Isabelle Coutand

Style and history of Andean deformation, Puna plateau, northwestern Argentina

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE-SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line-balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW-ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE-SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip-slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.

Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0‐1.8 mm/yr) than inside of it in the east (0.55‐0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region’s recent tectonic evolution.

Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes

Orogenic plateaus are extensive, high-elevation areas with low internal relief that have been attributed to deep-seated and/or climate-driven surface processes. In the latter case, models predict that lateral plateau growth results from increasing aridity along the margins as range uplift shields the orogen interior from precipitation. We analyze the spatiotemporal progression of basin isolation and fi lling at the eastern margin of the Puna Plateau of the Argentine Andes to determine if the topography predicted by such models is observed. We fithat the timing of basin fi lling and reexcavation is variable, suggesting nonsystematic plateau growth. Instead, the Airy isostatically compensated component of topography constitutes the majority of the mean elevation gain between the foreland and the plateau. This indicates that deep-seated phenomena, such as changes in crustal thickness and/or lateral density, are required to produce high plateau elevations. In contrast, the frequency of the uncompensated topography within the plateau and in the adjacent foreland that is interrupted by ranges appears similar, although the amplitude of this topographic component increases east of the plateau. Combined with sedimentologic observations, we infer that the low internal relief of the plateau likely results from increased aridity and sediment storage within the plateau and along its eastern margin.

Fragmentation of a foreland basin in response to out-of-sequence basement uplifts and structural reactivation; El Cajon-Campo del Arenal Basin, NW Argentina

The style and mechanisms by which a foreland region is incorporated into an orogen depends on the tectonic style, effectiveness of uplift, and dynamic subsidence. Classical foreland-basin models reflect a self-similar propagation of deformation into the foreland in a thin-skinned thrust-belt setting governed by wedge mechanics. Thick-skinned foreland regions, which are characterized by high-angle reverse-fault–bounded basement uplifts and intervening basins, however, do not fit this idealized model. Unlike thin-skinned tectonic provinces, deformation and uplift in these regions may be highly variable in time and space. Furthermore, deformation patterns may be complicated by the presence of preexisting structures, particularly those which lie at orientations that enable them to be reactivated and utilized to accommodated deformation under compression. The Neogene El Cajon–Campo del Are-nal basin is one of a series of basins located along the eastern margin of the Puna Plateau within the Sierras Pampeanas, a region that is composed of a thick-skinned foreland fragmented by reverse-fault–bounded basement uplifts that regionally characterize an eastward-younging trend. This region is superimposed onto the Cretaceous Salta Rift province, which provides a series of pre-existing structures that may potentially be reactivated. The basin is located along the eastern margin of the Puna Plateau, an integral component of the Andean orogen, which includes several filled, uplifted, and internally drained Cenozoic intraplateau basins. Structural and sedimentological similarities exist between basins along the margin of the Puna and those within it. Understanding the evolution of foreland basins, such as the El Cajon–Campo del Arenal basin, provides possible mechanisms for the development and incorporation of marginal basins into orogenic belts, and in the case of the Andean orogen, the potential for these basins to be incorporated into the plateau. Our analysis, which integrates seismic, sedimentary, and thermochronological data, characterizes the evolution of this basin and surrounding ranges. The appearance in the sedimentary section of a distinct grain-age population derived from the basement erosion surface constrains the uplift and erosion of an out-of-sequence intrabasin high to ca. 6 Ma. The basin fill, therefore, records an evolution from an undeformed foreland to one that is compartmentalized by basement uplifts and that is incorporated into the greater orogenic structure. The data reveal the importance of the reactivation of preexisting structures along the basin margin in creating east-dipping structures in a generally west-dipping domain. These opposing faults on the basin margin consequently caused the out-of-sequence uplift of the intrabasin range, the Sierra de Quilmes. The Sierra de Quilmes fragments the foreland and, because its position is locked by loads to the west and east, creates increased deformation within the basin, basin fill, uplift, and incorporation into the orogen. Unlike basins within the plateau, however, the El Cajon–Campo del Arenal basin has been re-excavated and integrated once more into the foreland drainage network.

Geometry and kinematics of the Main Himalayan Thrust and Neogene crustal exhumation in the Bhutanese Himalaya derived from inversion of multithermochronologic data

Both climatic and tectonic processes affect bedrock erosion and exhumation in convergent orogens, but determining their respective influence is difficult. A requisite first step is to quantify long-term (~106 year) erosion rates within an orogen. In the Himalaya, past studies suggest long-term erosion rates varied in space and time along the range front, resulting in numerous tectonic models to explain the observed erosion rate distribution. Here, we invert a large data set of new and existing thermochronological ages to determine both long-term exhumation rates and the kinematics of Neogene tectonic activity in the eastern Himalaya in Bhutan. New data include 31 apatite and five zircon (U-Th)/He ages, and 49 apatite and 16 zircon fission-track ages along two north-south oriented transects across the orogen in western and eastern Bhutan. Data inversion was performed using a modified version of the 3-D thermokinematic model Pecube, with parameter ranges defined by available geochronologic, metamorphic, structural, and geophysical data. Among several important observations, our three main conclusions are as follows: (1) Thermochronologic ages do not spatially correlate with surface traces of major fault zones but appear to reflect the geometry of the underlying Main Himalayan Thrust; (2) our data are compatible with a strong tectonic influence, involving a variably dipping Main Himalayan Thrust geometry and steady state topography; and (3) erosion rates have remained constant in western Bhutan over the last ~10 Ma, while a significant decrease occurred at ~6 Ma in eastern Bhutan, which we partially attribute to convergence partitioning into uplift of the Shillong Plateau.

Differential structural and geomorphic mountain-front evolution in an active continental collision zone: The northwest Pamir, southern Kyrgyzstan

Western, central, and eastern segments of the Trans Alai mountain front in the northern Pamir of Kyrgyzstan have accommodated varying degrees of approachment of the Pamir orogen with respect to the Tien Shan (Shan = Mountains) to the north. Ongoing collision between the north-western corner of the Indian indenter and Eurasia has resulted in closure of the intramontane Alai Valley, which separates the Tien Shan and Trans Alai (Pamir) ranges. The different segments highlight the processes of shaping tectonically active mountain fronts in a semiarid environment. In this study, we have characterized this variation in processes with compilations of regional tectonic information, detailed geologic and geomorphic maps, topographic analyses, and interpretation of seismic reflection data. Along the sinuous western segment of the mountain front, dextrally oblique thrusting has created a wide (>500 m) zone of highly erodible fault gouge. This fault zone impinges on the southern Tien Shan, but complete basin closure is prevented by erosion due to the westward-flowing Kyzilsu River; the Kyzilsu valley forms the only outlet and is the vestige of a formerly contiguous sedimentary basin linking the Tarim Basin of China with the Tadjik Depression in the west. Numerous large landslides rooted in the fault zone have covered the active fault, which is partially undercut by the Kyzilsu River. Older, large landslides in this setting are associated with different levels of fluvial terraces of the former or present course of the Kyzilsu River, suggesting a causative relationship between lateral fluvial scouring, failure of mechanically weak mountain fronts, ongoing faulting, and mass transfer. Along the linear central segment, deformation is confined to a narrow single south-dipping thrust fault that juxtaposes Pliocene-Pleistocene and Holocene conglomerates. In this sector, the mountain front has numerous Holocene offsets. This prevailing structural style and the long-term deformation are underscored by multiple flights of gently sloping pediments and glaciogenic terrace surfaces that abruptly terminate at the steep mountain front, which also forms the boundary with the wide regraded piedmont. In contrast, closure between the Pamir and Tien Shan is complete along the eastern segment. The eroded and sinuous mountain front has been tectonically inactive during late Quaternary time. Small drainage-basin areas and low stream power apparently were not conducive to maintaining an eastern outlet to the Tarim Basin. Active deformation has stepped back into the orogen and now is concentrated along the Markansu Fault and within the Tien Shan to the north. The large drainage-basin area of the Kyzilsu River and the constant, glacially fed runoff guarantee that an effective interplay between tectonic uplift and erosion is maintained. Therefore, the geomorphically different mountain-front segments highlight the relationships between tectonic uplift and geomorphic processes, which in turn are controlled by lithology, topography, and the history of sediment routing throughout the landscape.

Vertical axis rotations across the Puna plateau (northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks

Between 10°S and 30°S, the central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward concave geometry whose origin remains controversial. The arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and northern Bolivia) and clockwise rotations on the southern limb (throughout southern Bolivia, northwestern Argentina, and northern Chile). To fill a gap in data from northern Argentina and to contribute to the ongoing debate on the origin of rotations in the central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in northwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central Andes. However, significant variations in the amount of rotation occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the central Andes remains to be demontrated. Even if such faulting has locally influenced rotations, Cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central Andes.

What controls the growth of the Himalayan foreland fold-and-thrust belt?

We provide empirical evidence for the impact of surface processes on the structure of the present-day foreland fold-and-thrust belt of the Himalaya. We have reconstructed and analyzed ten balanced cross sections distributed along the entire length of the Himalayan arc. Here, we focus on the Siwalik Group, which represents the deformed part of the foreland basin and consists of synorogenic Middle Miocene to Pleistocene sediments that form the youngest and frontal part of the Himalayan orogen. We make two important observations: (1) a distinct west-to-east increase in strain and strain rate correlates with plate convergence rates, and (2) belt morphology is inversely correlated with rainfall amount. According to the predictions of the critical taper model, an eastward increase in convergence rate would induce higher rates of material accretion. Thus, the Himalayan fold-and-thrust belt should widen eastward, yet we have observed the opposite. However, higher annual rainfall amounts and specific stream power appear to favor a narrower belt. Thus, we suggest that the morphology of the Himalayan foreland fold-and-thrust belt is controlled primarily by surface processes, in accordance with the critical taper model.

Tectonics, Climate, and Landscape Evolution of the Southern Central Andes: the Argentine Puna Plateau and Adjacent Regions between 22 and 30°S

The history of the Puna Plateau and its marginal basins and ranges in the Eastern Cordillera and the northern Sierras Pampeanas structural provinces in northwestern Argentina impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the successive evolution of orographic barriers, as well as their influence on erosion and sedimentation processes. In addition, this region exemplifies that there are several pathways by which tectonic activity may be coupled to the effects of climate and erosion. Apatite fission track and sedimentologic data indicate that distributed, diachronous uplift of ranges within the present Puna Plateau of NW Argentina began as early as Oligocene time, compartmentalizing a foreland region similar to tectonically active sectors along the current eastern plateau margins. However, fission track data from detrital apatite in sedimentary basins and vertical profiles along the eastern plateau margin document that wholesale plateau uplift probably affected this region in mid to late Miocene time, which may have been associated with mantle delamination. This coincided with the establishment of humid conditions along the eastern Puna margin and a sustained arid to hyper-arid climate within the plateau region. A common feature of the Puna Plateau is that its location corresponds to hyper-arid areas of the landscape in which channels fail to incise deeply into basin sediments or surrounding basement ranges. Importantly, the local base-level is hydrologically isolated from the foreland. This isolation occurs where the incising power of regional drainage systems has been greatly reduced due to a combination of diminished precipitation related to regional climate and local orography, and exposure of resistant bedrock. Hydrologic isolation of the plateau from the foreland permits deposition within basins as material is eroded from the surrounding ranges, reducing the relief between basins and adjacent peaks. While a variety of deformation styles and possibly combinations of different processes may have generated the high elevations observed in the Puna Plateau, the observed low-relief morphology requires evacuation of material via regional fluvial systems to be restricted. Therefore, the low-relief character of the orogenic plateau may be a geomorphic, rather than a tectonic phenomenon. At the eastern plateau margins similar basin histories can be observed in fault-bounded intramontane depressions that straddle the eastern Puna border. However, these basins remain only transiently isolated and internally drained due to their proximity to the high precipitation gradients which were established by orographic barriers in the course of Pliocene uplift. These outlying barriers focus precipitation, erosion, promote headward erosion, stream capture, and ultimately basin exhumation. This conspiring set of processes thus prevents these areas to become incorporated into the plateau realm, while the interior of the orogen conserves mass and may influence deformation patterns in the foreland due to high lithostatic stresses. Sustained aridity in the core of the orogen may thus be responsible for the creation, maintenance and potential for future lateral growth of the plateau, thus emphasizing the coupling between tectonics, climate and erosion.

Late Miocene-Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong Plateau: New evidences from foreland basin deposits along the Dungsam Chu section, eastern Bhutan

The Shillong Plateau is a unique basement-cored uplift in the foreland of the eastern Himalaya that accommodates part of the India-Eurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1600 m, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern Himalaya-Shillong Plateau-ISM is suited to investigate effects of tectonics, climate, and erosion in a mountain range-broken foreland system. This study focuses on a 2200 m thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong Plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1 Ma in a marginal marine deltaic transitioning into continental environment after 5 Ma, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong Plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills, and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest Miocene-Pleistocene, in response to increasing partitioning of the India-Eurasia convergence into the active faults bounding the Shillong Plateau.