珠江口盆地新生代古热流重建及热史恢复——来自AFT和Ro的证据

As one of the frontiers and difficult problems in basin dynamics, basin analysis and petroleum geology, paleo-geothermal reconstruction has become a hotspot of current Earth science domain. Paleo-water depth restoration, compaction restoration, erosion thickness restoration, calculation of tectonic subsidence, balanced cross-section restoration, etc. were applied in this project to figure out the paleo-heat flow reconstruction and thermal evolution since the Cenozoic in the Pearl River Mouth Basin, which under the restriction of the vitrinite reflectance (Ro) and apatite fission track (AFT). The key scientific problems not only involved the establishment of thermal evolution models of continental shelf and continental slope, but also discussed the influence of tectonic-thermal events on the paleo heat flow in passive continental margin basin. Two improvements were made in this project: Firstly, multiple techniques were applied in this project to obtain the information of paleo-water depth, which break the current situation of paleo-water depth restoration by single method. Detailed study of paleo-water depth was not only used to calculate the tectonic subsidence of different tectonic units, but also describe the evolution of the continental slope and optimize the tectonic-thermal evolution model in the Pearl River Mouth Basin. Secondly, because of the difficulties in sample collection, single geothermometer was always used in the thermal history reconstruction of the Pearl River Mouth Basin. In this project, the paleo-heat flow and thermal history was reconstructed by paleo-geothermal gradient method and paleo-heat flow method, which under the constraints of vitrinite reflectance (Ro) and apatite fission track (AFT).

作为盆地动力学、盆地分析及石油地质领域研究的前沿及难点问题之一，古地温场恢复已经成为当前国际地球科学领域中倍受关注的研究热点。本项目选取珠江口盆地，拟通过古水深恢复、压实恢复、剥蚀恢复、构造沉降量计算、平衡剖面等技术方法，开展裂变径迹和镜质体反射率约束下的新生代古热流重建，不仅涉及南海扩张影响下陆架、陆坡热演化模式的建立，还对构造-热事件对被动陆缘盆地热流变化的影响进行了探讨。本次研究的创新点主要体现在以下两个方面：其一，打破单一方法推测古水深的研究现状，创新性的利用多种方法获取水深信息相互校正，计算盆地构造沉降量，精细刻画陆坡演化的全过程，优化构造-热演化模型；其二，开展基于AFT与Ro双重约束下的盆地构造-热演化研究，突破因深水盆地样品采集和获取困难所导致的珠江口盆地热史反演采用单一古温标的现状，采用古地温梯度法和古热流法重建盆地古热流。

本项目选取珠江口盆地，拟通过古水深恢复、压实恢复、剥蚀恢复、构造沉降量计算、平衡剖面等技术方法，开展裂变径迹和镜质体反射率约束下的新生代古热流重建，不仅涉及南海扩张影响下陆架、陆坡热演化模式的建立，还对构造-热事件对被动陆缘盆地热流变化的影响进行了探讨。取得成果认识如下：.打破单一方法推测古水深的研究现状，创新性的利用多种方法获取水深信息相互校正，计算盆地构造沉降量，精细刻画陆坡演化的全过程，优化构造-热演化模型。珠江口盆地主要经历了古近纪裂谷阶段和新近纪被动陆缘阶段，渐新世白云凹陷主要以内浅海环境为主，随着南海扩张脊向南跃迁，陆架坡折带跃迁至番禺低隆起，白云凹陷由渐新世晚期的浅海陆架环境转为陆坡深水环境。.开展基于AFT与Ro双重约束下的盆地构造-热演化研究，突破因深水盆地样品采集和获取困难所导致的珠江口盆地热史反演采用单一古温标的现状，采用古地温梯度法和古热流法重建盆地古热流。构造-热演化揭示了两个阶段的沉降期次和加热过程存在明显差异，海底扩张造成南海北部陆架和陆坡不同的热演化模式。大陆架模式：在早期裂谷阶段多期拉张应力的影响下，古热流值两次快速增加，并在随后的被动陆缘阶段逐渐降低。大陆坡模式：裂谷阶段两期拉张活动导致岩石圈减薄增温，大地热流值快速增加。随着南海扩张的开始，南海北部陆坡岩石圈迅速减薄，大量深部岩浆上涌使莫霍面在海底扩张过程中持续抬升，大地热流值在被动陆缘阶段持续增加，并于10.0 Ma达到最大值。.南海海底扩张不仅控制了珠江口盆地的构造格局，同时也影响了主力烃源岩的受热过程。裂谷阶段，在区域拉张应力影响下，白云凹陷逐渐成为珠江口盆地的沉降-沉积中心，凹陷中心发育巨厚的文昌组深湖-半深湖相和恩平组浅湖-沼泽相烃源岩，并在海底扩张后经历持续热沉降，于珠海组沉积晚期逐渐进入生烃窗，开始大量生烃。