Radiometric Dating of the Rocky Mountain Foreland Thrust Belt

尽管通过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有效，但地壳脆性区浅层断层的放射性测年仍然很困难，因为这些断层的典型低温阻止了较深断层的同构造重结晶或复位。含粘土断层岩在许多断层带中很常见，但这些岩石通常既保存了原生（碎屑），也保存了新生长（自生）的细粒层状硅酸盐。因此，这些断层带的放射性年龄反映了两种矿物的混合，而我们只对新生长阶段的年龄感兴趣。第二个与粘土矿物年代测定有关的问题是中子辐照过程中39 Ar的损失，这会产生错误的年龄。利用X射线模拟粒度总体和样品封装fordating样品中碎屑粘土部分的特性，PI已经开发并测试了一种可靠的地质年代学方法来确定近地表断层。通过对加拿大落基山脉南部最前缘冲断层（刘易斯冲断层）~52 Ma断层泥的测年，说明了这种方法的可行性。皮氏提出将他们的测年方法应用于北美西部的一个地质上重要的地区，即落基山脉，该地区既具有区域构造意义，又具有基本的逆冲带意义。其他断层的初步结果证明了该项目的可行性，并指出了区域地质和基本构造地质的几个令人兴奋的影响。拟议的研究将由两名PI进行，他们联合收割机结合了所需的结构，矿物学和同位素专业知识，以及一名博士生，他们将完成这项工作作为其论文的一部分。X射线、电子显微镜和Ar设备都在密歇根大学提供。

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.