| dc.description.abstracteng | The major aim of this PhD thesis is to explore the post-Early Mesozoic thermo-tectonic evolution and provenance analysis of the area of satellite basins to the east of the Songliao basin in NE China, covering a huge area, over 200,000 km2. Comparing with the Songliao basin, the thermo-tectonic evolution of the basins and basement highs separating them is still less understood. Therefore, an integrated evaluation of the thermal history of the basement highs and the basin remnants was firstly performed using low-T thermochronology and burial/thermal modelling based on vitrinite reflectance data. The studied Mesozoic sedimentary formations and the basement are penetrated and partly covered by Cenozoic mafic volcanic rocks. As supplementary research to the regional thermal evolution study, a case study was performed on the thermally influenced substrate of a basalt lava flow. Raman spectroscopy and zircon (U-Th)/He thermochronology were applied to detect the thermal effect of the lava flow and determine the eruption age. Detrital zircon U-Pb age distributions from modern sands provide useful insights to detect, verify or re-classify the ages of the zircon-bearing units in the catchments. Moreover, combining the modern age data with a regional compilation of ages from the basement units and some Mesozoic sedimentary formations allows for refining the Cretaceous provenance history of the region. Finally, the inferred provenance evolution is checked against the thermo-tectonic evolution. Sand samples from five modern rivers whose catchments drain most of the currently elevated basement blocks of eastern NE China were investigated with the detrital zircon U-Pb geochronology method. The Cretaceous supply's temporal change is well discussed by carefully considering multiple influencing factors to the modern sediments' provenance analysis and referring to the well-studied igneous basement units. The supposed regional geological evolution model is mutually verified with our regional thermo-tectonic evolution.
After a short introduction to the subject (Chapter 1), the new data from the study area is presented and discussed in Chapters 2 to 4. In Chapter 2, apatite and zircon (U-Th)/He and apatite fission-track results from most basement highs in eastern NE China are presented. The low-T thermochronometers show mostly Late Cretaceous - early Paleogene apparent ages, younger than the onset of the Early Cretaceous burial in the Songliao and related satellite basins. These age constraints are in harmony with the thermal modelling of vitrinite reflectance data from the basins, which indicates that the maximum burial depth occurred in mid-Cretaceous. The following primary basin inversion leads to erosion from ca. 110 ca. 40 Ma. The modelling indicated that in the Jiamusi Uplift, the central part experienced deeper erosion than the marginal areas. Combining the above modelling results, we suggest a single united down-warped basin which formed in the Early Cretaceous and covered the currently elevated western Zhangguangcai Range and eastern Mishan Uplift at the time of its maximum extent. The Late Cretaceous - Paleogene exhumation of the Jiamusi Uplift gradually destroyed the formerly continuous, 1.6 to 4.8 km thick sedimentary cover. Only isolated, deeply eroded basin remnants have been preserved.
Chapter 3 focuses on the dating of young mafic lava with an unconventional method. Mafic lavas of the Cenozoic age are widely distributed in NE China and received much attention as an important part of the Circum-Pacific volcanic belt. We present new zircon (U–Th)/He ages obtained on the thermally overprinted sands directly underlying basaltic lava. This thermochronometer is insensitive to weathering and cannot be biased by, e.g., excess argon; thus, it can accurately express the age of the lava flow's thermal effect. As a regional cooling age reference, three granite samples were dated from basement units away from the basalt lavas at different distances. The reference granite samples revealed well-defined Cretaceous (U–Th)/He-ages, while 20 zircon crystals from the sand below the basalt lava revealed a prominent Miocene (U–Th)/He age component of 9.33 ± 0.24 Ma. Raman spectroscopy of these zircon crystals supports their thermally overprinted character. We infer that the sand sample has experienced a significant thermal overprint by the overlying basalt lava, leading to most of the detrital zircon crystals' thermal reset. The obtained age is thus interpreted as the eruption age of the basalt lava. The dating results provide strict constraints on the thermal influence of the regional volcanic units on reconstructing the study area's thermo-tectonic evolution history.
Chapter 4 provides detrital zircon U-Pb data from modern sand samples of five rivers draining catchments of variable size (~500 to ~40.000 km2), dominated by Carboniferous to Jurassic granitoids, Proterozoic to Early Paleozoic siliciclastic (meta-)sediments, and/or Jurassic to Cenozoic volcanic rocks from the Lesser Xing'an-Zhangguangcai Range and the Jiamusi block. Our results show low consistency between the age spectra and the potential source units' areas in the catchment. A part of the differences can be explained by variation in fertility and sediment yield among the source units. Additionally, we detected a consequent mismatch between the obtained and expected ages. It can be explained only by re-considering some igneous suites' emplacement ages and assuming that some metasedimentary units have much younger depositional ages. Although the proportion of the identified age components is highly different from the areal proportions of the igneous suites in the catchments, the mean ages of the age components in the modern sand samples and the age components isolated from the compiled U-Pb ages of the former basement studies show excellent agreement. By including the zircon U-Pb age patterns of the studied catchments and the region-wide compilation of the basement ages, it is possible to refine the Cretaceous provenance of the Songliao Basin and its strongly inverted eastern satellite basins. In the Early Cretaceous, the Songliao Basin mainly received sediment from the Great Xing'an Range, North China Craton, and Zhangguangcai Range. The Lesser Xing'an Range and Jiamusi block provided minor or no sediment as these currently exhumed basement areas were buried at the time. In the early Late Cretaceous, the Jiamusi block became the primary sediment provider for the eastern satellite basins.
To conclude, the low-temperature thermochronology studies on the currently exposed basement areas in eastern NE China area revealed that late Early Cretaceous to Late Cretaceous continuous subsidence primarily led to the reset of the thermochronometers from the basement highs and basin sediments; the volcanic thermal influence was minor or negligible. Using the detrital zircon U-Pb data from this study, both the thermal-tectonic evolution model and the temporal change in Cretaceous sediment provide evidence for the forming of a huge Early Cretaceous united down-warped basin that covered most of the current eastern satellite basins and basement highs in the eastern NE China area. From ca. 110 to 40 Ma, the exhumation of the Jiamusi Uplift has gradually destroyed the formerly continuous sedimentary cover, and only basin remnants have been preserved. By the end of the major exhumation in the Eocene, both the major uplift areas and the basin remnants developed towards a slow uplift and erosion stage which continued until recent times. | de |