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

Karlstrom 0003500康奈利 0003528威廉姆斯 0003569西多威 0101314科罗拉多中部的元古宙岩石保存了一个从半脆性上地壳到普遍韧性中地壳的过渡，该过渡在大约1.4 Ga时冻结。 这种流变性的转变表现为一个地区性的重大变化，在风格的ca。1.4克拉通内构造作用 现场观察表明，一个近水平的边界分离域在北方和科罗拉多州中部（例如科罗拉多州矿物带），其中1.4 Ga变形的特点是离散糜棱岩带，从域在科罗拉多州南部和北方新墨西哥州为主的渗透性韧性变形。 陆内造山带（如西藏、高原-普纳和塞维尔-拉拉姆迪）的地球动力学模型表明，这种机械解耦可能是控制陆内变形构造演化的关键因素。 像它的一些年轻的类似物一样，科罗拉多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。这些深挖出的岩石提供了直接研究在年轻造山带中无法获得的深地壳过程的有趣机会。 具有普遍影响的问题包括：（1）脆/韧性转变深度和厚度的控制（例如温度、岩性、流体），（2）流变转变附近变形的运动学和动力学，（3）调节不同变形状态之间的应变不相容性的机制，以及（4）花岗岩熔体和/或广泛的熔体-流体层的作用，中地壳流动和上下地壳构造拆离的局部化。 除了利用科罗拉多中部的深部地壳暴露来研究陆内变形的一般过程之外，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，“非造山”（伸展）与“造山”（压扭）构造环境为1.4 Ga构造作用的替代模型，也许可以调和的造山高原模型，区域水平收缩平衡更本地化的重力塌陷。这项研究使用了一个综合的数据集，包括结构，岩石学和U-Pb地质年代学数据，以评估这一重要的构造转换的运动学和演化。 重点是一个充分暴露的横断面跨越结构过渡。具体而言，研究黑峡谷的甘尼森和邻近地区的科罗拉多矿产带的特征的样式，范围和P-T条件的糜棱岩剪切带和侵位的同运动1.4 Ga岩体以上的过渡。 在湿山和Sangre de Cristo范围内的工作是表征普遍韧性变形和织物复活内和以下的过渡。 地质年代学的重点是科罗拉多中部同运动岩体的U/Pb锆石定年和沿沿着断面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.