Constraining Deformation in the North Pamir and the Westernmost Tarim Basin

Abstract HKT-ISTP 2013 A

Regional deformation of the Southern Puna Plateau, Central Andes, recorded by basin evolution and bedrock exhumation history

The regional deformation history of the southern Puna Plateau remains poorly constrained but is key to understanding the growth and dynamics of the central Andes, an important example of orogeny along a non-collisional plate boundary. Several lines of evidence lead us to propose that the southern Puna Plateau was occupied by an uninterrupted sedimentary basin during the late Eocene to early Oligocene (~38-28 Ma). First, oldest strata in the Antofagasta de la Sierra region (SW Puna) and the Pasto Ventura region (SE Puna) both contain little volcanic material, suggesting they predate the most recent arc activity at ~28 Ma. Second, detrital zircons from the Antofagasta de la Sierra region yield youngest U-Pb ages of ~39 Ma and detrital apatites from the Pasto Ventura region yield youngest fission-track ages of ~38 Ma, giving a maximum depositional age. Third, provenance analysis reveals a single, western source for the Antofagasta de la Sierra region (SW Puna) and dual, eastern and western sources for the Pasto Ventura region (SE Puna), supporting the presence of a regional basin. This regional basin was disrupted and compartmentalized by uplift of ~N-S trending bedrock ranges starting as early as in late Oligocene time. Bedrock samples from the eastern foot of the Sierra de Calalaste (SW Puna) yield an apatite (U-Th-Sm)/He age of 19.92.0 Ma, consistent with modeled apatite fission-track data that show onset of exhumation at ~25-20 Ma. Modeling of apatite fission-track and (U-Th-Sm)/He data shows that the Sierra Laguna Blanca (SE Puna) experienced exhumation at ~15-10 Ma, the youngest bedrock exhumation documented in the plateau region, implying that deformation and erosion of basement-bounding structures continued into the middle Miocene. We suggest that the post-late Eocene regional deformation history of the southern Puna Plateau documents an important dynamic shift from flexure-controlled foreland dynamics to flexure-limited broken foreland dynamics during the Oligocene

Slow NE-SW to NNE-SSW extension in the Pasto Ventura region of the southern Puna Plateau

Recent extension on the Puna Plateau of NW Argentina has been linked to lithospheric foundering, gravitational spreading, and edge effects. However, the timing, kinematics and rate of extension are poorly constrained. In the Pasto Ventura region, along the southern margin of the plateau, we map out two different groups of faults: (1) recently formed Quaternary normal faults and strike-slip faults; and (2) pre-Quaternary reverse faults reactivated in the Quaternary. The faults in Group (1) are relatively short (~1-2 km) normal and strike-slip faults that offset Quaternary geomorphic features. The orientation of these faults indicates NE-SW to NNE-SSW extension. The faults in Group (2) bound exposures of basement rock and are associated with basaltic cinder cones and lava flows. Previous studies indicate they were reverse faults which have been reactivated as normal faults. We applied kinematic GPS surveying and 40Ar/39Ar dating of three cinder cones displaced by two of Group-(2) faults. Kinematic analysis on vertical and horizontal offsets obtained by GPS survey shows that the one fault is now undergoing NE-SW to NNE-SSW extension, consistent with Group (1) fault kinematics. A cinder cone has been displaced 34-40 meters horizontally along this fault, yielding a slow extension rate of 0.02-0.04 mm/yr since 0.8-0.5 Ma. The shift from contraction to extension in the Pasto Ventura region is estimated to be between 7.8 and 0.5 Ma, but more likely between 7.8 and 4 Ma. A regional compilation of kinematics on the southern plateau from this study and existing data, although sparse, shows two spatial groups: the extension directions are N-S to NE-SW south of 26°S latitude, while they are NW-SE to NNW-SSW north of 26°S latitude. Mafic volcanism, thought to indicate the timing of the onset of extension in the Puna, shows a similar pattern, with the oldest ages (up to 7.3 Ma) clustered near 26°S latitude, becoming younger to both the north and the south. Kinematic and geochronologic data from the Pasto Ventura region are consistent with this trend. The pattern of ages of mafic volcanism and the fault kinematics imply that the removal of the lower lithosphere beneath the Puna Plateau occurred through the formation of a Rayleigh-Taylor type instability, or “driplet,” located around 26°S at about 7.3 Ma. This driplet is probably relatively small since the extension rate observed on the surface is very slow. However, the pattern of extension directions indicates that the “driplet” located around 26°S was probably not perfectly cylindrical and/or the surficial extension pattern is also affected by other drivers, such as gravitational collapse, back-arc extension or other “driplets” located in the other regions

Cooling history for the Sierra Laguna Blanca (NW Argentina) on the Southern Puna Plateau, Central Andes

Various dynamic models have been proposed to explain deformation history and topographic evolution for the southern Altiplano-Puna Plateau, including inversion of the Cretaceous Salta rift structures, formation of an orogenic wedge, flat subduction, climate-tectonic coupling, and lithospheric foundering. Controversies persist in the southern Puna Plateau, where preexisting rift structures are unknown and Cenozoic shortening events are sparsely documented. The 6-km high Sierra Laguna Blanca (LB) (NW Argentina) is among the most outstanding topographic features in the interior of the southern Puna Plateau. We document cooling history for LB with apatite (U-Th)/He, apatite fission-track and zircon (U-Th)/He thermochronometers for a vertical profile from 3.6-5.6 km on its eastern flank. Preliminary results from apatite fission-track (AFT) analysis yield ages ranging from 45-65 Ma, with top samples being the oldest. Dpar values for all samples are low (1.54 to 1.74), suggesting a relatively low-temperature partial annealing zone. All samples have shortened mean track lengths ranging from 10.9 to 12.3 micrometers, suggesting partial resetting. Preliminary apatite U-Th/He (AHe) ages are compatible with AFT ages but are widely dispersed, perhaps due to U zoning and small U-rich inclusions which have been observed on AFT external detectors. Inverse modeling of AFT data and selected AHe data using the HeFTy program reveal two major cooling events for LB. All models start ~90-70 Ma and immediately decrease their temperatures to ~60°C before ~50 Ma. Samples may have stayed ~60°C without additional thermal events until ~15-10 Ma, when the most recent cooling event took place, bringing all samples to surface temperature. Our first finding is that the interior of the southern Puna Plateau may have been influenced by the Salta Rift during the Cretaceous, extending the known zone of influence further west. Second, the most recent cooling phase (mid-late Miocene) is consistent with out-of-sequence deformation in the southern Puna Plateau, which might be genetically linked to a proposed lithospheric dripping event

Non-monotonic cooling history in the southern Central Andes recorded by multisystem low-temperature thermochronology

Three low-temperature thermochronometers are used to study the temperature and exhumation history of the Sierra Laguna Blanca, a major basement range that rises over 6 km in the backarc region of the Central Andes. Five samples analyzed with zircon (U-Th-Sm)/He thermochronology yield early Carboniferous to early Triassic dates. Ten apatite fission-track samples provide ~50 to 70 Ma ages with shortened, ~11-12 µm track lengths. Ten apatite (UTh-Sm)/He samples yield highly dispersed dates ranging from ~30 Ma to 120 Ma, with eU values ranging from ~50 to 500 ppm. Time-temperature inverse and forward models reveal three major cooling events at the late Paleozoic, late Cretaceous and mid-late Miocene, and a heating event during late Eocene to early Oligocene. This study demonstrates that accumulated radiation damage, in this case caused by very high eU instead of a prolonged low-temperature history, may lead to significant apatite (U-Th-Sm)/He date dispersion

Late Cenozoic exhumation history of the Gaoligong and Chongshan shear zones (SE Tibet)

The Gaoligong, Chongshan, and Ailaoshan-Red River shear zones (GLSZ, CSSZ, and ALRRSZ) are interpreted as the boundary structures, accommodating Cenozoic block or sub-block extrusion and rotation in the southeastern Tibetan Plateau. Their orogenic processes and landscape evolution have significant implications for understanding the geodynamics of intracontinental deformation. In this study, we present new low-temperature thermochronologic data from the GLSZ and CSSZ, and quantitative analyses of fluvial longitudinal profiles along the Salween valley which lies between the shear zones in order to examine their exhumation histories. The results reveal that the GLSZ and CSSZ experienced two phases of exhumation in the late Cenozoic. The first episode of rapid cooling commenced in the early Miocene caused by transpressional movements along the shear zones. The nearly contemporaneous shearing between the GLSZ and CCSZ continued until ~11 Ma. The northward migration of the tectonic events along the Mogok metamorphic belt and GLSZ, and synchronous dextral displacement along the Jiali fault indicate the dominant role of the north advancing eastern Himalayan syntaxis on the surrounding structures. The second phase of rapid exhumation, albeit of lower magnitude, occurred in the late Miocene-early Pliocene. During this interval, the tributaries along the GLSZ and CSSZ may have formed lower steep channels, compared to the upper relatively low-gradient segments. The tributary transient response could result from temporal changes in uplift or adjustments of the trunk channel to climatic change. Different drivers for the two exhumation events may reflect distinct stages of plateau growth characterized by different crustal deformation patterns

Erosion rate study at the Allchar deposit (Macedonia) based on radioactive and stable cosmogenic nuclides (26Al, 36Cl, 3He, and 21Ne)

This paper focuses on constraining the erosion rate in the area of the Allchar Sb-As-Tl-Au deposit (Macedonia). It contains the largest known reserves of lorandite (TlAsS2), which is essential for the LORanditeEXperiment (LOREX), aimed at determining the long-term solar neutrino flux. Because the erosion history of the Allchar area is crucial for the success of LOREX, we applied terrestrial in situ cosmogenic nuclides including both radioactive (26Al and 36Cl) and stable (3He and 21Ne) nuclides in quartz, dolomite/calcite, sanidine, and diopside. The obtained results suggest that there is accordance in the values obtained by applying 26Al, 36Cl, and 21Ne for around 85% of the entire sample collection, with resulting erosion rates varying from several tens of m/Ma to ∼165 m/Ma. The samples from four locations (L-8 CD, L1b/R, L1c/R, and L-4/ADR) give erosion rates between 300 and 400 m/Ma. Although these localities reveal remarkably higher values, which may be explained by burial events that occurred in part of Allchar, the erosion rate estimates mostly in the range between 50 and 100 m/Ma. This range further enables us to estimate the vertical erosion rate values for the two main ore bodies Crven Dol and Centralni Deo. We also estimate that the lower and upper limits of average paleo-depths for the ore body Centralni Deo from 4.3 Ma to the present are 250–290 and 750–790 m, respectively, whereas the upper limit of paleo-depth for the ore body Crven Dol over the same geological age is 860 m. The estimated paleo-depth values allow estimating the relative contributions of 205Pb derived from pp-neutrino and fast cosmic-ray muons, respectively, which is an important prerequisite for the LOREX experiment