從礦物低溫熱定年與應力分析來探討臺灣脊樑山脈剝蝕速率及臺灣南部大地構造演化

Abstract

臺灣造山帶一直以來被認為是標準穩定造山的例子，由北而南碰撞，且5百萬年以來保持地形、地熱及快速剝蝕的穩定狀態。在穩定且快速剝蝕下，形成高海拔的中央山脈，接著發生重力垮塌造成造山帶中心呈現拉張的應力場。本研究利用同樣本中多種不同封存溫度的低溫熱定年法，來分析中央山脈東翼與玉山的剝蝕速率變化，並整合中央山脈晚期斷層野外調查及其古應力反演結果、GPS 應變速率與地震機制解的應力解釋，來討論前人所提出大地構造解釋的問題，再進一步針對臺灣南部山脈擡升的過程提出可能的演化假說。本研究的低溫熱定年結果，顯示中央山脈東翼不同緯度的橫斷面，雖然速率略有不同但呈現相似的三期剝蝕速率變化：第一期，早於2百萬年前，剝蝕速率相當緩慢(~0.1 mm/yr)；第二期，在2-1.5百萬年前到0.5百萬年前，剝蝕速率呈現明顯加速(2.6–5 mm/yr)；第三期，0.5百萬年前到現今，剝蝕速率呈現更進一步地加速(3.7–5 mm/yr)。此結果表示中央山脈呈現斷斷續續的剝蝕且其速率逐漸加快而非之前認為的穩定快速地剝蝕。本研究野外量測再配合GPS資料 及地震機制解，得到橫跨臺灣南部中央山脈與山脈西南部的應力場分布，結果反映了臺灣南部的西南向擠壓突出構造(extrusion)，不過三度空間上的運動方式是中央山脈底下的下部地殼，向西南及垂直方向運動來調適北港高地與呂宋島弧碰撞的西北-東南向壓縮，下部地殼同時向西南及抬升運動造成中央山脈快速地剝蝕及上部地殼呈現東北-西南的伸張。最後本研究根據前人研究西南部擠壓構造始於晚更新世的結果與本研究低溫熱定年數據，推測此西南向擠壓構造的發生可能年輕於0.5百萬年內。

For a long time Taiwan orogen has been regarded as a classic example of the steady southward propagation of the collision model, having been reaching a topographic, thermal, and exhumational steady state for over 5 million years. The high topography of the Central Range is resulted from the steady high exhumation and was proposed to follow by gravitational collapse that caused the extension in the core of the Taiwan orogen. In order to challenge this classic collision model, this study estimates the exhumation rate along the eastern Central Range and Mountain Jade by multiple different low-temperature dating by the same rock samples. Moreover, this study integrated the data from field observations and paleostress inversion of the regional late-stage faults together with GPS strain rate and earthquake focal mechanisms. It helps reevaluate the previous tectonic interpretations and further provide a new hypothesis for the tectonic evolution of southern Taiwan. In low-temperature thermochronological data, three different latitudes that transect the eastern Central Range gain similar results and indicate three stages of exhumation: 1) slow cooling prior to ca. 2–1.5 Ma (~0.1 mm/yr); 2) an acceleration in exhumation rates from ca. 2–1.5 Ma to ca. 0.5 Ma (2.6–5 mm/yr); 3) a further acceleration in exhumation from ca. 0.5 Ma to present (3.7–5 mm/yr). Each transect shows punctuated exhumation with accelerated rates rather than steady exhumation. The scenarios of the tectonic evolution are based on the stress states in the deep crust southwest of the range and along the eastern and western flanks of the range, which are obtained from the paleostress inversion of 141 outcrop-scale faults and the strain rate and stress axes derived from GPS and earthquake focal mechanisms. This study interprets these stress states as reflecting a forceful extrusion of the southern Central Range where the lower crust is being pinched between the Peikang High and the Luzon Arc. Under such a stress state southern Taiwan accommodates the compression by southwest-directed lateral movement and uplift of the lower crust and results in the northeast extension of normal faults in the southern Central Range. Combining the results of the onset timing in the late Pleistocene for the tectonic extrusion in southwest Taiwanand the exhumation rate speeding up after 0.5 Ma, this study further infers that this southwestward extrusion process might occur younger than 0.5 Ma.