Collaborative Research: Contrasting Styles of Ca. 1.4 Ga Tectonism in the Southern Rockies: Evidence for a Fossil Rheologic Transition in a Deeply Exhumed Intracontinental Orogen

科罗拉多州中部的Karlstrom 0003500Connelly 0003528 Williams 0003569 Siddoway 0101314元古代岩石保存了从半脆性上地壳到渗透韧性中地壳的转变，冻结时间约为1.4Ga。这种流变性转变表现为1.4Ga左右的克拉通构造作用样式的区域显著变化。野外观察表明，科罗拉多州北部和中部(以科罗拉多矿带为例)的一个域与科罗拉多州南部和新墨西哥州北部的一个以穿透性韧性变形为主的域分开，科罗拉多州北部和科罗拉多州北部的域以离散的糜棱岩带为特征。针对西藏、高原-普纳和塞维埃-拉兰迪等陆内造山带建立的地球动力学模型表明，这种机械去耦合作用可能是控制陆内变形构造演化的关键因素。就像它的一些年轻的类似物一样，科罗拉多州中部的结构转变显然与富含岩浆和流体的高应变层相对应。这些深挖出来的岩石提供了有趣的机会，可以直接研究在年轻造山带中无法接触到的地壳深部过程。具有普遍影响的问题包括：(1)对脆性/韧性转变深度和厚度的控制(如温度、岩性、流体)，(2)在流变转变附近的变形运动学和动力学，(3)不同变形制度之间应变不相容的调节机制，以及(4)花岗岩类熔体和(或)广泛的熔体-流体层在中地壳流动和上、下地壳结构脱钩中的作用。除了利用科罗拉多州中部的地壳深部暴露来研究陆内变形的一般过程外，这项研究还有助于了解美国西南部的元古界构造演化。特别是，1.4Ga构造作用的“造山”(伸展)和“造山”(转压)构造环境的替代模式也许可以在造山高原模式中得到协调，在该模式中，区域水平收缩被更局部化的重力坍塌所平衡。这项研究使用了一个涉及构造、岩石学和U-Pb地质年代学数据的综合数据集，以评估这一重要构造转变的运动学和演化。焦点集中在结构过渡期间暴露良好的横断面上。具体地说，对Gunnison黑峡谷和科罗拉多矿带邻近地区的研究正在表征糜棱岩剪切带的样式、范围和P-T条件，以及过渡上方同运动型1.4Ga深成岩体的侵位。湿润山脉和Sangre de Cristo山脉的工作特征是过渡期内和过渡期下方普遍存在的韧性变形和组构重新激活。地质年代学的重点是科罗拉多州中部关键的同运动学深成岩体的锆石U/Pb定年和沿断面的高级变质作用的微探针独居石定年。

Karlstrom 0003500Connelly 0003528Williams 0003569Siddoway 0101314Proterozoic rocks in central Colorado preserve a transition, from semi-brittle upper crust to pervasively ductile middle crust, that was frozen in at about 1.4 Ga. This rheologic transition is expressed as a regionally significant change in the style of ca. 1.4 Ga intracratonic tectonism. Field observations suggest that a subhorizontal boundary separates a domain in northern and central Colorado (exemplified by the Colorado Mineral Belt), where 1.4 Ga deformation is characterized by discrete mylonite zones, from a domain in southern Colorado and northern New Mexico dominated by penetrative ductile deformation. Geodynamic models developed for intracontinental orogens such as Tibet, Altiplano-Puna, and the Sevier-Laramdie indicate that such mechanical decoupling may be a critical factor in controlling the tectonic evolution of intracontinental deformation. Like some of its younger analogs, the structural transition of central Colorado apparently corresponds with a magma- and fluid-rich, high-strain layer. These deeply exhumed rocks present intriguing opportunities to directly investigate deep-crustal processes that are inaccessible in younger orogens. Issues of general impact include characterization of: (1) the controls on the depth and thickness of the brittle/ductile transition (e.g. temperature, lithology, fluids), (2) the kinematics and dynamics of deformation near the rheological transition, (3) mechanisms of accommodating strain incompatibility between different deformation regimes, and (4) the role of granitoid melts and/or a widespread melt-fluid layer, in localizing middle crustal flow and structural decoupling of upper and lower crust. In addition to exploiting the deep crustal exposures of central Colorado to study general processes of intracontinental deformation, this study is contributing to the understanding of the Proterozoic tectonic evolution of the southwestern United States. In particular, alternate models for "anorogenic" (extensional) versus "orogenic" (transpressional) tectonic settings for 1.4 Ga tectonism can perhaps be reconciled in an orogenic plateau model where regional horizontal contraction is balanced by more localized gravitational collapse. The study uses an integrated data set involving structural, petrologic, and U-Pb geochronologic data to evaluate the kinematics and evolution of this important structural transition. Focus is on a well exposed transect across the structural transition. Specifically, studies of the Black Canyon of the Gunnison and adjacent areas within the Colorado Mineral Belt are characterizing the style, extent, and P-T conditions of mylonite shear zones and emplacement of syn-kinematic 1.4 Ga plutons above the transition. Work in the Wet Mountains and Sangre de Cristo Range is characterizing pervasively ductile deformation and fabric reactivation within and below the transition. The geochronology is emphasizing both U/Pb zircon dating of critical syn-kinematic plutons in central Colorado and microprobe monazite dating of high-grade metamorphism along the transect.