全球海床冷卻之沉降率變化

Abstract

大洋之海床水深和其年齡開根號成正比線性關係，其物理機制可用一維熱傳導-半空間冷卻模型來解釋。半空間冷卻模型(以及後續的二維板塊模型、GDH1、GDH2、PSM等修正)可解釋洋脊擴張方向上的觀測，但對沿著中洋脊頂走向的地形或沉降率變化卻沒有完整的討論。 截軸選取廊道結果顯示在海床深度和年齡的開根號的確有線性關係，但沿著洋脊脊軸存在沉降率變化差異，定性上將沉降率變化歸因於洋脊下方的地幔溫度不同，若將沉降率變化完全歸因於溫度差異的話，則溫度異常可達400~600度不等。若進一步避免選取廊道的主觀影響而系統化將區域內逐點計算沉降率變化，則發現太平洋地區受熱柱、小尺度對流、轉型斷層等影響，呈現高沉降率外且擴張歷史複雜無法以單一半空間冷卻模型來解釋。大西洋及東南印度洋脊有清楚的沿脊沉降率變化，以赤道大西洋、澳洲-南極錯亂帶地區有低沉降率。半空間冷卻模型奠基於擴張被動流場(passive spreading flow field)的基本假設，將海洋岩圈視為單純的一維熱擴散向下冷卻，忽略如果地幔存在溫度差異會造成地幔對流動力流場(dynamic flow)的可能影響。但我們發現東南印度洋脊擴張速率均一，地形受冷異常影響而有低沉降率，應為被動流場所主導。

Bathymetry is linearly proportional to the square root of the seafloor age according to decades of observations. It is well explained by the essentially one-dimensional thermal contraction such as that demonstrated in the classical half-space cooling model and the subsequent 2-D modifications such as the plate model, PSM model, GDH1 model and GDH2 model, etc. However, much less efforts have been undertaken on study of variation of seafloor cooling along the ridge axis. The subsidence rates vary along ridges, suggesting that there might be heterogeneous mantle temperature along the ridge. It would require a range of 400 to -600°C for sub-ridge mantle temperature anomalies. To avoid subjective bias that might be embedded in our preliminary corridors selection described above, we propose to take advantage of the moden global data set to perform global systematic subsidence calculation. The history of East Pacific Rise (EPR) sea-floor spreading is complicated, that makes a single cooling model interpretation difficult. The Mid Atlantic Ridge (MAR) and Southeast Indian Ridge (SEIR) indicate clear along strike subsidence rate variation. The SEIR’s sea-floor spreading rate is uniform whereas lower topography and lower subsidence are likely manifestation of the passive flow field.