on the evolution of the southern sector of the Aconcagua fold and thrust belt, southern Andes

Abstract The Aconcagua fold and thrust belt, located in the Andean mountains at 32°30′ to 34°S, has been described as a classic model of a thin-skinned thrust belt. However, new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple Mesozoic extensional phases, overprinted by structural inversion, as well as thin- and thick-skinned tectonics. Two major superimposed extensional structural styles have been identified for the Mesozoic characterized by distinctly oriented stress fields. A key role in the evolution of this part of the fold and thrust belt was played by a Late Triassic to Early Jurassic depocentre and by Late Jurassic block faulting. Shortening was accommodated by a combination of inversion of pre-existing normal faults, development of footwall short cuts and both thin and thick-skinned thrusting. Synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults, with further shortening along newly formed thin-skinned thrust faults. The geometry of thin-skinned fault systems is controlled by the architecture of the rift basin, competent footwalls forming barriers to the lateral propagation of detachments.

Pre-Andean deformation of the Precordillera southern sector, southern Central Andes

This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent fold-and-thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The west-vergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip–dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Precordillera involves a north-northwest–trending weakness zone affected by north-northeast–directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a north-northwest–trending late Paleozoic belt, and Permian–Triassic rift-related magmatism indicates the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.

Evolution of shallow and deep structures along the Maipo-Tunuyán transect (33°40'S): From the Pacific coast to the Andean foreland

Abstract We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

Influence of pre-Andean history over Cenozoic foreland deformation: Structural styles in the Malargüe fold-and-thrust belt at 35°S, Andes of Argentina

The Andes are the classic example of a subduction-related orogen. Segmentation of the orogenic belt is related to dynamics of the subduction zone and to upper plate thermomechanical properties. Understanding the controlling factors on deformation along the orogen requires studying cross sections at different latitudes and determining the respective roles of plate interactions, upper plate weakness zones, and crustal architecture. A newly constructed balanced cross section of the Argentinean Andes at 35°S, in the transition between a flat-slab and a normal subduction segment, shows tectonic inversion of Mesozoic normal faults and development of new thrusts during Andean shortening. Estimated shortening of 26.2 km, equivalent to 22% of the initial length, is lower than previous estimates obtained from partial cross sections using non-inversion structural models. Comparison of this estimate with crustal area balance constrained by geophysical data indicates that (1) crustal thickness was varied across the transect before Andean shortening, with a thick (∼45 km) crustal block to the west related to late Paleozoic orogeny, and a thinner block (∼32 km) in the east related to Mesozoic stretching; and (2) a structural model incorporating tectonic inversion is consistent with regional shortening and crustal thickness trends. Our results underscore the role of the inherited characteristics of the upper plate in subduction-related orogens, including preexisting faults and preorogenic crustal thickness variations.

True three-dimensional trishear: A kinematic model for strike-slip and oblique-slip deformation

Most structural/kinematic models are inherently two-dimensional; even several recent three-dimensional models are “pseudo–three-dimensional” in that they consist of a series of parallel two-dimensional cross sections. Lack of a true three-dimensional formulation hampers our abilities to simulate three-dimensional structures such as oblique- and strike-slip faulting and displacement gradients perpendicular to the slip vector. The mathematical formulation of trishear deformation using incompressibility of flow is well suited to a solution in three dimensions. We derive one plausible velocity field for true three-dimensional flow in a triangular shear zone. This formulation allows us to simulate the deformation in oblique-slip deformation zones as well as flower structures associated with strike-slip fault zones. The strain distribution in flower structures combined with some simple mechanical assumptions suggests that faults in these zones would have a helicoidal geometry. The results of the kinematic model compare well to well-described structures in the Colorado Plateau, Andaman Sea, and Death Valley, as well as to data from analogue experiments.

Near pure surface uplift of the Argentine Frontal Cordillera: insights from (U–Th)/He thermochronometry and geomorphic analysis

Abstract Apatite (U–Th)/He thermochronology from palaeosurface-bounded vertical transects collected in deeply incised river valleys with >2 km of relief, as well as geomorphic analysis, are used to examine the timing of uplift of the Frontal Cordillera and its relation to the evolution of the proximal portions of the Andean foreland between 32° and 34°S latitude. The results of apatite (U–Th)/He (AHe) analyses are complex. However, the data show positive age-elevation trends, with higher elevation samples yielding older AHe ages than samples at lower elevation. Slope breaks occur at c. 25 Ma in both profiles, separating very slow cooling and or residence within a partial retention zone (slope of c. 10 m/Myr) at the highest elevations from a slope of c. 60–100 m/Myr cooling rate at lower elevations. The older AHe ages suggest either (1) minimal burial of the Frontal Cordillera and/or (2) significant pre–middle Miocene local relief. Geomorphic analysis of the adjacent, east-draining Río Mendoza and Río Tunuyán catchments reveals a glacial imprint to the landscape at elevations above 3000 m, including greater channel steepness and lower profile concavities developed during glacial erosion. Detailed analysis of headwall heights provides evidence of ongoing rock uplift along the entire eastern flank of the Frontal Cordillera and in the eastern flank of the Principal Cordillera south of the slab dip transition.

Temporal variation of the stress field during the construction of the Central Andes: Constrains from the volcanic arc region (22°- 26°S), Western Cordillera, Chile, during the last 20 Ma†

In order to understand the response of the stress field state to intrinsic processes during the construction of the Andes, such as thickening of the continental crust, lithospheric delamination, and/or thermal weakening, we investigate the stress field evolution of the arc region since the last 20 my, in the Central Andes (22°-26.5°S). The 43 reduced paleostress tensors derived from inversion of 682 fault-slip data reveal a complex pattern of stress states during the last episode of orogenic construction and topographic uplift. We identify two geodynamic stages; the first stage corresponds to the construction of the Altiplano/Puna plateau and the second one to its gravitational collapse. Four stress states that have prevailed in the Altiplano/Puna plateau since middle Miocene times, characterize the transition from one stage to the other. Along the study latitudes, a spatiotemporal change in stress state is clearly observed, which led to an understanding that a change in the stress field may be related not only to the boundary conditions but also to intrinsic factors associated with the construction of the Andean orogeny. Our results suggest that ca. 13-10 Ma and ca. 8-5 Ma, in the southern Altiplano and northern Puna, and in the southern Puna, respectively, regional elevation and crustal thicknesses reached threshold values necessary to generate the orogenic collapse.

Thermochronologic evidence for late Eocene Andean mountain building at 30°S

The Andes between 28°-30°S represent a transition between the Puna-Altiplano Plateau and the Frontal/Principal Cordillera fold-and-thrust belts to the south. While significant early Cenozoic deformation documented in the Andean Plateau, deciphering the early episodes of deformation during Andean mountain building in the transition area is largely unstudied. Apatite fission track (AFT) and (U-Th-Sm)/He (AHe) thermochronology from a vertical and a horizontal transect reveal the exhumation history of the High Andes at 30°S, an area at the heart of this major transition. Interpretation of the age-elevation profile, combined with inverse thermal modeling indicate that the onset of rapid cooling was underway by ~35Ma, followed by a significant decrease in cooling rate at ~30-25Ma. AFT thermal models also reveal a second episode of rapid cooling in the early Miocene (ca. 18Ma) related to rock exhumation to its present position. Low exhumation between the rapid cooling events allowed for the development of a partial annealing zone (PAZ). We interpret the observed Eocene rapid exhumation as the product of a previously unrecognized compressive event in this part of the Andes that reflects a southern extension of Eocene orogenesis recognized in the Puna/Altiplano. Renewed early-Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also impacted the core of the range in this transitional sector. The major episode of Eocene exhumation suggests the creation of significant topographic relief in the High Andes earlier than previously thought.

Cenozoic Orogenic Evolution of the Southern Central Andes (32–36°S)

This review explores the complex interactions of endogenic and exogenic processes in the segment of the Andes that straddle a transition from the Pampean flat slab to a normal subduction segment (32°–36°S). This segment shows remarkable along-strike variations in topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. In the flat-slab segment, high elevations, the development of several tectonic provinces (mountain systems) and the lack of active volcanism characterize the orogen. Deformation and uplift advanced to the east, together with arc-related magmatic activity, sequentially uplifting the Principal Cordillera (20 to ~8 Ma), the Frontal Cordillera (12–5 Ma), the Precordillera (<10 Ma) and the Sierras Pampeanas (<5 Ma). In the normal subduction segment to the south, the Andes are characterized by a decrease in elevation, with a big step in topography at ~35°S and the development of an active magmatic arc straddling the Argentina-Chile border. The Frontal Cordillera is only in the northern part of the normal subduction segment, disappearing at 34°S; only the Principal Cordillera remains south of this latitude. Similarly, deformation progressively advanced to the east, uplifting the Principal Cordillera (20–8 Ma), the Frontal Cordillera (<10 Ma) and the San Rafael basement block (<5 Ma). The amount of shortening systematically decreases from north to south along these two segments, but at the transitional zone between flat and normal subduction zones, there is a sharp decline from ~180 km of shortening (32°S) to ~70 km (33°40′S). South from this latitude, the amount of shortening lineally decreases until it reaches ~30 km at 35°S. Yet, interestingly, the amount of late Miocene surface uplift is opposite to the trend in crustal shortening . These along-strike variations are best explained by boundary conditions of the subduction system related to interplate dynamics controlling the overall pattern of tectonic shortening. However, local variations in mean topographic elevation, deformation styles and crustal root geometry are more likely to be due to upper-plate lithospheric strength variations. These strength variations have governed the coupling degree between brittle upper crust and ductile lower crust deformation. In the flat-slab segment, an initial thick and felsic crust favors the coupling model; while in the normal subduction segment, a thin and mafic lower crust allows the uncoupling model.