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1. Kinematic Evolution of the Tangra Yumco Rift, South-Central Tibet

   https://doi.org/10.55575/tektonika2024.2.1.42  

   Reynolds, Aislin; Laskowski, Andrew; Howlett, Caden; Orme, Devon; Sundell, Kurt; Taylor, Michael; Forte, Adam; Cai, Fulong; Guo, Xudong; Ding, Lin (July 2024, Tektonika)

   We investigate rifting during continental collision in southern Tibet by testing kinematic models for two classes of rifts: Tibetan rifts are defined as >150 km in length and crosscut the Lhasa Terrane, and Gangdese rifts are <150 km long and isolated within the high topography of the Gangdese Range. Discerning rift kinematics is a crucial step towards understanding rift behavior and evolution that has been historically limited. We evaluate spatiotemporal trends in fault displacement and extension onset in the Tangra Yumco (TYC) rift and several nearby Gangdese rifts and examine how contraction and rift exhumation relate to evolution of the Gangdese drainage divide. Igneous U-Pb and zircon (U-Th)/He (ZHe) results indicate rift footwall crystallization between ~59-49 Ma and cooling between ~60-4 Ma, respectively, with ZHe ages correlating with sample latitude. Samples from Gangdese latitudes (~29.4-29.8°N) yield predominantly Oligocene-early Miocene ages, whereas samples north of ~29.8°N yield both late Miocene-Pliocene ages and Paleocene-Eocene ages. Thermal history models indicate two-stage cooling, with initially slow cooling followed by accelerated cooling during late Miocene-Pliocene time. From spatial distributions of ZHe ages we interpret: (1) ~28-16 Ma ages from Gangdese latitudes reflect exhumation along contractional structures, (2) ~8-4 Ma ages reflect rift-related exhumation, and (3) ~60-48 Ma ages indicate these samples experienced lesser rift exhumation. Our data are consistent with a segment linkage evolution model for the TYC rift, with interactions between rifts and contractional structures likely influencing the evolution of topography and location of the Gangdese drainage divide since Miocene time 

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2. Record of Crustal Thickening and Synconvergent Extension from the Dajiamang Tso Rift, Southern Tibet

   https://doi.org/10.3390/geosciences11050209  

   Burke, William; Laskowski, Andrew; Orme, Devon; Sundell, Kurt; Taylor, Michael; Guo, Xudong; Ding, Lin (May 2021, Geosciences)

   Carosi, Rodolfo; da Costa Campos Neto, Mario; Fossen, Hakkon; Montomoli, Chiara; Simonetti, Matteo; Martinez-Frias, Jesus (Ed.)

   North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension. 

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3. Dynamics of Continental Rifting in Southern Tibet: Interpretations from Spatiotemporal Trends in Thermochronology Data

   Reynolds, Aislin R; Laskowski, Andrew K (December 2023, AGU Fall Meeting Abstracts)

   Evaluating spatial and temporal patterns of rifting is an essential step towards disentangling the complex tectonic evolution of southern Tibet from Oligocene to Miocene time. Here we examine spatial trends in thermochronology data for two classes of rifts: Tibetan rifts that are generally >150 km in length and crosscut the Lhasa Terrane, and Gangdese rifts that are typically <50 km long and isolated within the high topography of the Gangdese Range. Analysis of compiled ZHe data from two Tibetan rifts and three Gangdese rifts suggests initiation along Tibetan rifts occurred between ~19-14 Ma, consistent with previous studies that interpret a northward sweep of extension onset related to northward underthrusting of the Indian plate. Conversely, results indicate Gangdese rift initiation at around ~28 Ma, prior to the recent episode of India underthrusting beginning at ~20 Ma. We suggest Gangdese rift initiation was driven by exhumation and uplift of the Gangdese Range, with ZHe ages overlapping timing estimates for contraction along the Great Counter Thrust from ~28-16 Ma. These results suggest the interactions and feedbacks between contractional and extensional structures in southern Tibet are more complex than previously recognized. 

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4. Assessing drivers of high exhumation magnitudes and young cooling ages in the eastern central Andes, southern Peru (13–18°S)

   https://doi.org/10.1016/j.epsl.2023.118281  

   Glover, Chloë O; McQuarrie, Nadine; Falkowski, Sarah; Ehlers, Todd A (October 2023, Earth and Planetary Science Letters)

   The potential structural controls on exhumation across the southern Peruvian Andes are not well understood, in part due to limited structural studies that co-locate with thermochronometric datasets. We integrate these two datasets and evaluate the relative contribution that fault geometry, magnitude, and shortening rate have on predicted cooling ages. Here we present a balanced cross-section constructed using new structural observations. This section, combined with existing thermochronometer data and a thermokinematic model, investigates the drivers of high exhumation and young canyon thermochronometric ages along the deeply incised Marcapata canyon in southern Peru. Together, these approaches constrain the timing and magnitude of exhumation in this portion of the southern Peruvian Andes and provide a mechanism for documenting how the internal architecture changes along strike. The balanced cross-section (oriented N30E) covers the Subandean Zone to the northeast, the Marcapata canyon on the eastern flank of the southern Peruvian Andes, and the Altiplano-Eastern Cordillera boundary to the southwest (13–18◦ S). Exhumation is constrained by four low-temperature thermochronometer systems, including apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and fission-track (AFT and ZFT, respectively). The youngest AHe (∼1–3 Ma), AFT (∼3–7 Ma), ZHe (∼4–7 Ma), and ZFT (∼14–17 Ma) ages are located in the center and valley bottom of the Marcapata canyon. The thermokinematically modeled cross-section produces cooling ages determined by fault geometry and kinematics. Reset ZFT ages require burial of Ordovician rocks in excess of 5.5 km above the original 6.5 km depositional depth. We find that the ZFT and ZHe ages in the Eastern Cordillera are sensitive to the history and magnitude of burial, age and location of uplift, and canyon incision. Canyon incision is required to reproduce the youngest canyon thermochronometric ages while slow shortening rates from ∼10 Ma to Present are required to reproduce interfluve thermochronometric ages. Shortening is accommodated by basement faults that feed slip up through three different décollement levels before reaching the surface. The proposed stacked basement geometry sets the first-order cooling signal seen in modeled ages. We determined that the total shortening in this section from the Subandean Zone to the Altiplano is 147.5 km, similar to shortening estimates in an adjacent thermo-kinematically modeled section in the San Gabán canyon 50 km to the southeast. Both the ZHe and ZFT ages in the Marcapata section (4–5 and 14 Ma) are noticeably younger than cooling ages from the San Gabán section (16 and 29 Ma). The Marcapata section’s higher magnitude of exhumation is due to a repetition of basement thrusts that continues to elevate the Eastern Cordillera while active deformation occurs in the Subandean Zone. The youngest thermochronometric ages in all four systems are co-located with the overlapping basement thrust geometry. This basement geometry, kinematic sequence of deformation, and canyon incision co-conspire to produce the young cooling ages observed in the Eastern Cordillera. 

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5. Uplift and exhumation of the Russell Fiord and Boundary blocks along the northern Fairweather transform fault, Alaska

   Schartman, A.; Enkelmann, E.; Garver, J.I.; Davidson, C.M. (January 2019, Lithosphere)

   Cooling ages of tectonic blocks between the Yakutat microplate and the Fairweather transform boundary fault reveal exhumation due to strike-slip faulting and subsequent collision into this tectonic corner. The Yakutat and Boundary faults are splay faults that define tectonic panels with bounding faults that have evidence of both reverse and strike-slip motion, and they are parallel to the northern end of the Fairweather fault. Uplift and exhumation simultaneous with strike-slip motion have been significant since the late Miocene. The blocks are part of an actively deforming tectonic corner, as indicated by the ~14–1.5 m of coseismic uplift from the M 8.1 Yakutat Bay earthquake of 1899 and 4 m of strike-slip motion in the M 7.9 Lituya Bay earthquake in 1958 along the Fairweather fault. New apatite (U-Th-Sm)/He (AHe) and zircon (U-Th)/He (ZHe) data reveal that the Boundary block and the Russell Fiord block have different cooling histories since the Miocene, and thus the Boundary fault that separates them is an important tectonic boundary. Upper Cretaceous to Paleocene flysch of the Russell Fiord block experienced a thermal event at 50 Ma, then a relatively long period of burial until the late Miocene when initial exhumation resulted in ZHe ages between 7 and 3 Ma, and then very rapid exhumation in the last 1–1.5 m.y. Exhumation of the Russell Fiord block was accommodated by reverse faulting along the Yakutat fault and the newly proposed Calahonda fault, which is parallel to the Yakutat fault. The Eocene schist of Nunatak Fiord and 54–53 Ma Mount Stamy and Mount Draper granites in the Boundary block have AHe and ZHe cooling ages that indicate distinct and very rapid cooling between ca. 5 Ma and ca. 2 Ma. Rocks of the Chugach Metamorphic Complex to the northeast of the Fairweather fault and in the fault zone were brought up from 10–12 km at extremely high rates (>5 km/m.y.) since ca. 3 Ma, which implies a significant component of dip-slip motion along the Fairweather fault. The adjacent rocks of the Boundary block were exhumed with similar rates and from similar depths during the early Pliocene, when they may have been located 220–250 km farther south near Baranof Island. The profound and significant exhumation of the three tectonic blocks in the last 5 m.y. has probably been driven by uplift and erosional exhumation due to contraction as rocks collide into this tectonic corner. The documented spatial and temporal pattern of exhumation is in agreement with the southward shift of focused exhumation at the St. Elias syntaxial corner and the southeast propagation of the fold-and thrust belt. 

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