Upper crustal evolution across the Juan de Fuca ridge flanks
Upper crustal evolution across the Juan de Fuca ridge flanks
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DOI：
10.1029/2008gc002085
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发表时间：
2008-09
期刊：
影响因子：
3.7
通讯作者：
M. Nedimović;S. Carbotte;J. Diebold;A. Harding;J. Canales;G. Kent
中科院分区：
文献类型：
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作者：
M. Nedimović;S. Carbotte;J. Diebold;A. Harding;J. Canales;G. Kent
最新的大洋地壳P波速度汇编表明，在地壳形成后10 Ma，最上层2A的速度增加一倍或达到∼4.3公里/S。这种速度变化通常归因于与脊侧热液循环有关的喷发岩石中低温蚀变矿物的沉淀。沉积物覆盖被认为是一种绝热材料，可以通过促进矿物沉淀来进一步加速第2A层的演化。我们对2002年胡安德·富卡岭多道地震资料中的常见中点超道集进行了一维旅行时模拟，以确定沿奋进、北方对称和裂谷脊段的300公里长的断面在∼3公里间隔处的上地壳结构。我们的结果表明，上地壳速度与地壳年龄之间存在区域相关性。在所调查的山脊两侧，测量到的速度随地壳年龄的增加并不均匀。在5-9 Ma的地壳年龄范围内，覆盖着封闭沉积盖层的海脊两侧的速度增长是稀疏和不连续沉积侧翼的两倍(∼增加60%，∼增加28%)。速度-年龄梯度的外推表明，在沉积盖层两侧，∼为8 Ma时，2A层的速度达到4.3km/S，而在沉积厚度较薄且不连续的两侧，∼为16 Ma。计算的厚度梯度表明，在Juan de Fuca地区，随着地壳年龄的增加或沉积物的覆盖，2A层并没有变薄和消失，而是作为一个相对较低的地震速度层存在，覆盖了更深的洋壳。然而，完全沉积的脊侧剖面上的2A层平均比稀疏和不连续沉积的侧面上的(330±80)m和(430±80)m薄。厚度的变化发生在10-20公里范围内，与沉积物埋藏的开始相一致。我们的结果还表明，传播子尾迹可能具有非典型的层2A厚度和速度。在沿奋进东段的ODP型钻孔采集的流体的化学特征中，传播子尾迹的影响很明显，这进一步表明这些地壳不连续面可能是局部流体流动和蚀变的地点。
Recent P wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2A doubles or reaches ∼4.3 km/s found in mature crust in <10 Ma after crustal formation. This velocity change is commonly attributed to precipitation of low‐temperature alteration minerals within the extrusive rocks associated with ridge‐flank hydrothermal circulation. Sediment blanketing, acting as a thermal insulator, has been proposed to further accelerate layer 2A evolution by enhancing mineral precipitation. We carried out 1‐D traveltime modeling on common midpoint supergathers from our 2002 Juan de Fuca ridge multichannel seismic data to determine upper crustal structure at ∼3 km intervals along 300 km long transects crossing the Endeavor, Northern Symmetric, and Cleft ridge segments. Our results show a regional correlation between upper crustal velocity and crustal age. The measured velocity increase with crustal age is not uniform across the investigated ridge flanks. For the ridge flanks blanketed with a sealing sedimentary cover, the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks (∼60% increase versus ∼28%) over the same crustal age range of 5–9 Ma. Extrapolation of velocity‐age gradients indicates that layer 2A velocity reaches 4.3 km/s by ∼8 Ma on the sediment blanketed flanks compared to ∼16 Ma on the flanks with thin and discontinuous sediment cover. The computed thickness gradients show that layer 2A does not thin and disappear in the Juan de Fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust. However, layer 2A on the fully sedimented ridge‐flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks (330 ± 80 versus 430 ± 80 m). The change in thickness occurs over a 10–20 km distance coincident with the onset of sediment burial. Our results also suggest that propagator wakes can have atypical layer 2A thickness and velocity. Impact of propagator wakes is evident in the chemical signature of the fluids sampled by ODP drill holes along the east Endeavor transect, providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration.