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

科罗拉多中部的元古代岩石保存了一个过渡，从半脆的上地壳到普遍韧性的中地壳，这是在大约1.4 Ga时冻结的。这种流变转变表现为约1.4 Ga克拉通内构造活动样式的区域性显著变化。野外观测表明，在科罗拉多北部和中部（以科罗拉多矿物带为例），1.4 Ga变形以离散糜棱岩带为特征，而在科罗拉多南部和新墨西哥北部，一个以渗透韧性变形为主的区域被亚水平边界分隔开。西藏、Altiplano-Puna和Sevier-Laramdie等陆内造山带的地球动力学模型表明，这种机械解耦可能是控制陆内构造演化的关键因素。像一些较年轻的类似物一样，科罗拉多中部的结构转变显然与岩浆和流体丰富的高应变层相对应。这些深挖的岩石提供了有趣的机会，可以直接研究在较年轻的造山带无法进入的深层地壳过程。一般影响的问题包括：(1)对脆性/韧性转变深度和厚度的控制（如温度、岩性、流体）；(2)流变转变附近变形的运动学和动力学；(3)不同变形机制之间应变不相容的调节机制；(4)花岗岩类熔体和/或广泛的熔融流体层在局部化中地壳流动和上下地壳结构解耦中的作用。除了利用科罗拉多中部的深部地壳暴露来研究大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代构造演化。特别是，1.4 Ga构造的“造山带”（伸展）和“造山带”（跨伸展）构造背景的交替模式，也许可以在造山带高原模式中得到调和，在造山带高原模式中，区域水平收缩被更局部的重力塌陷所平衡。该研究使用了综合数据集，包括构造、岩石学和U-Pb地质年代学数据，以评估这一重要构造转变的运动学和演化。重点是整个结构过渡的一个暴露良好的样带。具体而言，通过对科罗拉多矿带内甘尼森黑峡谷及邻近地区的研究，确定了糜糜岩剪切带的样式、范围和P-T条件，以及过渡带上方同动1.4 Ga岩体的侵位。在Wet Mountains和Sangre de Cristo Range的工作表明，在过渡带内部和下面，普遍存在韧性变形和织物再活化。地质年代学强调了科罗拉多中部关键同动岩体的U/Pb锆石定年和沿样带高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.