Late Cretaceous-Cenozoic Source To Sink System Of The Central Myanmar Basin In An Active Continental Margin Setting

The western Myanmar hosts a typical transitional continental margin where the Himalayan orogen produced by continental collision in the north and the Sunda trench-arc system produced by oceanic subduction in the south.The active continental margin of Myanmar,therefore,documented not only the complete geological history of the trench-arc system but also the initiation collision between India and Asia,and early record of Himalayan erosion.Moreover,the active continental margin of Myanmar was characterized by several kinds of sedimentary basins,including trench,trench-slop,forearc and backarc,which have experienced a complex evolution process that probably influenced accumulation of the hydrocarbon in different basins.Therefore,the study of the active continental margin of Myanmar is very important for better understanding the geological theory and for the developing of the petroleum industry.The Western Myanmar Arc?WMA?and Central Myanmar Basin?CMB?developed together along the active continental margin of Myanmar where the WMA divided the CMB into a forearc and a backarc depression.The magmatic belt of the WMA is,today,largely buried beneath sedimentary rocks and exposed are a few isolated volcanoes?e.g.Mt.Popa and Mt.Monywa?,and the Late Cretaceous-Eocene aged Wuntho batholith in the north and the middle Cretaceous aged Salingyi batholith in the south.Different models have been proposed to explain the magmatic and tectonic evolution of the WMA,with some proposed that it was suited on an Andean-type continental margin that has been expected to extend along the Eurasian margin from south Tibetan Plateau to at least central Myanmar,and others however proposed that the WMA was formed in a westward migrating and east-dipping subduction zone,above which lies an ocanic crust.These models were established primarily based on evidencs from studies of the petrlogy,geochemistry and a few bulk-rock Sr-Nd samples,lacking single mineral isotopic data.It is for these reasons that authors have two end-member interpretations for the detrital zircon Hf data from the CMB.While some authors proposed that the Upper Cretaceous to Eocene sediments were totally derived from the WMA,others proposed that the Eocene and Oligocene sediments were derived from the Gandese Arc through the palaeo Yarlung-Irrawaddy River.Such disputes on both origin of the WMA and provenance of the CMB restricted our understanding of the geological evolution the active continental margin of Myanmar but also our interpretation for the geological events related to the India-Eurasia collision and subsequent fast growth of the Tibetan Plateau.For example,some authors proposed that the upper Yangtze,Mekong,Salween,Irrawaddy and Yarlung were once all tributaries of the palaeo Red river and prior to capture by the Brahmaputra in the Miocene the palaeo Yarlung was connected with the palaeo Irrawaddy.If such connection was once existed,the CMB must have sedimentary records in some extent of the Bomi-Ranwu-Chayu magmatic belt,however if any of the sedimentary rocks in the CMB show no provenance signatures similar to the magmatic belt,does the continental-scale palaeo Red River existed?And can we use the drainage reorganization to constraint the timing of uplift of the Tibetan Plateau?In this study,all these important issues were taken into consideration and source to sink system of the CMB was used as a proxy to study origin of the WMA magmas and its relationships with the Central Myanmar Magmatic Belt?CMMB?,Dianxi-Burma Magmatic Belt?DBMB?,Bomi-Ranwu-Chayu Batholiths?BRCB?and Gandese Arc,and explained the provenance of the CMB which we think was controlled by transverse rivers during the Late Cretaceous-Eocene times and by axis trunk river of the Irrawaddy in the Oligocne-Quaterary and discussed the dynamical evolution and basin transition along the active continental margin of Myanmar.This study was benefited from our samples collected from both ourcrops and drillings,and supported by some new techniques,such as detrital rutile and apatite U-Pb geochronology.Three bedrock samples?core drilling?and one ourcrop samples collected from the WMA were analyzed at the State Key Laboratory of Geological Processes and Mineral Resouces?SKLGPMR?,China University of Geosciences,Wuhan for petrology,whole rock geochemistry,zircon U-Pb dating with Hf,and whole rock Sr-Nd isotope analyses.The WMA contains I-and S-type granites and basaltic to rhyolitic volcanics which show characters similar to medium-to high-K calc-alkalic and metaaluminou to peraluminous rocks,and enriched large ion lithphile elements and depledted most high-field-strength elements of the magmatic belt suggest that magmas were originated from an island arc setting.Samples YB1-84,Y2-86,Y3-84,and M3A exhibt crystalized ages 64.5±0.8 Ma,103.6±0.8 Ma,106.1±0.9 Ma,and 69.0±2.6 Ma,respectively,which together with published data suggest that the WMA has four stages of arc magmatism in the middle Cretaceous?106-98 Ma?,Late Cretaceous?70-65 Ma?,Early-Middle Eocene?53-38Ma?,and Miocene-Quaternary?13.6-0.15 Ma?.Also,?Hf_?t?values of these four samples are in the range of+2.6 to+16.0,+1.3 to+16.5,+1.3 to+14.5,and+7.1 to+15.0,and the sample Y3-84 has a ⁸⁷Sr/⁸⁶Sr_?t?ratio of 0.705439,?Nd_?t?value of 0.88,suggesting that magmas of the WMA are primarily derived from depleted mantle and partial melting of juvenile crust.The zircon Hf and bulk-rock Sr-Nd compositions of the WMA are very dissimilar to those of the CMMB,DBMB,and BRCB but exhibit similar signatures to those of the Gangdese Arc.An integrated provenance techniques,including sandstone petrography,whole rock geochemistry,detrital zircon U-Pb dating and Hf,detrital rutile and apatite U-Pb dating,and bulk-rock Sr-Nd,are utilized on the upper Cretaceous to Cenozoic sedimentary rocks of the CMB,which were colleted from both the core drilling and outcrops.We compared the petrographic and isotopic characteristics of the CMB sedimentary rocks with the equivalent characteristics from the potential sources regions as we have summarized above.The sedimentary provenance evidences suggest that the upper Cretaceous-Eocene are totally derived from the WMA but the Miocene-Quaternary are likely derived from the Dianxi-Burma magmatic belt with minor contributions from the CMMB.The Oligocene sedimentary rocks of the CMB show similar provenance signatures both to the WMA and CMMB and DBMB,thus we interpreted this time as a transitional period that the source region from the local WMA to the distal regions of the CMMB and DBMB.The Gangdese Arc is probably not the source of the CMB during the Late Cretaceous to Eocene times,because?1?the majority magmatism of the Gangdese Arc occurred at the Paleogene?65-45 Ma?and age peaks of the detrital zircons in the Xigaze forearc and Indus-Yarlung Suture Zone at the¹10 Ma and¹20Ma?or⁸5 Ma?are very different age distribution patterns of the CMB;?2?Zricon?Hf_?t?values of the middle Cretaceous Gangdese Arc are in the ranges of+10 and+15,whereas those of the CMB are in the ranges of-1.0 and+15;and?3?detrital rutile U-Pb ages are characterized by significant peaks at the 400-600 Ma and/or¹00 Ma for the Paleocene and Eocene rocks but those in the Indus-Yarlung Suture Zone that were derived from the Gangdese Arc are at 100-200 Ma.We also propose that the CMB was influenced by the transverse rivers in the Late Cretaceous to Eocene time and by the axis trunk river,i.e.,the Irrawaddy River,in the Oligocene to Quaternary times.The transition of the drainage of the CMB was thought mainly controlled by tectonically driven force,related to the growth of the Indo Burman Ranges and SE Tibetan Plateau,and basin subdidence.For further conforming that provenance of the CMB are derived from the WMA and DBMB?with minor contributions from the CMMB?,we reconstructed uplift history of the source regions by detrital apatite fission track?AFT?age and track-length modelling analyses of sedimentary rocks of the CMB.The upper Cretaceous-Eocene show two AFT age populations of 75-68 Ma and 53-42 Ma respectively,with two samples experienced basin burial and heating>80-90?,resulting the AFT age resetting,and thus such AFT ages?21.8±2.1 Ma and 21.3±5.3 Ma?were interpreted as a consequence of basin shorting.The Miocene and Quarternary samples show one major AFT age populations of 26-21 Ma and three subordinate AFT age populations of139±25 Ma,66.1^+49.6\_-28.4 Ma,and 57.8-33.1 Ma,consistent with Oligocene uplift of the SE Tibetan Plateau.Two Miocene samples show AFT age populations of 17.1±1.5 and15.6±1.6 Ma,which are younger than their depositional ages,and are interpretated as a consequence of basin shorting.Our results combined with published data helped us proposed that?1?there were three uplift and exhumation processes at the end of the Lat Cretaceous?75-68 Ma?,Early-Mid Eocene?53-42 Ma?,and Late Oligocne?30-20 Ma?for the WMA;?2?the SE Tibetan Plateau must has been uplift before 26 Ma with an erosion ratio of 139.4-287.4 m/m.y.;and?3?the growth of Indo-Burman Ranges started at the Oligocene/Miocene boundary?²2 Ma?and propagated eastward from western flanks to eastern flanks of the forearc depression?17-15.6 Ma?.The apatite fission track-length modelling gives the similar results help us believe that the timing of the provenance transition,growth of the Indo-Burman Ranges,and initiation of the palaeo Irrawaddy trunk river constraint by the sedimentary provenance data are robust.The study of the source to sink system of the CMB makes a great contribution to the dynamical evolution of the active continental margin of Myanmar.We documented the oldest crystalized ages of¹06 Ma in the WMA,which is much younger than those of the Gangdese Arc?²10 Ma?,indicating that the trigger mechanism of Neo-Tethyan oceanic lithosphere beneath the Lhasa and West Burma blocks are very different.We thus agreed with previous model that prior to eastward subduction of the Neo-Tethyan oceanic lithosphere,there is a westward subduction system beneath the Mawgyi Arc.The oceanic arc thrusted onto the West Burma block at the late Early Cretaceous?¹15Ma?,followed by polarity-reversal of the subduction system.The model explain why there are no Jurassic and Early Cretaceous magmatism in the WMA and initial of the magmatism of the WMA¹0 Ma later than the timing of formation of the ophiolites in the Indo-Burman Ranges.Following eastward subduction of the Neo-Tethyan crust,the remnant basin between the West Burma and SIBUMASU blocks disappeared with time,thus the S-type granits with very negative?Hf_?t?and?Nd_?t?values could be explained by heating resulted from continental collision.The eastward subduction of the Neo-Tethyan oceanic crust also accounted for tectonic evolution of sedimentary basins along the active continental margin.Our provenance study of the CMB helpe us to propose that the Yarlung River has never flowed into the Irrawaddy River in the Paleocene or in the Miocene times,and if this is the case,we do not support the idea that large-scale rivers along eastern margin of the SE Tibetan Plateau were once tributaries of the Red river and then captured by their lower parts.And if our understanding is correct then the use of the timing of the palaeo drainage reorganization to constraint the timing of uplift of the SE Tibetan Plateau remains an open question and needs more studies.By constrast,our AFT ages and modelling results suggest that the SE Tibetan Plateau has uplifted to its present-day elevations before Late Oligocene?²6 Ma?,meeting the requirement that there must be at least 20 Ma after collision of the India and Eurasia plates to achieve the thermal state favoring crust flow.Therefore we believe the crust flow from the Tibetan Plateau must has played an important role in the growth of the Indo-Burman Ranges and tectonic reversal of the CMB,although oblique convergent between India and Asia is,undisputed,the major driven force for development of the CMB and the subsequent basin inversion.