Modes of Lithospheric Extensional Consumption and Magmatism: Testing for End-member Cases at the Great Basin-Colorado Plateau Transition with a Magnetotelluric Resistivity Transect

岩石圈伸展消耗和岩浆活动模式：犹他州大盆地-科罗拉多高原过渡端元案例的大地电磁电阻率测试[j]。Philip E. Wannamaker（犹他大学能源与地球科学研究所，423Wakara Way, Suite 300, Salt Lake City, UT 84108, pewanna@egi.utah.edu）摘要：将根据稳定岩石圈延伸对犹他州中南部大盆地-科罗拉多高原过渡带深部地壳和上地幔物理性质的影响，对稳定岩石圈延伸的三种可能性进行测试。要研究的主要地球物理性质是电导率，以及它对热流变状态的影响，这些将使用大地电磁（MT）技术进行成像。新的MT测量主要跨越过渡带（TZ）本身，将现有数据连接到一个近400公里长的东西向样带，深度范围从100米到大约250-400公里。数据将使用犹他大学和华盛顿大学的宽带和长周期记录仪获取，这些记录仪位于nsf支持的EMSOC仪器设施中。现有的地球物理指标在上地壳岩石圈尺度上显示出强烈的南北向走向，并且应该允许使用目前主要的二维正则化反演平台精确的模型电导率截面。根据MT现场数据推断的下地壳和上地幔深度的电阻率值大幅降低，很可能反映了熔体和溶解的离子流体。以往的调查表明，地壳深部低电阻率的起始点为高温~500 ~ 550℃的高品位变质岩。因此，低电阻率的深度变化将近似于等温线，在均匀TZ变暖的情况下，等温线应该从GB逐渐演变到CP，或者在深度非均匀，增强扩展的情况下，在小扩展的TZ下表现出上升流。溶解的流体通过扩散蠕变极大地削弱了岩石圈，因此电阻率至少可以提供相对流变对比的清晰视图。电导率、热流以及伸展的组成和时间限制将用于随时间变化的二维热模拟，以改进上地幔热场的投影。

EAR-0230027WannamakerEAR02-30027: Modes of lithospheric extensional consumption and magmatism: testingfor end-member cases at the Great Basin-Colorado Plateau transition, Utah, with amagnetotelluric resistivity transectP.I.: Philip E. Wannamaker (University of Utah, Energy & Geoscience Institute, 423Wakara Way, Suite 300, Salt Lake City, UT 84108, pewanna@egi.utah.edu)AbstractEnd-member possibilities for stable lithospheric extension will be tested according totheir influence on physical properties of the deep crust and upper mantle across the GreatBasin-Colorado Plateau transition zone, south-central Utah. The main geophysicalproperty to be investigated is electrical conductivity, together with its implications forthermal and rheological state, which will be imaged using the magnetotelluric (MT)technique. New MT surveying primarily across the transition zone (TZ) per se will joinexisting data into an E-W transect nearly 400 km in length, with a depth range ofsensitivity from 100's of m to 250-400 km approximately. Data will be acquired usingwideband and long-period recorders of the University of Utah and University ofWashington, which reside in the NSF-supported EMSOC instrument facility. Existinggeophysical indicators show a strong N-S strike orientation at upper crustal tolithospheric scales and should allow accurate cross sections of model conductivity usingmainly current 2-D regularized inversion platforms. Substantially reduced electricalresistivity values at lower crustal and upper mantle depths inferred from MT fielddata most likely reflect melts and exsolved ionic fluids. Previous surveying indicatesthat the onset of low resistivity in the deep crust occurs for T ~500-550 o C in high-grademetaigneous host rocks. Depth variation in low resistivity thus will approximatean isotherm which should evolve gradually from the GB to the CP in the case ofuniform TZ warming, or show an upwelling under the little-extended TZ in the caseof non-uniform, enhanced extension at depth. Exsolved fluids weaken the lithospheregreatly through diffusion creep, so resistivity will provide a clear view of at leastrelative rheological contrasts. Conductivity, heat flow and constraints on compositionand timing of extension, will be used in time-dependent, 2-D thermal modeling for animproved projection of the thermal field into the upper mantle.