A Robust Estimation of the 3-D Intraplate Deformation of the North American Plate From GPS
A Robust Estimation of the 3-D Intraplate Deformation of the North American Plate From GPS
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DOI：
10.1029/2017jb015257
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发表时间：
2018-05-01
影响因子：
3.9
通讯作者：
Blewitt, Geoffrey
中科院分区：
文献类型：
--
作者：
Kreemer, Corne;Hammond, William C.;Blewitt, Geoffrey
冰川均衡调整（GIA）是北美板内变形的主要原因。在这里，我们使用多达3,271个全球定位系统站的速度来拍摄整个板块的三维变形。我们应用一种新的鲁棒应变率估计算法（局部变形的中值估计），它不需要假设板的一部分不受GIA的影响，假设我们证明是错误的。我们的研究结果表明，在劳伦泰德冰盖下的区域的延伸，与半环形带的水平收缩高达类似的4x 10（-9）年（-1）周围的前冰盖。这个收缩带在运动学上与一个类似于1-2 mmyr（-1）的远场水平运动有关，该运动指向冰盖。我们的研究结果，连同一个新的强大的成像垂直速度场，是一致的GIA作为变形的主要原因，虽然前冰盖周围的收缩应变率和远场水平速度显着高于ICE6G_C（VM 5a）模型预测的。这一发现表明，我们的研究结果，包括位置的反转标志的水平运动相对于冰盖，将是有用的，重新评估地幔粘度结构所使用的GIA模型。除了GIA归因的变形之外，我们几乎没有发现其他区域有显着的应变积累。应变率和地震活动之间缺乏板块尺度的空间相关性，这表明GIA是当代地震成因的有限驱动力，板内地震活动必须归因于长期应变积累以外的因素。简明语言摘要板块构造理论认为，构造板块作为刚性块体沿着地球表面移动，地壳只会在板块之间的边界变形。然而，最近高精度全球定位系统台站数量的激增使我们能够捕捉到北美板块内部的一些微妙变形模式，这些模式只有通过对数千个台站之间的相对运动进行非常仔细的分析才能变得明显。我们发现大部分板块正以每年1-2毫米的速度向加拿大中部移动。因此，加拿大大部分地区周围都存在地壳收缩区域。在加拿大境内，地壳正在向外延伸，并迅速向上移动。这些模式可以用地壳和地幔仍然从大约16,000年前被厚厚的冰盖覆盖的时候反弹的过程来解释。事实上，这导致陆地向前冰盖移动，这是一个意想不到的结果，将有助于理解底层地幔的松弛特性。此外，我们发现，北美板块内部的地震不会发生在我们看到地壳变形的地方，这使得这些事件仍然是个谜。
Glacial isostatic adjustment (GIA) is the main cause of deformation in intraplate North America. Here we use up to 3,271 Global Positioning System station velocities to image this 3-D deformation across the entire plate. We apply a new robust strain rate estimation algorithm (median estimation of local deformation), which does not require the assumption that part of the plate is unaffected by GIA, an assumption we show to be false. Our results show extension in the area underneath the Laurentide ice sheet, contrasted with a semiannular belt of horizontal contraction of up to similar to 4x10(-9)yr(-1) around the former ice sheet. This contractional belt is kinematically linked to an similar to 1-2 mmyr(-1) far-field horizontal motion directed toward the ice sheet. Our results, together with a new robustly imaged vertical velocity field, are consistent with GIA as the main cause of deformation, although the contractional strain rates around the former ice sheet and the far-field horizontal velocities are significantly higher than those predicted by the ICE6G_C(VM5a) model. This finding suggests that our results, including the location of reversal in sign of horizontal motion relative to the ice sheet, will be useful to reevaluate the mantle viscosity structure used by GIA models. Besides the GIA-attributed deformation, we find almost no other region with significant strain accumulation. A plate-scale spatial correlation between strain rate and seismicity is absent, suggesting that GIA is a limited driver of contemporary seismogenesis and that intraplate seismicity must be attributable to factors other than secular strain accumulation.Plain Language Summary The theory of plate tectonics says that tectonic plates move as rigid blocks along the Earth's surface and that the Earth's crust should only deform at the boundary between plates. However, the recent explosion in the number of high-precision Global Positioning System stations allowed us to capture some subtle deformation patterns inside the North American plate that only became apparent by a very careful analysis of the relative motions between thousands of stations. We found that most of the plate is moving at 1-2 mm/year towards central Canada. Consequently, around most of Canada there is a zone where the crust is contracting. Within Canada, the crust is extending outward and is moving upward rapidly. These patterns can be explained by the process of the crust and mantle still rebounding from a time when it was covered by a thick ice sheet about 16,000 years ago. The fact that this causes the land to move towards the former ice sheet is an unexpected result that will be useful in understanding the relaxation properties of the underlying mantle. Moreover, we found that earthquakes inside the North American plate do not occur where we see the crust deform, which leaves these events still enigmatic.