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

EAR-0230027 Wannamaker EAR02-30027：岩石圈伸展消耗和岩浆作用的模式：用电磁电阻率横断面测试犹他州大盆地-科罗拉多高原过渡区的端员案例：Philip E.Wannamaker(犹他州大学能源与工程地球科学研究所，423 Wakara Way，Suite300，Salt Lake City，UT 84108，pewant@egi.utah.edu)摘要端员稳定伸展的可能性将根据它们对跨越犹他州中南部格雷阿特盆地-科罗拉多高原过渡带的地壳深部和上地幔物理性质的影响而进行测试。要研究的主要地球物理性质是电导率及其对热态和流变态的影响，这将使用大地电磁(MT)技术进行成像。新的大地电磁测量主要跨越过渡带(TZ)本身，将现有数据拼接成一条近400公里长的东西向断面，深度范围从100‘S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器获取，这些记录器驻留在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N-S走向，应该可以主要利用目前的二维正则化反演平台来精确地计算模型电导率的截面。从MT现场数据推断的下地壳和上地幔深度的显著降低的电阻率值很可能反映了熔体和出溶的离子流体。前人的测量表明，地壳深部低电阻率的起始温度为T~500-550℃，寄主岩石为高变质岩型。因此，低电阻率的深度变化将近似于一条等温线，在TZ变暖的情况下，该等温线应该从GB逐渐演化到CP，或者在深度不均匀、扩展增强的情况下，在较小延伸的TZ下显示上升流。溶解流体通过扩散蠕变严重削弱了岩石圈，因此，电阻率将提供至少相对流变学对比的清晰景象。传导性、热流和对伸展成分和时间的限制将被用于与时间相关的二维热模拟，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.