以熱機模型探討海溝後撤與弧後張裂的形成與其影響因素

Thermo-mechanical models on the trench retreat and back-arc spreading

李芷芹(Chih-Chin Lee)

指導教授 ： 譚諤 ； 洪淑蕙

國立臺灣大學/理學院/地質科學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201901933

摘要

隱沒系統常伴隨著弧後擴張，但並非每一個隱沒系統皆有弧後擴張的發生。弧後擴張發生的時機和影響因素至今仍是一個待解的問題。隨著板塊的隱沒，含水礦物在一定溫壓條件下釋放水分並降低了周圍岩漿的熔點，使其更容易部分熔融產生岩漿。這些液體與熔體大大降低了地函楔(mantle wedge)內的強度，形成一低黏滯性地函楔(low viscosity wedge, LVW)。LVW的存在誘發強烈的朝向海溝方向且向上的地函流。上升的高溫地函物質使上覆地殼被拉薄，最後產生弧後擴張。本研究利用FLAC (Fast Lagrangian Analysis of Continua)的演算方式做數值模擬，並以參數化模擬熔體與液體造成地函楔與其上板塊弱化的效應，以探討影響弧後擴張行為的因素。LVW屈服應力和黏滯度會依據當區體積熔化比例下降，且分別降至f_ys和f_v倍。在此研究中對f_ys和f_v的數值進行系統化測試。
　　模擬結果顯示，弧後張裂活動的時間點和隱沒板塊的總長度有關。在板塊隱沒至地函楔後，大量的流體在地函楔中累積並降低當區強度形成LVW。持續回捲的板塊和LVW促使地函流上升產生並拉薄上覆地殼，最終產生弧後擴張中心。弧後擴張中心會隨時間離海溝越來越遠，且在到達一定距離後轉移到距海溝較近的地方。
　　我們可根據f_ys和f_v的不同將弧後擴張模型結果分成四個類型。(一)無弧後擴張(f_ys≥0.5、f_v≥0.5)：此類型中岩石圈的強度下降幅度小，弧後延伸與擴張等行為不易發生；(二)弧前擴張(f_ys>0.5、0.5>f_v≥0.01)：破裂帶沿著隱沒界面發展並延伸到地面，擴張中心發生在弧前；(三)一次弧後擴張(0.5>f_ys≥0.1)和(四)二次弧後擴張(f_ys<0.1)：由於流體的大幅弱化，上覆岩石圈強度驟減並且容易脆性破裂，擴散中心將在火山島弧區形成。比較四個類型與現實中的弧後擴張行為，表明在實際的弧後擴張階段弧後岩石圈的屈服應力需要急劇下降到50％或以下。這項研究可以幫助我們了解弧後區岩石圈的真實強度，同時也為未來的弧後擴散模型提供了可靠的岩石圈強度範圍。

關鍵字

弧後擴張 ； 海溝後撤 ； 數值模擬 ； 低黏滯度楔型 ； 伊豆-小笠原-馬里亞納隱沒帶

並列摘要

Back-arc spreading is usually accompanied by slab rollback and trench retreat, which make overriding plate easier to extend. However, the occurrence and development of back-arc spreading have not been well studied. With the continually subduction of the slab, the aqueous mineral reaches higher temperature and pressure then releases water. The released water hydrates surrounding mantle and consequently induces partial melting of the surrounding mantle. The high concentrations of water in the mantle wedge could decrease the strength of the mantle, forming a low-viscosity wedge (LVW). The presence of LVW provides an opportunity for back-arc spreading to occur because of weak enough strength and induces a strongly upward mantle flow which also toward the trench. The rising hot mantle material causes the overlying crust to be thinned and eventually causes back-arc spreading.
　　In this study, we parameterize the bark-arc spreading processes using thermo-mechanical models with FLAC (Fast Lagrangian Analysis of Continua) technology in the subduction system, in order to explore the influencing factors affecting the back-arc spreading behavior. The generation of LVW is added to this model parametrically. The viscosity and yield stress is decreased by fv and fys times respectively depending on the regional volume melting ratio. We vary fys and fv systematically.
　　Our results show that the duration of back-arc spreading depends on the total length of subducted slab, regardless of convergence rate. With slab subducted, magma begins to build up and accumulates in mantle wedge, reducing the strength. The presence of the low viscosity wedge and the sinking slab induce strong upwelling in the wedge, which thins the overlying crust and eventually produces back-arc spreading. As time spreads, the spreading center will be farther away from the trench. A new center might form closer to the trench. The models with different fys and fv can roughly divide into 4 types. In type Ⅰ ( fys/ fv > 0.5), lithosphere strength is strong, resulting in no back-arc spreading. In type Ⅱ (fys >0.5, 0.5> fv ≥0.01), overriding plate still has great yield stress so the rupture zone develops along the subducting interface and then extends to the surface, forming a spreading center which is close to the trench. And for the type Ⅲ (0.5>fys≥0.1) and Ⅳ (0.1> fys), overriding plate get really weak due to magma and can rupture easily. The spreading center will form inside the overriding plate. Type 3 and 4 is closer to the reality, suggesting that the yield stress in back-arc need to drop sharply to 50-10-1% in the real back-arc spreading stage. This research can help us to understand the real strength of the back-arc region. It also provides a reliable parameter range for future back-arc spreading models.

並列關鍵字

back-arc spreading ； trench retreat ； numerical simulation ； low viscosity wedge ； Izu-Bonin-Mariana subduction system

參考文獻

Anderson, M. O., Chadwick Jr, W. W., Hannington, M. D., Merle, S. G., Resing, J. A., Baker, E. T., ... & Augustin, N. (2017). Geological interpretation of volcanism and segmentation of the M ariana back‐arc spreading center between 12.7° N and 18.3° N. Geochemistry, Geophysics, Geosystems, 18(6), 2240-2274.

Barazangi, M., & Isacks, B. (1971). Lateral variations of seismic‐wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island Arc: Deep anomaly in the upper mantle. Journal of Geophysical Research, 76(35), 8493-8516.

Billen, M. I., & Gurnis, M. (2001). A low viscosity wedge in subduction zones. Earth and Planetary Science Letters, 193(1-2), 227-236.

Brenan, J. M., Shaw, H. F., & Ryerson, F. J. (1995). Experimental evidence for the origin of lead enrichment in convergent-margin magmas. Nature, 378(6552), 54.

Calvert, A. J., Klemperer, S. L., Takahashi, N., & Kerr, B. C. (2008). Three‐dimensional crustal structure of the Mariana island arc from seismic tomography. Journal of Geophysical Research: Solid Earth, 113(B1).