Radiometric Dating of the Rocky Mountain Foreland Thrust Belt

尽管通过应用U/Pb、Ar矿物定年和其他同位素技术，对韧性剪切带进行直接定年越来越可行，但对地壳脆性区域的浅层断裂进行放射性定年仍然很困难，因为这些断裂典型的低温阻碍了深层断裂的完全同构造再结晶或重置。含粘土断层岩在许多断裂带中都很常见，但这些岩石通常保存着原生（碎屑）和新生（自生）的细粒层状硅酸盐。因此，这些断裂带的辐射年龄反映了两种矿物种群的混合，而我们只对新生长阶段的年龄感兴趣。与提取粘土矿物有关的第二个问题涉及中子辐照期间39Ar的损失，这可能会产生错误的年龄。通过对碎屑粘土组分样品进行表征，利用x射线粒度种群建模和样品包封进行定年，PI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裂的年代。通过对加拿大南部落基山脉最前缘逆冲（Lewis逆冲）~52 Ma断层泥的测年，证明了他们方法的可行性。Pi团队提出将他们的测年方法应用于北美西部一个重要的地质区域——落基山脉，该地区不仅具有区域构造意义，而且具有基本的冲断带意义。其他断层的初步结果证明了该项目的可行性，并对区域地质和基础构造地质提出了一些令人兴奋的启示。拟议的研究将由两名pi进行，他们结合了所需的结构、矿物学和同位素专业知识，以及一名博士生，他将完成这项工作，作为他论文的一部分。x射线、电子显微镜和Ar设备都可以在密歇根大学使用。

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.