GPS Measurements of Plate Interactions and Intra-Plate Deformation in the Western Mediterranean

在过去的三年里，麻省理工学院、康奈尔大学和他们的摩洛哥合作伙伴在摩洛哥开发并观测了一个由3个连续（CGPS）和22个测量模式（SGPS） GPS站点组成的网络，以更精确地确定非洲板块的运动，非洲和欧亚板块在地中海西部的相互作用，以及与非洲板块西北部分（摩洛哥）的现代构造相关的变形。在AMIGO项目（大西洋和地中海跨学科GPS观测）框架内，PI在西班牙和葡萄牙的合作伙伴正在进行补充性GPS研究，为这项工作提供了便利。他们在摩洛哥的工作提供了关于板块运动和相互作用的新运动学信息，有望约束大陆碰撞过程和板块内部变形的动态模型。在这项建议中，他们要求支助将摩洛哥全球定位系统网络的监测工作再延长3年，并扩大监测站的覆盖范围，以便在变形最活跃的地区确定更好的应变模式。从2年期间（10/99 - 10/01）的三次调查中获得的初步GPS站点速度已被分解为~2毫米/年（1西格玛，或5毫米/年95%）。这些不确定性相当于整个非洲-欧亚大陆的辐合速率（在摩洛哥为~4 - 5毫米/年），还不足以为非洲板块运动或板块内变形提供可靠的约束条件。然而，目前的速度场确实表明，在裂谷山脉以南的站点，相对于欧亚大陆的整体西北纬向运动约为6±2毫米/年（1西格玛），这与我们根据现有的整个非洲板块GPS数据对非洲板块运动的新GPS估计（约4.7毫米/年西北纬向）在不确定性范围内是一致的。相比之下，位于摩洛哥西北部裂谷山脉西部的遗址表明，向北运动明显缓慢，可能向南运动，表明阿尔博兰海今天的延伸。这两个观测结果与非洲板块持续的北西向运动、阿尔博兰海（摩洛哥和西班牙之间的地中海最西端）的活跃伸展以及西裂谷和东裂谷之间的左侧走滑变形相一致。这种总体上的一级模式与地震和新构造现场研究估计的变形一致。PI要求继续支持3年，以便：1)通过2002年、2003年、2004年和2005年的调查，在现有的SGPS站点上更好地求解速度（所有现场费用由摩洛哥方面承担），并将摩洛哥速度场与伊比利亚的GPS观测相结合；2)在裂谷山脉建立10个新的SGPS站点，以更好地约束明显的晚内变形（缩短和左侧，走滑）；3)开发并应用弹性块体和有限元模型来研究地中海西部的活动变形；4)进一步发展摩洛哥在gps地球动力学应用方面的国内能力。最后一项任务将使摩洛哥合作伙伴（保留我们联合项目提供的设备）能够在请求的赠款期限之后，在摩洛哥的支持下，自己进行今后所有的GPS观测。我们希望未来NSF对这项工作的支持请求，如果需要的话，将仅限于数据集成、建模和解释。

For the past three years, MIT, Cornell University, and their Moroccan partners have developed and observed a network comprising 3 continuous (CGPS) and 22 survey-mode (SGPS) GPS sites in Morocco to determine more precisely Africa plate motion, the interaction of the African and Eurasian plates in the western Mediterranean, and deformation associated with present-day tectonics of the NW part of the African plate (Morocco). This work is being facilitated by complementary GPS studies being pursuedby the PI's partners in Spain and Portugal with whom they are collaborating within the framework of the AMIGO Project (Atlantic and Mediterranean Interdisciplinary GPS Observations). Their work in Morocco is providing new kinematic information on plate motions and interactions that promise to constrain dynamic models for continental collision processes as well as intra-plate deformation. In this proposal, they request support to continue to monitor the Morocco GPS network for an additional 3 years, and to densify station coverage to determine better strain patterns in the most actively deforming regions.Preliminary GPS site velocities derived from three surveys over a 2 year period (10/99 - 10/01) have been resolved to ~2 mm/yr (1 sigma, or 5 mm/yr 95%). These uncertainties are on the order of the entire Africa-Eurasia convergence rate (~4 - 5 mm/yr in Morocco), and are not yet sufficient to provide reliable constraints on Africa plate motion or intra- plate deformation. However, the present velocity field does indicate overall WNW motion relative to Eurasia at about 6 +or- 2 mm/yr (1 sigma) for sites located south of the Rif Mountains, consistent within uncertainties with our new GPS estimates of Africa plate motion derived from available GPS data for the entire African plate (~ 4.7 mm/yr WNW). In contrast, sites located along the western Rif Mountains in NW Morocco indicate significantly slower northward motion, and possibly southward motion, suggesting present-day extension in the Alboran Sea. These two observations are consistent with the continuing NW motion of the African plate, active extension of the Alboran Sea (westernmost part of the Mediterranean between Morocco and Spain), and left-lateral, strike slip deformation between the western and eastern Rif. This overall, first-order pattern is consistent with deformation as estimated from earthquakes and neotectonic field studies. The PI's request continued support for 3 years to: 1) resolve better velocities on existing SGPS sites with surveys in 2002, 2003, 2004, and 2005 (all field costs borne by the Morocco side), and integrate the Morocco velocity field with GPS observations from Iberia, 2) establish 10 new SGPS sites in the Rif Mountains to constrain better apparent intra- late deformation (shortening and left lateral, strike-slip), 3) develop and apply elastic block, and finite element models for active deformation in the western Mediterranean, and, 4) further develop Moroccan in-country capabilities in GPS-geodynamic applications. This last task will allow the Moroccan partners (retaining the equipment provided by our joint project) to carry out all future GPS observations on their own and with Moroccan support beyond the requested grant period. We expect that future requests for support from NSF for this work, if required, will be restricted to data integration, modeling, and interpretation.