ABSTRACT New geological mapping in midwestern Nepal, complemented by thermochronological and geochronological data sets, provides stratigraphic, structural, and kinematic information for this portion of the Himalayan thrust belt. Lithofacies and geochronologic data substantiate five genetic (tectono)stratigraphic packages: the Lesser Himalayan (ca. 1900–1600 Ma), Greater Himalayan (ca. 800–520 Ma), Tethyan Himalayan (Late Ordovician–Cretaceous), Gondwana (Permian–Paleocene), and Cenozoic Foreland Basin (Eocene–Pleistocene) Sequences. Major structures of midwestern Nepal are similar to those documented along strike in the Himalaya and include a frontal imbricate zone, the Main Boundary and Ramgarh thrusts, the synformal Dadeldhura and Jajarkot klippen of Greater Himalayan rocks, and the hybrid antiformal-stack/hinterland-dipping Lesser Himalayan duplex. Total (probably minimum) shortening between the Main Frontal thrust and the South Tibetan detachment is 400–580 km, increasing westward from the Kaligandaki River region. The Main Central and Ramgarh thrusts were active sequentially during the early to middle Miocene; the Lesser Himalayan duplex developed between ca. 11 Ma and 5 Ma; the Main Boundary thrust became active after ca. 5 Ma and remains active in places; and thrusts that cut the Siwalik Group foreland basin deposits in the frontal imbricate belt have been active since ca. 4–2 Ma. The Main Central “thrust” is a broad shear zone that includes the boundary between Lesser and Greater Himalayan Sequences as defined by their protolith characteristics (especially their ages and lithofacies). The shape of the major footwall frontal ramp beneath the Lesser Himalayan duplex is geometrically complex and has evolved progressively over the past ~10 m.y. This study provides the basis for understanding the Himalayan thrust belt and recent seismic activity in terms of critical taper models of orogenic wedges, and it will help to focus future efforts on better documenting crustal shortening in the northern half of the thrust belt.

Spanning eight kilometers of topographic relief, the Himalayan fold-thrust belt in Nepal has accommodated more than 700 km of Cenozoic convergence between the Indian subcontinent and Asia. Rapid tectonic shortening and erosion in a monsoonal climate have exhumed greenschist to upper amphibolite facies rocks along with unmetamorphosed rocks, including a 5–6-km-thick Cenozoic foreland basin sequence. This Special Paper presents new geochronology, multisystem thermochronology, structural geology, and geological mapping of an approximately 37,000 km 2 region in midwestern and western Nepal. This work informs enduring Himalayan debates, including how and where to map the Main Central thrust, the geometry of the seismically active basal Himalayan detachment, processes of tectonic shortening in the context of postcollisional India-Asia convergence, and long-term geodynamics of the orogenic wedge.

We synthesize geologic observations with new isotopic evidence for the timing and magnitude of uplift for the central Andes between 22°S and 26°S since the Paleocene. To estimate paleoelevations, we used the stable isotopic composition of carbonates and volcanic glass, combined with another paleoelevation indicator for the central Andes: the distribution of evaporites. Paleoelevation reconstruction using clumped isotope paleothermometry failed due to resetting during burial. The Andes at this latitude rose and broadened eastward in three stages during the Cenozoic. The first, in what is broadly termed the “Incaic” orogeny, ended by the late Eocene, when magmatism and deformation had elevated to ≥4 km the bulk (~50%) of what is now the western and central Andes. The second stage witnessed the gradual building of the easternmost Puna and Eastern Cordillera, starting with deformation as early as 38 Ma, to >3 km by no later than 15 Ma. The proximal portions of the Paleogene foreland basin system were incorporated into the orogenic edifice, and basins internal to the orogen were enclosed and isolated from easterly moisture sources, promoting the precipitation of evaporites. In the third orogenic stage during the Pliocene–Pleistocene, Andean deformation accelerated and stepped eastward to form the modern Subandes, accounting for the final ~15%–20% of the current cross section of the Andes. About 0.5 km of elevation was added unevenly to the Western Cordillera and Puna from 10 to 2 Ma by voluminous volcanism. The two largest episodes of uplift and eastward propagation of the orogenic front and of the foreland flexural wave, ca. 50 (?)–40 Ma and <5 Ma, overlap with or immediately postdate periods of very rapid plate convergence, high flux magmatism in the magmatic arc, and crustal thickening. Uplift does not correlate with a hypothesized mantle lithospheric foundering event in the early Oligocene. Development of hyperaridity in the Atacama Desert by the mid-Miocene postdates the two-step elevation gain to >3 km of most (~75%) of the Andes. Hence, the record suggests that hyperarid climate was a consequence, not major cause, of uplift through trench sediment starvation.

The Arizaro Basin in northwestern Argentina sits today in the western Puna Plateau at elevations of 3800–4200 m along the eastern flank of the Miocene to modern magmatic arc. The basin is roughly circular in plan view and ~100 km in diameter, and it was filled during Miocene time (ca. 21–9 Ma) by >3.5 km of eolian, alluvial, fluvial, and lacustrine sediment in addition to ash-fall tuffs from the Andean magmatic arc. The basin fill was subsequently shortened in its central part, and it has been uplifted and topographically inverted. The Arizaro Basin is not obviously related to known faults, nor does it exhibit a peripheral belt of coarse-grained sedimentary rocks derived from flanking topographically higher regions. Sandstone modal framework compositions are arkosic, but not as rich in volcanic lithic fragments as typical intra-arc basins. Detrital zircon U-Pb age spectra implicate source terranes in locally exposed Ordovician granitoid rocks, more distal Upper Paleozoic–Mesozoic arc terranes in western Argentina and possibly northern Chile, and the local Miocene magmatic arc. Depositional-age zircons are present in most of the sandstones analyzed for detrital zircon U-Pb geochronology, and zircon U-Pb ages from volcanic tuff layers provide independent chronological control. The tectonic component of subsidence initiated at low rates, accelerated to ~0.6 mm/yr during the medial stage of basin development, and tapered off to zero as the basin began to shorten internally and experience topographic inversion after ca. 10 Ma. Together, the data presented here suggest that the Arizaro Basin could have developed in response to the formation and gravitational foundering of a dense Rayleigh-Taylor–type instability in the lower crust and/or mantle lithosphere. Insofar as hinterland basins of uncertain tectonic affinity are widespread in the high central Andes, the model developed here may be relevant for other regions of enigmatic subsidence and sediment accumulation in the Andes and other cordilleran hinterland settings.

Abstract Understanding time–temperature histories using apatite fission-track thermochronology involves sample preparation, analysis and then thermal modelling using an appropriate annealing algorithm. A subtle point in this sequence is ascertaining that the sample preparation utilized is compatible with the methodology used in obtaining the data for constructing the annealing data set. This issue is important if one wishes to utilize the relatively new multikinetic annealing algorithm of Ketcham et al. that is implemented in their AFTSolve and HeFTy models which is based on a different etching recipe than those previously used. A preliminary calibration step involves comparing published etch pit diameters for a suite of samples with those analysed by an operator. Results show that the operator can reliably reproduce the calibration data set. We then report a laboratory experiment using samples from Finland and Spain that compares the results obtained using two different etching methodologies (7% nitric acid with qualitative etching conditions and 5.5 M nitric acid at constant conditions). The two raw data sets yield variable results. Comparing the two etching methodologies reveals the influence of this procedure on the kinetic parameter D par .

The Tertiary Piedmont Basin is a synorogenic basin located on the internal side of the Western Alps. Because of its key position, the Tertiary Piedmont Basin represents an important record of processes occurring in the Alpine retrowedge for over the last 30 m.y. 40 Ar/ 39 Ar geochronology has been applied to detrital white micas as a provenance tool and to derive information on cooling and exhumation patterns within the surrounding orogen. The age distribution in the detritus shows that in the Oligocene the clastic sediments were fed mainly from a southern source area (Ligurian Alps) that widely records high pressure (HP) Alpine metamorphism (40–50 Ma) and, in part, Variscan metamorphism (ca. 320 Ma). From the Miocene, the main source area gradually moved from the south to a western Alpine provenance characterized by strong Late Cretaceous (70 Ma) and Early Cretaceous (120 Ma) signals. This enlargement in the source is likely linked to an evolution of the main paleodrainage system into the basin. From the Serravallian, Variscan ages reappear; this is attributed to the exposure of the Argentera Massif as a new source for the Tertiary Piedmont Basin. The lack of thermal overprinting of the main detrital signals through time suggests that the western Alpine orogen has been regulated by episodic fast cooling and exhumation events followed by periods of slower erosion. Also, detrital 40 Ar/ 39 Ar Cretaceous signals in Miocene and Present sediments suggest the presence of real Eoalpine events in the Alps.

Abstract The Oligocene to Miocene Tertiary Piedmont Basin (TPB) is located in the NW part of Italy at the junction between the Apennine and the Alpine thrust belts. The position of the TPB on top of the Alpine/Apennine Orogen poses fundamental questions as to the tectonics of the basin subsidence. Having undergone little deformation, the TPB sediments provide an insight into the stress regime and rotations in the kinematically very complex area surrounding the basin itself. In this study we integrate subsidence and structural analysis with measurements of magnetic susceptibility anisotropy (AMS) and natural remanent magnetization (NRM) in order to better constrain the tectonic kinematics of the basin evolution. A major important period of subsidence occurred in the Middle Miocene involving the whole basin. During this period the TPB experienced NE-SW-directed compression and limited shortening. Some NW-SE-directed compressional features have been identified and they were probably active during post Tortonian times. Structures associated with north-south tension are quite common, but the amount of strain that they accommodate is minor. In addition this research provides new preliminary data suggesting counterclockwise rotation in the TPB by c. 20° which has taken place during Middle Miocene time.