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Intramontane sedimentary basins along the margin of continental plateaus often preserve strata that contain fundamental information regarding the pattern of orogenic growth. The sedimentary record of the elastic Miocene-Pliocene sequence deposited in the Fiambala Basin, at the southern margin of the Puna Plateau (NW Argentina), documents the late Miocene paleodrainage evolution from headwaters to the west, towards headwaters in the ranges that constitute the border of the Puna Plateau to the north. Apatite Fission track (AFT) thermochronology of sedimentary and basement rocks show that the southern Puna Plateau was the source for the youngest, middle Miocene, detrital population detected in late Miocene rocks; and that the margin of the Puna Plateau expressed a high relief, possibly similar to or higher than at present, by late Miocene time. Cooling ages obtained from basement rocks at the southern Puna margin suggest that exhumation started in the Oligocene and continued until the middle Miocene. We interpret the basin reorganization and the creation of a high relief plateau margin to be the direct response of the source-basin system to a wholesale surface uplift event that may have occurred during the late Cenozoic in the Puna-Altiplano region. At this time coeval paleodrainage reorganization is observed not only in the Fiambala Basin, but also in different basins along the southern and eastern Puna margin, suggesting a genetic link between the last stage of plateau formation and basin response. However, this event did not cause sufficient exhumation of basin bounding ranges to be recorded by AFT thermochronology. Our new data thus document a decoupling between late Cenozoic surface uplift and exhumation in the southern Puna Plateau. High relief achieved at the Puna margin by late Miocene time is linked to Oligocene-Miocene exhumation; no significant erosion (< 3 km) has occurred since in this and highland.
The arid Puna plateau of the southern Central Andes is characterized by Cenozoic distributed shortening forming intramontane basins that are disconnected from the humid foreland because of the defeat of orogen-traversing channels. Thick Tertiary and Quaternary sedimentary fills in Puna basins have reduced topographic contrasts between the compressional basins and ranges, leading to a typical low-relief plateau morphology. Structurally identical basins that are still externally drained straddle the eastern border of the Puna and document the eastward propagation of orographic barriers and ensuing aridification. One of them, the Angastaco basin, is transitional between the highly compartmentalized Puna highlands and the undeformed Andean foreland. Sandstone petrography, structural and stratigraphic analysis, combined with detrital apatite fission-track thermochronology from a similar to 6200-m-thick Miocene to Pliocene stratigraphic section in the Angastaco basin, document the late Eocene to late Pliocene exhumation history of source regions along the eastern border of the Puna (Eastern Cordillera (EC)) as well as the construction of orographic barriers along the southeastern flank of the Central Andes. Onset of exhumation of a source in the EC in late Eocene time as well as a rapid exhumation of the Sierra de Luracatao (in the EC) at about 20 Ma are recorded in the detrital sediments of the Angastaco basin. Sediment accumulation in the basin began similar to 15 Ma, a time at which the EC had already built sufficient topography to prevent Puna sourced detritus from reaching the basin. After similar to 13 Ma, shortening shifted eastward, exhuming ranges that preserve an apatite fission-track partial annealing zone recording cooling during the late Cretaceous rifting event. Facies changes and fossil content suggest that after 9 Ma, the EC constituted an effective orographic barrier that prevented moisture penetration into the plateau. Between 3.4 and 2.4 Ma, another orographic barrier was uplifted to the east, leading to further aridification and pronounced precipitation gradients along the mountain front. This study emphasizes the important role of tectonics in the evolution of climate in this part of the Andes
[ 1] For the Puna Plateau and Eastern Cordillera of NW Argentina, the temporal and spatial pattern of deformation and surface uplift remain poorly constrained. Analysis of completely and partially reset apatite fission track samples collected from vertical profiles along an ESE trending transect extending from the plateau interior across the southern Eastern Cordillera at similar to 25 degrees S reveals important constraints on the deformation and exhumation history of this part of the Andes. The data constrain the Neogene Andean development of the Eastern Cordillera as well as rift-related exhumation for some of the sampled locations in the Late Jurassic/Early Cretaceous. An intervening Eocene-Oligocene exhumation episode in the southern Eastern Cordillera was probably related to crustal shortening. Subsequent reburial of the area by Andean foreland basin strata commenced between 30 and 25 Myr. Magnitude and duration of sedimentation, revealed by thermal modeling, differ between the sample locations, pointing to an eastward propagating basin system. In the southern Eastern Cordillera, Andean deformation commenced at 22.5 - 21 Myr, predating both the inferred formation of significant topography by 5 - 7.5 Myr and preservation of sediments in the adjacent Cenozoic basins by 6.5 - 8 Myr. Comparing the calculated structural depth of partially reset samples suggests that newly formed west dipping reverse faults along the former Salta Rift margin accommodated most of the Neogene tectonic movement. Late Cenozoic deformation at the southern Eastern Cordillera began earlier in the west and subsequently propagated eastward. The lateral growth of the orogen is coupled with a foreland basin system developing in front of the range and then becomes subsequently compartmentalized by later emergent topography.
The Cenozoic Tian Shan is one of the preeminent examples of an intracontinental orogen. However, there remains a significant controversy over when deformation related to the India-Asia collision commenced and therefore how shortening within the mountains has been partitioned over time. One approach has been to look at the modem shortening rate as measured by geodetic studies, combined with estimates of the total shortening across the range and extrapolate backwards. This approach suggests that the onset of range construction was ca. 10 Ma [K.Y. Abdrakhmatov, S.A. Aldazhanov, B.H. Hager, M.W Hamburger, T.A. Herring, K.B. Kalabaev, K.B. Kalabayev, V.I. Makarov, P. Molnar, S.V Panasyuk, M.T. Prilepin, R.E. Reilinger, I.S. Sadybakasov, B.J. Souter, Y.A. Trapeznikov, V.Y. Tsurkov, A.V. Zubovich, Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates, Nature 384 (6608) (1996) 450-453]. An alternate method is to determine the age of the onset of exhumation using thermochronology. We present 19 new apatite fission-track (AFT) results from the southwestern Chinese portion of the belt; this region represents the first area exhumed during the late Tertiary along a transect at ca. 76 degrees E. Exhumation commenced at the Oligocene-Miocene boundary (similar to 24 Ma) along the Maidan and Muziduke thrusts, which bound the southern side of the Kokshaal range. Subsequently, deformation propagated ca. 20 km south to the Kashi basin- bounding thrust (KBT), which was exhumed by no earlier than 18.9 +/- 3.3 Ma. Three detrital AFT samples from Plio- Pleistocene strata deposited ca. 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20.9 + 7.0/- 5.3 Ma to 15.9 + 5.4/- 4.0 Ma with a fairly constant lag time of 16 to 18 Ma. These ages, combined with structural data, suggest that both the hanging wall and the footwall of the KBT experienced a renewed episode of exhumation during the latest Cenozoic. The discrepancy between the Late Oligocene-Miocene initiation of significant exhumation shown herein and the 10 Ma initiation estimate from geodesy suggests that the Tian Shan has undergone a complex Late Cenozoic shortening history. Assuming that the present shortening rate could account for the total amount of Cenozoic shortening in 10 Ma and realizing that shortening initiated at least 15 Myr earlier, we conclude that the shortening rate must have varied over time, possibly in pulsed-southward migrating events, and that the present rate may not reflect the average rate since initiation of range uplift. (c) 2006 Elsevier B.V. All rights reserved
[1] The Kyrgyz Range, the northernmost portion of the Kyrgyzstan Tien Shan, displays topographic evidence for lateral propagation of surface uplift and exhumation. The highest, most deeply dissected segment lies in the center of the range. To the east, topography and relief decrease, and preserved remnants of a Cretaceous regional erosion surface imply minimal amounts of bedrock exhumation. The timing of exhumation of range segments defines the lateral propagation rate of the range-bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology. New multicompositional apatite fission track ( AFT) data from three transects from the eastern Kyrgyz Range, combined with published AFT data, demonstrate that the range has propagated over 110 km eastward over the last similar to 7 - 11 Myr. On the basis of the thermal and topographic evolutionary history, we present a model for a time-varying exhumation rate driven by rock uplift and changes in erodability and the timescale of geomorphic adjustment to surface uplift. Easily eroded, Cenozoic sedimentary rocks overlying resistant basement control early, rapid exhumation and exhibit slow surface uplift rates. As increasing amounts of resistant basement are exposed, exhumation rates decrease while surface uplift rates are sustained or increase, thereby growing topography. As the range becomes high enough to cause ice accumulation and to develop steep river valleys, fluvial and glacial erosion becomes more powerful, and exhumation rates once again increase. Independently determined range-normal shortening rates also varied over time, suggesting a feedback between erosional efficiency and shortening rate
Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new field mapping, structural data, and cooling ages from the western flank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post-middle Miocene structural development. The western flank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of high-grade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. Ar- 40/Ar-39 mica and apatite fission track (AFT) ages constrain cooling and exhumation pathways front 350 to 60 degrees C and suggest that the dome cooled in three stages since the middle Miocene. Ar-40/Ar-39 white mica ages of 16-14 Ma suggest a first phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased significantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may reflect more pronounced regionally distributed and erosion-driven exhumation