扎格罗斯前陆盆地晚中新世快速剥露证据——来自磷灰石(U-Th)/He的约束

Evidence of rapid Miocene exhumation of the Zagros Foreland basin:constraints from apatite (U-Th)/He ages

伊朗扎格罗斯造山带是世界上最年轻的造山带之一,前人对其构造演化历史的研究尚存争议。前陆盆地蕴藏着丰富的造山带热史信息,为研究造山带的隆升和剥露历史提供了重要途径。本研究选取位于扎格罗斯前陆盆地前缘中部的Kuh-E Bedush向斜和Kuh-E Murdeh向斜翼部出露的第三纪陆相红层(Agha Jari组)为研究对象,共采集11个粗砂岩样品进行磷灰石(U-Th)/He测试分析。11个样品共得到38个单颗粒年龄,年龄区间为0.8~79.9Ma。大部分样品的单颗粒年龄比较分散,表明这些样品没有发生完全热重置。相反,位于Murdeh向斜剖面最底部的样品,(U-Th)/He年龄小于其对应的地层年龄,并且集中分布在8.3~6.8Ma,表明该样品发生了完全重置并记录了最后一次剥露事件的时间。因此,我们认为扎格罗斯前陆盆地于晚中新世~7.5 Ma经历了一期快速剥露事件。此外,我们发现未重置的(U-Th)/He年龄大致分布在四个时间段:晚白垩纪—早古新世、早—中始新世、渐新世、早—中中新世。根据所得热年龄并结合前人研究设定了三种不同的热史,正演模拟结果与实际样品拟合较好,由此推断出前陆中部Agha Jari陆相红层沉积物可能来自于:(1)晚白垩世以来以蛇绿岩套为主的仰冲体持续剥露;(2)萨南达季-锡尔詹变质带(SSZ)渐新世和中新世逐渐加速的剥露;(3)渐新世末堆积在仰冲体之上的同造山期砾岩,在中新世的大规模逆冲作用下重新剥露。

The Zagros orogenic belt is one of the youngest orogenic belts in the world, but previous studies of its tectonic evolution history are still controversial. The foreland basin is rich in thermal history information, which provides an important way to explore the uplifting and exhumation history of the orogenic belt. In this study, we collected 11 coarse sandstone samples of the Kuh-E Bedush syncline andthe Kuh-E Murdeh syncline in the forefront of the middle Zagros foreland basin. A total of 38 single-grain ages were obtained by using apatite(U-Th)/He thermochronometry, which range from 79.9 Ma to 0.8 Ma. Single-grain age of most samples is relatively scattered, indicating that these samples did not undergo a complete thermal reset. In contrast, sample LS10 which located at the bottom of the Murdeh syncline has a(U-Th)/He age that is much younger than its corresponding stratigraphic age, and clusters at 8.3~6.8 Ma, indicating sample LS10 has been completely reset and recorded the last cooling event. Therefore, we believe that the Zagros foreland basin experienced a rapid exhumation event during the Late Miocene ~7.5 Ma. In addition, we found that the unreset AHe ages roughly distributed in four periods: Late Cretaceous-Early Paleozoic, Early-Middle Eocene, Oligocene, early-middle Miocene. Three different thermal histories were set based on the obtained He ages and previous studies, and the forward modeling results fit well with the actual samples. We suggest that the Agha Jari Formation in the central foreland basin may source from:(1) continuous exhumation of obduction body mainly dominated by ophiolites since the late Cretaceous;(2) the Sanandaj-Sirjan metamorphic zone(SSZ) with gradual acceleration of cooling rate during the Oligocene and Miocene;(3) thesyn-orogenic conglomerates accumulated above the thrusting body of High Zagros at the end of the Oligocene, then exhumed again during the Miocene thrusting period.