紅河斷裂帶位於兩個大陸地殼的交界，呈西北－東南走向，從中國境內綿延至北越。整個北越地區由兩個地塊所組成，以紅河斷層為界，東北方為南中國地台，西南方則屬於越南－寮國褶皺系統的一部份。由於當地的構造複雜，地質及地物資料較少，因此本研究將利用重力及磁力的資料來探討當地的地下構造。所使用的重力及磁力資料為越南科學研究院地質科學所所提供，分別為布蓋重力異常值以及磁力異常值，再加上Geosat和ERS1兩顆衛星在海上測得的自由空間異常以及地形起伏。我們利用布蓋異常值計算得出區域重力異常及剩餘重力異常，進行構造的探討。 區域重力異常值在紅河凹地呈現相對的高值，從前人研究發表得知該區屬於大陸斷裂帶，推測重力高值應該是受到莫荷面變淺所影響。從重力異常的等值線變化也可看出紅河斷裂帶有往Tonkin灣延伸的趨勢，剩餘異常值反應出北越地區構造斷層的分布情形；磁力異常值則反應出北越地區火成岩出露以及岩漿侵入的位置。本研究亦使用三個經驗公式，由重力異常值估算出地殼厚度，推測北越地區的地殼厚度為32公里至45公里，由西北向東南漸薄。 由於在北越地區所做的探勘有限，對於該區地下構造的探究範圍也因此受限，用重力做出的解釋彼此間仍有很大的差異。若能有更多的震測、鑽井或是其他地質、地物相關的資料，對於地下構造的解釋將會有很大的幫助。 The Red River fault zone has been considered to be a profound geological discontinuity separating the South China platform and the Indochina plate. The consequences of the large scale displacement and major tectonic features in Vietnam, such as rifting and folding, were caused by active of the above two tectonic units. In this study, we combined the Bouguer gravity and geomagnetic anomalies of northern Vietnam, and the free-air gravity anomaly of Gulf of Tonkin. We calculated the regional gravity anomaly and the residual gravity anomaly. We also calculated the crustal thickness over my study region from the gravity and terrain data. Comparison between the calculating results and the previous researches, we have obtained better results beneath the Red River fault zone and its vicinity. The area of gravity high was due to the continental rift zone. The extension of the gravity high revealed the extension of the Red River fault zone. From magnetic anomaly map, the distributions of intrusive and igneous rocks were easily distinguished. The locations of tectonic faults could also be verified from the residual gravity anomaly map. Since we don’t have enough reliable data, the constraints to the deeper structure were limited. Seismic profile and detailed prospecting will help to understand the tectonic of northern Vietnam in the future.
