美國奧勒岡州奧懷希高原與High Lava Plains岩石圈構造之接收函數分析

Abstract

High Lava Plains與奧懷希高原（Owyhee Plateau）位於美國西北傍太平洋地區（Pacific Northwest）奧勒岡（Oregon）州東部。自中新世（Miocene）以來，此區域發生大量劇烈且持續的火山活動，包括於一千六百六十萬年前（16.6 Ma）所噴發的哥倫比亞河洪流玄武岩（Columbia River flood basalts）。之後於約一千兩百萬年前（12 Ma）開始發展出兩條隨時間遷徙的火山軌跡，即著名的蛇河平原-黃石公園（Snake River Plain-Yellowstone）與High Lava Plains，此兩條火山軌跡約以奧懷希高原為中心，分別往東北與西北方向發展。先前的研究發現High Lava Plains地殼中體波的泊松比（Poisson’s ratio）較高，並且在此區域地殼與淺部地幔中S波波速皆較低。另一方面，奧懷希高原相較於周圍火山活動地區則相對平靜。過去的震波成像研究指出此處有較厚的地殼，環境噪訊成像亦顯示奧懷希高原地殼中的S波波速相對於周圍區域高，支持其為一未受火山活動干擾的微型陸塊。散射波移位成像與Sp接收函數均發現High Lava Plains與奧懷希高原區域中有較薄的岩石圈。本研究利用接收函數（receiver functions）進一步分辨High Lava Plains與奧懷希高原地殼與淺部地幔內不連續面的分布情形。我們選用146個High Lava Plains與USArray寬頻地震網在此區域的測站，在2004年到2009年間記錄的遠震資料進行接收函數分析，觀察Ps轉換波相的到時與波形特徵，推算其對應的不連續面深度與分布。我們發現High Lava Plains岩石圈中普遍存在明顯的低速不連續面的訊號，暗示地殼仍受到熱作用的影響，並可能有部分熔融。High Lava Plains的淺部地幔約50公里深處與奧懷希高原下75公里深皆發現顯著低速不連續面，可能代表岩石圈與軟流圈的交界。我們認為High Lava Plains火山軌跡的起源與胡安•德富卡隱沒板塊roll back所牽動弧後擴張的作用有關。

The High Lava Plains and the Owyhee Plateau are located in the eastern Oregon, Pacific Northwest, USA. The Pacific Northwest has experienced continuous intraplate volcanism from the mid-Miocene to present, including the Columbia River flood basalts and the Steens Mountains flood basalts which erupted 16.6 Ma ago. In addition, there are two prominent age-progressive volcanic tracks, the Snake River Plain-Yellowstone and the High Lava Plains, both of which appear to have originated from near the Owyhee Plateau at approximately 12 Ma. Previous seismic studies have characterized the High Lava Plains with thinned crust, high Poisson’s ratio and low S-wave velocities in the crust and the uppermost mantle. In contrast, the Owyhee Plateau possesses thick crust with low Poisson’s ratio, and high intracrustal S-wave velocities, as well as distinct, albeit thin, mantle lithosphere. Recent ambient noise tomography and scattered-wave imaging reveal the presence of low velocity in the crust of the High Lava Plains and the high velocity layering in the Owyhee mid-crust. On the other hand, low velocities dominate the uppermost mantle. In this study, we perform detailed single station Ps receiver function analysis to better constrain the discontinuity structures within the High Lava Plains and the Owyhee lithosphere. We use teleseismic waveform data recorded at 115 High Lava Plains seismic array and 31 USArray Transportable Array broadband stations from 2004 to 2009. Our results show the coherent signals of low velocity discontinuity in the curst of the High Lava Plains, suggesting the presence of weakened crust and partial melt. We observe coherent signals of high velocity discontinuity at ~20 km depth beneath the Owyhee Plateau, possibly marking the upper bound of the high velocity layer imaged in previous studies, and suggesting that the Plateau represents an isolated block of older, and less modified, continental crust. Our results also reveal low velocity discontinuity in the uppermost mantle, at ~50 km depth beneath the High Lava Plains and ~75 km depth beneath the Owyhee Plateau. We interpret this low velocity discontinuity as the lithosphere-asthenosphere boundary (LAB), and thus both regions have relatively thin mantle lithospheres. We suggest that the origin of the High Lava Plans volcanic track is associated with the Cascadian back-arc extension, induced by the change of mantle flow field related to subduction processes.