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

WannamakerEAR 02 -30027：岩石圈伸展消耗和岩浆作用的模式：在大盆地-科罗拉多高原过渡区（犹他州）用大地电磁电阻率剖面P.I.测试端元情况：Philip E. Wannamaker（犹他州大学，能源地球科学研究所，423 Wakara Way，Suite 300，湖城，UT 84108，pewanna@egi.utah.edu）摘要将根据端元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，测试岩石圈稳定伸展的可能性。研究的主要物理性质是电导率，以及其对热状态和流变状态的影响，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，将把现有的数据加入一个近400公里长的东西向样带，深度范围从100米到250-400公里左右。数据将使用犹他州和华盛顿大学的宽带和长周期记录器获取，这些记录器位于NSF支持的EMSOC仪器设施中。地球物理指标表明，在上地壳岩石圈尺度上，南北走向方向较强，应允许使用主要是当前的二维规则化反演平台模型电导率的准确横截面。从MT场数据推断，下地壳和上地幔深度的电阻率值大幅降低，这很可能反映了熔体和出溶离子流体。以往的测量表明，地壳深部的低电阻率起始于T ~500-550 oC的高级变火成岩中。因此，低电阻率的深度变化将近似于等温线，在TZ变暖均匀的情况下，该等温线将从GB逐渐演变到CP，或者在深度上不均匀的增强伸展的情况下，在小伸展TZ下显示出上升流。出溶流体通过扩散蠕变大大削弱了岩石圈，因此电阻率将提供至少相对流变学对比的清晰视图。传导性、热流和对成分和延伸时间的限制将用于与时间相关的2-D热模拟，以改进热场向上地幔的投影。

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.