Radiometric Dating of the Rocky Mountain Foreland Thrust Belt

虽然通过U/Pb、Ar矿物测年和其他同位素技术的应用，韧性剪切带的直接测年越来越可行，但地壳脆性状态下的浅层断裂的放射性测年仍然困难，因为这些断裂的典型低温阻碍了深部断裂实现完全的同构造重结晶或重结晶。含粘土的断层岩在许多断裂带中很常见，但这些岩石通常保存着原生(碎屑)和新生长(自生)的细粒层状硅酸盐。因此，这些断裂带的放射性年龄反映了两种矿物的混合，而我们只对到目前为止断裂活动的新生长阶段的年龄感兴趣。与粘土矿物测年相关的第二个问题涉及中子辐射过程中39Ar的损失，这可能会产生错误的年龄。利用粒度群体X射线建模和样品包裹法对样品中碎屑粘土组分的表征，PI已经开发并测试了一种可靠的近地表断层测年方法。他们方法的可行性通过测定加拿大落基山脉南部最前缘逆冲(刘易斯逆冲)约52 Ma的断层泥来说明。Pi夫妇建议将他们的测年方法应用于北美西部一个具有重要地质意义的地区-落基山脉，该地区既具有区域构造意义，又具有基本的逆冲带含义。其他断层的初步结果证明了该项目的可行性，并表明了对区域地质和基础构造地质的几个令人振奋的意义。拟议的研究将由两名结合了所需的结构、矿物学和同位素专业知识的私人助理和一名博士生进行，这名博士生将完成这项工作，作为其论文的一部分。X光、电子显微镜和Ar设备都可以在密歇根大学获得。

Whereas direct dating of ductile shear zones is increasingly available through theapplication of U/Pb, Ar mineral dating and other isotopic techniques, radiometric datingof shallow faulting in the crust's brittle regime has remained difficult, because the lowtemperatures typical of these faults prevent complete syntectonic recrystallization orresetting achieved in deeper faults. Clay-bearing fault rocks are common in many faultzones, but these rocks typically preserve both primary (detrital) and newly-grown(authigenic), fine-grained phyllosilicates. Radiometric ages of these fault zonestherefore reflect a mixture of both mineral populations, while we are only interested inthe age of the newly-grown phase to date faulting. A second problem associated withdating clay minerals involves the loss of 39Ar during neutron irradiation, which canproduce erroneously old ages. Using characterization of the detrital clay fraction insamples using X-ray modeling of grain size populations and sample encapsulation fordating, the PI's have developed and tested a reliable geochronologic method to date near-surface faulting. The feasibility of their approach was illustrated by dating ~52 Ma fault gouge from the most frontal thrust (Lewis Thrust) of the southern Canadian Rockies. The Pi's propose a application of their dating method to a geologically important region of western North America, the Rocky Mountains, that has both regional tectonic significance as well as fundamental thrust belt implications. Preliminary results from other faults demonstrate the feasibility of the project and indicate several exciting implications for regional geology and basic structural geology. The proposed research will be conducted by two PIs who combine the needed structural, mineralogic and isotopic expertise, and a PhD student who will complete the work as part of his dissertation. The X-ray, electron microscopy and Ar facilities are all available at the University of Michigan.