低溫定年法在造山帶之大地構造應用 (II)

行政院國家科學委員會專題研究計畫 成果報告

低溫定年法在造山帶之大地構造應用 (II)

研究成果報告(精簡版)

計 畫 類 別 ： 個別型 計 畫 編 號 ： NSC 95-2116-M-002-009- 執 行 期 間 ： 95 年 08 月 01 日至 96 年 07 月 31 日 執 行 單 位 ： 國立臺灣大學地質科學系暨研究所 計 畫 主 持 人 ： 楊燦堯 計畫參與人員： 學士級-專任助理：張佳菱 博士班研究生-兼任助理：莫妮卡 碩士班研究生-兼任助理：鍾靈 報 告 附 件 ： 出席國際會議研究心得報告及發表論文 處 理 方 式 ： 本計畫可公開查詢

中 華 民 國 96 年 11 月 16 日

行政院國家科學委員會補助專題研究計畫

■ 成 果 報 告

□期中進度報告

低溫定年法在造山帶之大地構造應用 (II)

計畫類別：

■

個別型計畫 □ 整合型計畫

計畫編號：NSC 95-2116-M-002 -009 -

執行期間：95 年 8 月 1 日至 96 年 7 月 31 日

計畫主持人：楊燦堯

共同主持人：

計畫參與人員：莫妮卡、鍾靈

成果報告類型(依經費核定清單規定繳交)：

■

精簡報告 □完整報告

本成果報告包括以下應繳交之附件：

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

■

出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、

列管計畫及下列情形者外，得立即公開查詢

□涉及專利或其他智慧財產權，

■

一年□二年後可公開查詢

執行單位：

II

第一章 緒論

1.1 研究動機與目的

近幾十年來許多人投注心力在研究青藏高原與喜馬拉雅山的形成歷史上。前人

的研究多著力於地體構造模式與高原抬升時間的探討（Tapponnier et al., 1982;

England and Houseman, 1989; Houseman et al., 1981; Harrison et al., 1992; Platt and

England, 1993），這些問題雖已有相當多的年代資料佐證，但之前的研究多採用封存

溫度較高的定年礦物來分析，故本研究選擇以低溫定年法中的核飛跡定年分析，來

探討青藏高原東南部晚期的熱歷史。

喀拉崑崙－嘉黎斷裂帶是由數個右移的錯動帶所組成的巨大斷裂帶，也是向東

脫逸之北部青藏高原塊體的南界。嘉黎斷層位於此東西橫貫之斷裂帶的最東端，並

且是新生代以來構造活動劇烈的東喜馬拉雅構造體系（南迦巴瓦山結）的北界，其

活動不只與向東脫逸的青藏高原塊體有關，更與至今仍構造活動強烈的南迦巴瓦山

結息息相關。正因為它位於一個新構造運動擠壓錯動強烈的地區，因此以核飛跡定

年法來研究嘉黎斷裂帶，希望能對解釋西藏東部的熱歷史提供進一步的制約。本研

究將結合核飛跡定年法的結果與前人已發表的定年結果，探討西藏東部自中新世以

來的熱歷史。

本研究區域的嘉黎斷層位於在青藏高原上東西綿延

2000 公里的喀拉崑崙－嘉

黎斷裂帶的東緣，自

30°42’N, 92°50’E 處起向 N100°-105°E 的方向延伸超過 200 公

里（Armijo et al., 1989）。嘉黎斷裂帶在經過了南迦巴瓦山結之後向東南方分成了兩

個分支，位於較北邊的帕隆斷層（Parlung Fault）沿著帕隆河谷發育，位於較南邊的

布曲斷層（Puqu Fault）沿著布曲河谷發育（Armijo et al., 1989）。嘉黎斷裂帶的活動

方式與喀拉崑崙斷裂帶的活動方式相似，皆是右移剪切並結合逆衝斷層的特性

（

Armijo et al., 1989; Molnar and Deng, 1984）。嘉黎斷裂帶位於向東脫逸的西藏塊體

脫逸方向由東轉成東南之處，而且正好位於東喜馬拉雅山結的北界，是山結北緣活

動最強烈的一條斷層。同時受到劇烈擠壓與旋轉的應力，這個地區的構造歷史成為

學者們關切的重要課題。本研究之樣本即採自於嘉黎斷裂帶及其兩分支斷層，期望

能更瞭解嘉黎斷裂帶新生代的冷卻歷史與其在印度板塊、歐亞大陸板塊碰撞過程中

所扮演的角色。

第二章 結果與綜合討論

2.1 察隅地區嘉黎帕隆與布曲斷層之異時活動

本研究中察隅地區的核飛跡定年分析結果呈現非常明顯的兩群分布，來自察隅

河上游波密－察隅岩體的核飛跡年齡均老於察隅河中游波密－察隅岩體的核飛跡年

齡，顯示出察隅地區自中新世以來北區與南區有著不同的熱歷史演化。

採集自察隅北區帕隆斷裂帶的樣本

ET117 與 ET106，其磷灰石與鋯石核飛跡定

年結果為

22.7 ± 1.9 Ma 與 19.6 ± 5.0 Ma。樣本雖然採集於斷裂帶的岩體中，但岩體

本身並沒有被斷層活動所產生的應力扭曲而變形，故推斷這兩個樣本的核飛跡年代

所代表的地質意義，應為伴隨斷層活動產生的岩體剝蝕抬升所造成的冷卻事件年齡，

表示在約

23 Ma 時此岩體已被抬升至約 3 公里深處，之後帕隆斷裂帶的活動漸趨平

緩。

Ding et al. (2001) 對一個在南迦巴瓦山結北方的嘉黎斷裂帶變形區中採集的石榴

子石－黑雲母－白雲母淡色花崗岩進行鈾－鉛定年分析。結果顯示此岩體的生成年

齡為

22.6 ± 0.3 Ma。Ding et al. (2001) 指出此花崗岩體可能為嘉黎斷裂帶剪切運動發

生時同期產生的淡色花崗岩體，故此鈾－鉛年齡表示嘉黎斷裂帶錯移的時間可能早

於

23 Ma，較 Lee et al. (2003)所發表之嘉黎斷裂帶活動高峰期（18－12 Ma）更早。

Lee et al. (2003)對察隅南區的糜嶺岩氬－氬定年分析結果顯示，布曲斷裂帶的主

要活動時間為

18－12 Ma。他們推論藏東的岡底斯岩體在 110 Ma 生成之後經歷了兩

個階段的冷卻歷史，首先是岩體形成後的一段漫長冷卻時期，18 Ma 之後進入快速

冷卻的階段，岩體的冷卻速率也從

2°C/Ma 增加至>40°C/Ma。這次的快速冷卻事件

應該是導因於布曲斷裂帶

18－12 Ma 的右移剪切運動，同時將其破裂帶中的岩體擠

壓抬升出地表所致。

Lin et al. (2005)也對波密至沙馬、察隅沿線的嘉黎－布曲斷裂帶

岩體進行了系統性的採樣，其氬－氬定年分析結果顯示嘉黎－布曲斷裂帶的活動時

間為

26.8－11.0 Ma，較 Lee et al. (2003)所推論的活動期間更長，而與本研究之結果

相符。由上述結果可推論嘉黎斷裂帶可能自～27 Ma 開始活動，但～27－～19 Ma

時可能為嘉黎斷裂帶與其南北兩分支斷層皆有活動，而～18 Ma 之後，帕隆斷裂帶

活動減緩，主要活動的斷層剩下嘉黎斷層與其南分支布曲斷裂帶，並持續活動至～

11 Ma。

本研究中採集自察隅南區的四個樣本只有

ET115 有受到斷層構造運動的影響而

變形，其餘三個標本皆是未受擾動的花崗岩體。但這四個樣本的核飛跡年齡皆落在

相同的時間範圍內（鋯石：

9.0 ± 0.8－10.0 ± 0.7 Ma；磷灰石：5.5 ± 2.5、5.8 ± 0.6 Ma），

表示這些核飛跡年代記錄的應該是斷層活動後期地體抬升的冷卻事件，並不代表斷

層活動的時間。

綜合上述的結果可推知，嘉黎斷裂帶的活動年齡最早可追溯至約

27 Ma。由本

研究的核飛跡定年結果可看出約

23－19 Ma 的時候此區北部的嘉黎－帕隆斷裂帶有

一次快速抬升的時期，將原本在深處的波密－察隅花崗岩體抬升至離地表約

3 公里

處。此次的快速剝蝕抬升事件應與嘉黎－帕隆斷裂帶的錯移有關，但

18 Ma 之後帕

隆斷裂帶的活動趨於平靜，剩下南區分支的嘉黎－布曲斷層活動持續活躍至晚中新

世（～11 Ma）。

由於嘉黎斷裂帶位在北西藏塊體脫逸方向由東轉為東南的關鍵轉折位置，此斷

裂帶在印度板塊與歐亞大陸板塊碰撞聚合的框架中所扮演的角色無疑地是相當重要

的。若我們將眼光放大至整個東亞地區，與嘉黎斷裂帶活動時間（27－11 Ma）相當

的斷層還有雲南至越南北部的紅河－哀勞山斷裂帶（

27－17 Ma，Wang et al., 1998）、

由雲南延伸至緬甸境內的高黎貢剪切帶（17－9 Ma，許，2000）

、Sagaing 斷層（20.7

－16.6 Ma，Betrand et al., 1999），以及藏東的仙水河斷裂帶（～15 Ma，Roger et al.,

1995）

。紅河－哀勞山斷裂帶的左移時期與嘉黎斷裂帶活動能量南移之前的時代（～

23 Ma）重疊，暗示了此兩斷裂帶的活動與中南半島沿著紅河－哀勞山斷裂帶脫逸的

活動有關。但進行此運動的塊體界線與塊體內部的變形機制還需更多的地質定年資

料與野外調查來證實。而嘉黎斷裂帶主要活動南移之後的時期與高黎貢剪切帶、

Sagaing 斷層以及仙水河斷裂帶錯移年代相當。若結合 Sato et al. (1999)於雲南西部的

古地磁研究與

1991－1998 年間青藏高原東部的 GPS 觀測資料（Chen et al., 2000），

我們可以推論自從嘉黎斷裂帶開始活動以來（～27 Ma 之後），其活動可分為兩個階

IV

脫逸。～18 Ma 之後，紅河－哀勞山斷裂帶的活動趨於平靜，但印度持續碰撞歐亞

大陸板塊，故此時期的能量改由藏北塊體順時鐘旋轉運動所吸收，這個時期嘉黎斷

裂帶中主要活動的斷層剩下嘉黎－布曲斷裂帶。而此運動的塊體東界可能是仙水河

斷裂帶（Xianshuehe Fault），西界是嘉黎－布曲斷裂帶與高黎貢剪切帶（Gaoligong

Fault）、Sagaing 斷層（許，2000）。因為有這兩期運動塊體的轉變，使得我們在察隅

地區的帕隆斷層與布曲斷層之核飛跡定年結果記錄到非常不同的熱歷史。

2.2 通麥與察隅地區中新世以來相異的熱歷史演化

本研究中採集自通麥地區的樣本皆分析出相當年輕的核飛跡年齡，顯示自

6 Ma

以來察隅地區與通麥地區不同的熱歷史演化。

通麥位於南迦巴瓦山結的北邊，這個地區岩體的歷史除了與嘉黎斷裂帶活動關

係密切之外，還與鄰近的東喜馬拉雅構造結息息相關。南迦巴瓦峰地區屬於喜馬拉

雅構造體系，但南迦巴瓦峰群的變質程度要比高喜馬拉雅結晶岩系大，印度板塊在

此呈現鋸齒狀嵌入歐亞大陸板塊，因此南迦巴瓦峰周圍的地區是碰撞變形得最嚴重

的地段，雅魯藏布縫合帶在這裡劇烈的錯移與轉折，因聚合擠壓所造成的強烈抬升

與侵蝕作用使得青藏高原下部的地殼物質得以裸露至地表。嘉黎斷裂帶正好位於這

樣一個碰撞前緣的北界，許多研究者對南迦巴瓦山結進行低溫定年法的分析工作，

所得研究成果顯示出構造結內的年代年輕於周圍地區的年齡（Burg et al., 1998；Ding

et al., 2001）。Burg et al. (1998)整理了前人對於南迦巴瓦山結及其周圍區域已發表之

鉀－氬定年成果。整體說來，整個區域內的鉀－氬定年年代分布於

39－3 Ma，但所

有小於

14 Ma 的年代都集中在山結的核心區域。不止鉀－氬定年數據記錄到南迦巴

瓦核心區域年輕的活動記錄，

Burg et al. (1998)針對南迦巴瓦山結核心區域與外圍區

域進行鋯石與磷灰石的核飛跡定年法分析，其山結構造區內的核飛跡年代也小於周

圍地區的核飛跡年代。鋯石與磷灰石核飛跡年代皆小於

3 Ma，與

潘裕生與孔祥儒（1998）

所得之結果相同，說明了南迦巴瓦峰地區自

3 Ma 以來經歷了加速抬升的時期，3 Ma

以來的平均抬升速率可達

3.3 mm/yr，近期（0.08 Ma 至今）抬升速率更高達 30 mm/yr，

是目前世界上抬升最快速的地區之一（

潘裕生與孔祥儒，1998

）

。

Burg et al. (1998)同時也在通麥地區得到一個非常年輕的磷灰石核飛跡定年結果

（0.5 ± 0.4 Ma），雖然通麥地區不在山結構造區內，但因地處

嘉黎斷裂帶之上，是南迦巴瓦山結北方最活躍的斷層，當印度板塊前緣不

斷以鋸齒狀嵌入歐亞大陸板塊時，凝聚的應力除了由擠壓抬升的山結與河流下切的

深谷所表現出來外，部分還由嘉黎斷層的剪切活動所吸收掉，因此通麥地區的核飛

跡年代才會與南迦巴瓦峰群內的年代相近。本研究在通麥地區兩個樣本之鋯石與磷

灰石核飛跡年代皆小於

3 Ma，與 Burg et al. (1998)、

潘裕生與孔祥儒（1998）

結果相

近，代表

3 Ma 以來，南迦巴瓦峰的快速剝蝕抬升事件同樣活化了其北方的嘉黎斷裂

帶，造成破碎帶中的岩體快速抬升。但根據本研究於察隅地區的核飛跡定年結果顯

示，此影響是局部性的，活化的動力並沒有傳送到嘉黎斷裂帶向東南延伸出去的兩

個分支斷層上。

綜合前面討論可推知藏東地區在

11 Ma 之後至少有兩期快速抬升的時期，分別

為

11－6 Ma 與 3 Ma 至現今。同樣的兩次快速抬升事件也被紀錄在孟加拉灣水下扇

的沈積歷史中。由於孟加拉灣水下扇的沈積物主要來自印度東北注入孟加拉灣中的

布拉馬普特拉河，而布拉馬普特拉盆地中的沈積物絕大部分是來自高喜馬拉雅

（Higher Himalayas）與青藏高原的碎屑物質（Garzanti et al., 2004)，本研究區域中

的察隅河以及鄰近區域的雅魯藏布江、布拉馬普特拉河與帕隆藏布河也發源與此。

故研究孟加拉灣水下扇的沈積物特性與沈積速率應能得到喜馬拉雅山與青藏高原抬

升的相關訊號，對於解開剝蝕抬升歷史應可提供更進一步的資訊。

Amano and Taira (1992)根據 ODP Leg 116 於孟加拉灣水下扇所鑽探的岩心樣本，

指出喜馬拉雅山與青藏高原在

10.9－7.5 Ma 與 0.9 至現今有兩次快速抬升的事件發

生，因為源區的地體抬升造成河流下切速率增加，孟加拉灣水下扇的沈積速率於

10.9

Ma 時第一次急速增加，至 6.5 Ma 左右趨於平緩；而源自高喜馬拉雅的沈積物也在

10.9－7.5 Ma 達到高峰。本研究於察隅河中游的鋯石樣本所記錄到的抬升事件與此

沈積速率增加的時期吻合，表示

10.9－7.5 Ma 除了喜馬拉雅山有大規模的地體抬升

之外，影響區域還達到嘉黎斷裂帶東南方的布曲斷層與察隅地區。孟加拉灣水下扇

於

0.9 Ma 開始第二次加速沈積，顯示喜馬拉雅山在 ca. 0.9 Ma 時可能開始大規模的

抬升。但根據

Burg et al. (1998)、Ding et al. (2001)與本研究的結果，南迦巴瓦峰群應

在

3 Ma 時即開始快速抬升，並且帶動其北方的嘉黎斷裂帶擠壓抬升出破碎帶內的岩

體。本研究推斷

3 Ma 開始的冷卻事件應為南迦巴瓦峰區域性的構造事件，這個時期

東南邊的察隅岩體應該是相對平靜無活動的。

中新世晚期以來，不止喜馬拉雅山有抬升活動發生，許多西藏高原的東緣地區

也在～11 Ma 之後有迅速隆升的紀錄。位於青藏高原與四川盆地交界處之龍門山 11

－4 Ma 時有一次快速抬升的紀錄（Kirby et al., 2002），龍門山西南部的仙水河斷裂

帶（Xu and Kamp, 2000）自～130－～22 Ma 也是先經歷了緩慢的冷卻時期（～

1°C/m.y），在中新世晚期才開始快速隆升。位於龍門山與仙水河斷裂帶南方的安寧

河地區（Clark et al., 2005）也在 13－9 Ma 記錄到快速剝蝕事件。 雖然許多研究顯

示藏東地區在中新世晚期有快速抬升的紀錄，但是青藏高原東緣新生代的侵蝕與隆

起並不是大區域性的，而是有集中在某個狹小區域內定點侵蝕速率上升的現象，特

別是沿著高原的邊緣地勢落差大的地區（Kirby et al., 2002）

。將本研究與其他研究之

數據做對比，推知青藏高原東部與東南部邊緣（本研究區域）地區在～11 Ma 開始

快速隆起，這個時期的抬升可能為高原邊緣達到現今高度的關鍵時期，但每個地區

的抬升強度不一，強烈抬升特別發生在東喜馬拉雅山結與其北界的嘉黎斷裂帶內。

2.4 地表抬升對於河流溯源襲奪影響之討論

藏東地區最引人注目的地質景觀除了雄偉聳立的南迦巴瓦峰之外，就是環繞峰

群做奇特大拐彎轉而南向奔流的雅魯藏布江了。南迦巴瓦山結是構造抬升非常快速

的地區這點是無庸置疑的，撇除現有的許多地質定年與野外調查資料不提，只從南

迦巴瓦峰的最高點（7767 m）至鄰近的雅魯藏布江峽谷有著近 5000 公尺的落差便可

推知端倪。但是什麼樣的機制造就了奇特的雅魯藏布大拐彎卻引發了許多人研究的

興趣。Clark et al. (2004) 提出了西藏東部河川襲奪的框架來解釋雅魯藏布江呈現馬

VI

在構造活動頻繁、地殼岩石變形劇烈的地區，地表河川系統的分布對於地形的

抬升是相當敏感的（Clark et al., 2004）。印、歐兩板塊的聚合不止造就了高聳的喜馬

拉雅山與青藏高原，還促使中南半島沿著紅河－哀勞山斷裂帶向東南脫逸，影響中

國南海的張裂。藏東地區正好位於劇烈擠壓與抬升的變形前緣。中國最著名的三條

河川：揚子江（金沙江）

、瀾滄江（湄公河）與怒江（薩爾溫江）發源自青藏高原後

向東南方蜿蜒而下，來到藏東地區後三條河川的流域面積頓時縮減，三江的主流呈

現近乎平行轉而南流，直到流出藏東地區三江的流域才又伸展開來。Hallet and

Molnar (2001)指出這三條大河在藏東地區的流域寬度比上主流長度的比例是不合乎

常理的低。以三江中的湄公河為例，與世界上主要河川的流域寬度來比較，湄公河

預期的流域寬度應為

437 ± 152 公里，但其在藏東地區的流域寬度實際上是 32 ± 7

公里。如此小的寬長比在世界上的大型河川中是相當少見的。造成三江流域面積縮

減的可能解釋為因板塊聚合導致地殼岩石擠壓變形，由於藏東地區正好位於變形前

緣，板塊聚合造成了地體在垂直方向上的增厚以及水平方向上距離的縮減，絕大部

分累積的能量由抬升的地表與緊縮的河川流域反應出來，同時因為岩體抬升而加速

河川下切的動力，所以藏東才會有如此山高水急的峽谷地形產生。

Clark et al. (2004)所提出的河川襲奪假說並無可靠地質證據來支持其理論，所有

的理論基礎是建立在現今「奇異」的水系分布圖上。依照

Clark et al. (2004) 的假說，

雅魯藏布江原本並未繞過南迦巴瓦山結，而是向東北流經帕隆藏布河後注入依洛瓦

底江，之後察隅河溯源侵蝕襲奪依洛瓦底江的源頭，最後布拉馬普特拉河自南迦巴

瓦山結旁襲奪了雅魯藏布江而形成現今所見之奇異大拐彎地形。但

Liang et al. (2006)

研究雅魯藏布江和其支流的河沙鋯石以及依洛瓦底江江畔的沈積物樣本，發現可靠

同位素以及定年證據指出雅魯藏布江的確曾經流入現在的依洛瓦底江。這個發現強

力地支持了藏東河川襲奪的假說。

本研究於嘉黎斷裂帶所採之樣本雖然不是直接的河川襲奪證據，但提供了河川襲奪

先後順序上的可能證據。原本的雅魯藏布－帕隆－依洛瓦底江可能因為察隅地區

10

－6 Ma 的快速抬升加速了河川下切的力量，使得古察隅河的源頭（即本研究察隅河

中游採樣點的鄰近區域）溯源侵蝕，襲奪了依洛瓦底江，於是雅魯藏布江變成順著

帕隆－察隅河再匯入布拉馬普特拉河。但由於南迦巴瓦山結在

3 Ma 之後快速的抬升，

察隅地區河川下切的動力慢慢消失，溯源侵蝕的動力轉移到後期持續劇烈活動的南

迦巴瓦峰地區，促使了布拉馬普特拉河襲奪雅魯藏布江。直到目前為止還沒有明確

的證據可以指出藏東地區河川襲奪的確切時間，地形抬升促使河川下切的反應時間

有多快速也需要更多的研究來解答。本研究提供了一個時間上可能的順序來解答藏

東河川襲奪的假說。

本研究結果已準備投稿於國際期刊。

另外，本研究另一部分於喜馬拉雅山西北部的工作，亦已經被接受發表於國際期刊

(如後附)。

Walia, M, Yang, T.F., Liu, T.K., Kumar, R. and Chung, L. (2007) Fission-track dates of the Mandi Granite and adjoining tectonic units in Kulu-Beas valley, NW Himalaya, India. Radiation

Fission-track dates of Mandi Granite and adjacent tectonic units in

Kulu-Beas valley, NW-Himalaya, India

Monika Waliaa, Tsanyao Frank Yanga*, Tsung-Kwei Liua, Ravindra Kumarb, Ling Chunga

a_{Department of Geosciences, National Taiwan University, Taipei, Taiwan} b_{Centre of Advanced Study in Geology, Panjab University, Chandigarh, India}

Abstract

We present zircon fission track (FT) ages from the lower parts of the High Himalayan Crystalline (HHC) ranging from 3.9 Ma to 4.3 Ma and apatite FT age as 1.9±0.4 Ma. Combined with previous radiometric dates, these ages provide the low temperature data for the area and enable us to draw temperature-time plot which shows significant increase in cooling rates from 25ºC/Ma in Oligocene and 4ºC/Ma in Miocene to about 64ºC/Ma during Pliocene. For the Mandi granite, the zircon FT ages range from 3.9 Ma to 4.9 Ma while apatite FT obtained is 2.8±0.5 Ma. The rapid Pliocene exhumation is characterized by a rapid increase in cooling rates from ~3.4ºC/Ma in Miocene to ~77ºC/Ma after 5.0 Ma. Our results confirm the rapid Pliocene to Pleistocene cooling of rocks of Kulu-Beas valley similar to that of the neighboring units from the frontal parts of HHC.

Keywords: Fission track dating; NW-Himalaya; Exhumation

*_{Corresponding author}

E-mail: [email protected] (T. F. Yang)

2

1. Introduction

The ongoing collision between Indian Plate and the Eurasian Plate causes deformation, crustal thickening and surface uplift of the Himalayan orogen. Various thrust zones divide the Himalayan orogen into its major tectonic units (Fig. 1a). From north to south these are: Transhimalaya, Indus Tsangpo Suture Zone (also known as Indus-Yarlung Suture Zone), Higher Himalaya, Lesser Himalaya and Sub Himalaya (Gansser 1964). Thrusting within the Himalaya as a response to the collision is considered to have propagated from north to south: the northern most suture zone was active at ca. 50-55 Ma (Searle et al., 1990), the Main Central Thrust (MCT) was active at ca. 20-22 Ma (Hubbard and Harrison, 1989) and the Main Boundary Thrust (MBT) was active after 10 Ma (Meigs et al., 1995). Moreover, south directed thrusts including the MCT and the MBT accommodated about 1400 km of north-south shortening between the two continents (Burbank et al., 1996). Immense size and high elevation of Himalaya-Tibet orogen is considered to have played a critical role in controlling the global climate change. This climate change is reported to have affected erosion rates (Yin and Harrison, 2000).

The Himalayan Metamorphic Belt (HMB) is the Proterozoic Indian crust which was deformed and remobilized in Cenozoic due to the collision. It is bounded to the south by the MCT and to the north by the South Tibetan Detachment System. Exhumation of this metamorphic belt was speeded up after ~20 Ma. HMB is intruded by numerous Paleozoic granitoids. One of these is the Mandi Granite of batholithic dimensions.

Very little published geochronological data pertain to the low temperature thermal history of the Mandi Granite and its adjoining units in HMB. Here we present

fission track ages of zircon and apatite along a section crossing major structures like MBT and HMB. Interpreting the spatial age variation in comparison with the data available from neighboring areas strengthens the relation of tectonic activity on erosion.

2. Regional Geology

The investigated area lies in the Kulu-Beas valley in NW-Himalaya, Himachal Pradesh, India. Shali Belt of Proterozoic metavolcanics-quartzite-dolomite is thrust over the Subhimalaya along Main Boundary Thrust (Fig. 1b). This belt is bounded by the Chail Thrust, a splay of the MCT, along its northeastern boundary. Further north of the Chail Thrust is the Himalayan Metamorphic Belt comprising of Jutogh nappe belonging to Higher Himalayan Crystallines (HHC). Various units in the studied area from north to south are (Fig. 1b):

Higher Himalayan Crystallines (HHC): The Main Central thrust (MCT) is a

shear zone of a few kilometers to >10 km thickness. The hanging wall of the MCT is the High Himalayan slab of Indian plate (Fig. 1). The mineral assemblages quartz + plagioclase + muscovite + biotite + garnet + staurolite + kyanite are typical of the medium-pressure, high temperature metamorphism.

Mandi Granite: Foliated and non-foliated granitic rocks of Mandi Granite belong

to the Lesser Himalaya. The unit is intruded into Chail Metamorphics (Gupta, 1970) and contains quartz, plagioclase, muscovite, biotite as the major constituents and epidote, monazite, zircon, apatite and tourmaline are accessory minerals. The intrusive body is considered to be of Paleozoic age based on the Rb-Sr whole rock isochron age of 545±12 Ma (Mehta, 1977).

4

Main Boundary Thrust (MBT): A shuppen zone, i.e., a zone of parallel thrusts,

marks the contact between the Lesser Himalaya and the underlying Siwalik Formation and is termed as the MBT. The slip on the MBT began at around 11 Ma as reported by Burbank et al. (1996) based on magnetostratigraphy records of the Himalayan foreland.

3. Fission Track Analyses:

Apatite crystals were mounted in epoxy resin and polished. Tracks were revealed by etching with 4% HNO3 at 21±1ºC for 25±2 sec. U-free muscovite was used as external detector attached to each sample mount. Zircon crystals were mounted in teflon sheets. Etching was done using KOH-NaOH-LiOH eutectic solution for 7-8 hours at 230±5ºC. Dosimeters used with apatite and zircon were NBS-612 and NBS-610, respectively. The samples were irradiated at Hsing-Hua University, Hsinchu, Taiwan. After irradiation, muscovite sheets were etched using 48% HF for 20 minutes at room temperature. Counting was performed at 2500 X total magnification (oil immersion). Grain-by-grain and external detector method with error of 1σ is considered for age calculations. The detailed analytical method adopted is given in Yang et al. (1999, and 2003).

4. Results and Discussion

The analytical data of zircon and apatite samples is presented in Table 1. Ages were calculated using the Binomfit program (Brandon 1996). The probability-density plots are given in Figure 2. For calculating the cooling rates, closure temperatures of 110ºC for apatite, 250ºC for zircon, 300ºC for Rb-Sr biotite and 500ºC for Rb-Sr

muscovite (Lal et al., 1999) have been considered. The slope of each line segment in the plots provides the cooling rates for different time periods.

The zircon FT ages from the HHC are 3.9±0.8 Ma and 4.3±0.9 Ma and apatite FT age is 1.9±0.4 Ma. Zircon FT ages from Mandi granite range from 4.9±1.1 to 3.9±0.9 Ma, while apatite FT age is 2.8±0.5 Ma. A quartzite sample from the Main Boundary Thrust zone has yielded zircon FT age as 4.1±1.0 Ma and a zircon FT age from the hanging wall of MBT is 9.2±1.7 Ma.

Very young apatite and zircon FT ages reported in previous studies from the Sutlej valley lie in the frontal parts of HHC. The area is characterized by very high precipitation due to high topography and lies in the zone of high erosion. The FT ages from this study point towards wide spread deep erosion in NW Himalaya. Monsoonal precipitation controlled by topography exerts a strong control on erosional processes mainly due to landslides, deep fluvial incision and sediment removal (Thiede et al., 2004).

For the Higher Himalayan Crystallines (HHC) near Manali Town, Himachal Pradesh, the newly reported zircon and apatite fission track ages along with the Rb-Sr biotite age of 16.7±0.5 Ma and Rb-Sr muscovite age of 26.5± 1 Ma (Mehta 1977) help in determining the cooling rates (Fig. 3). These are 25ºC/Ma for late Oligocene to mid-Miocene, decreasing to about 4ºC/Ma during mid-Pliocene and again increasing during Pliocene to about 64ºC/Ma.

New FT ages along with Rb-Sr Biotite ages (18.6±0.5 Ma and 19.3±0.5 Ma) reported by Mehta (1977) for Mandi granite have been used in determining the cooling rates (Fig. 3). From the temperature-time plot it is evident that during early Miocene to early Pliocene this granitic body had a slow cooling rate of 3.4ºC/Ma, which increased to

6

about 77ºC/Ma during Pliocene. A zircon fission track age reported in this article from MBT zone in NW Himalaya is 4.1±1.0 Ma. Apatite FT ages from MBT zone in the Western Himalaya, Pakistan, (Meigs et al., 1995), in the range 8.2±2.0 to 9.6±1.2 Ma have been suggested to indicate activation of MBT in middle-late Miocene. Although the data is too sparse to define its age precisely, it seems likely that the thrust was still active in NW-Himalaya until late Miocene-Pliocene.

5. Conclusions

Cooling rates of ~64ºC/Ma from the lower parts of the High Himalaya Crystallines (HHC) and ~77ºC/Ma from the Lesser Himalaya (Mandi Granite) obtained from the new FT data added to the previous data give evidence for increased exhumation rates during Pliocene in this part of the Himalaya. The obtained young ages may be the result of rapid erosion coupled with high precipitation. The zone of high erosion related to high regional precipitation lies in the frontal part of HHC as reported from Sutlej valley (Thiede et al., 2004). The young rapid Pliocene-Pleistocene cooling of the Mandi Granite is similar to the rapid cooling of Bandal Granitic gneiss intrusive body of Larji Kulu-Rampur Window, as evidenced by Jain et al. (2000). Young zircon FT age obtained from the MBT zone indicates that the thrust zone cooled through ~250ºC at around 4 Ma.

Acknowledgements:

The authors would like to thank Mr. Fu, C. C. for his help during field work. Prof. C-H Chen and Prof. S. L. Chung are thanked for their guidance and fruitful discussions. This

research work was funded by National Science Council (TFY/NSC 94-2116-M-002-010; NSC 95-2116-M-002-009).

References:

Brandon, M. T., 1996. Probability density plot for fission track grain-age samples. Radiat. Meas. 26, 663-676.

Burbank, D. W., Beck, R. A., Mulder, T., 1996. The Himalayan foreland basin, In: Yin, A., Harrison, T.M. (eds.) The tectonics of Asia, New York: Cambridge University Press, 149-188.

Gansser, A., 1964. Geology of the Himalaya. Interscience Publication, John Wiley and Sons Ltd., London, 289pp.

Gupta, L. N., 1970. Petrology of the metabasites and associated rocks north-east of Mandi (H. P.). Jour. Indian Geos. Assoc. 12, 9-14.

Hubbard, M. S., Harrison, T. M., 1989. 40Ar/39Ar age constraints on deformation and metamorphism in the MCT zone and Tibetan Slab, eastern Nepal Himalaya. Tectonics 8, 865-880.

Jain, A. K., Kumar, D., Sigh, S., Kumar, A., Lal, N., 2000. Timing, quantification and tectonic modeling of Pliocene-Quaternary movements in the NW Himalaya: evidence from fission track dating. Earth Planet. Sci. Lett. 179, 437-451.

Lal, N., Mehta, Y.P., Kumar, A., Jain, A.K., 1999. Cooling and exhumation history of the Mandi granite and adjoining tectonic units, Himachal Pradesh, and estimation of closure temperature from external surface of zircon. In: Jain, A. K., Manickavasagam R. M. (Eds.), Geodynamics of the NW Himalaya, Gondwana Res. Group Memoir 6, 207-216.

8

Mehta, P. K., 1977, Geochronology of the Kulu-Mandi belt: Its implications for the Himalayan tectogenesis. Geol. Rundschau 6, 156-188.

Meigs, A. J., Burbank, D. W., Beck, R. A., 1995. Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya. Geology 23, 423-426.

Neumayer, J., Wisemayr, G., Janda, C., Grasemann, B., Draganits, E., 2004. Eohimalayan fold and thrust belt in the NW-Himalaya (Lingti-Pin Valleys): Shortening and depth to detachment calculation. Austr. Jour. Earth Sci. 95/96, 28-36.

Pandey, A. K., Sachen, H. K., Virdi, N. S., 2004. Exhumation history of a shear zone constrained by microstructural and fluid inclusion technique: an example from the Satluj valley, NW Himalaya, India. Jour. Asian Earth Sci. 23, 391-406..

Searle, M. P., Parrish, R. R., Tirrul, R., Rex, D. C., 1990. Age of crystallization of the K2 gneiss in the Baltoro Karakorum. Jour. Geol. Soc. 147, 603-606.

Thiede, R. C., Bookhagen, B., Arrowsmith, J.R., Sobel, E. R., Strecker, M. R., 2004. Climate control on rapid exhumation along the Southern Himalayan Front. Earth Planet. Sci. Lett. 222, 791-806.

Yang, T. F., Chen, C-H, Tien, R. L., Song, S. R., Liu, T. K., 2003. Remnant magmatic activity in the Coastal Range of East Taiwan after arc-continent collision: fission-track data and 3He/4He ratio evidence. Radiat. Meas. 36, 343-349.

Yang, T. F., Wang, J. R., Lo, C. H., Chung, S. L., Tien, J. L., Xu, R., Deng, W. M. 1999. The thermal history of the Lhasa Block, South Tibetan Plateau based on FTD and Ar-Ar dating. Radiat. Meas. 31, 627-632.

Yin, A. and Harrison, T. M., 2000. Geologic evolution of the Himalyan-Tibetan orogen. Ann. Rev. Earth Planet. Sci. 28, 211-280.

10 Table 1: Fission track analytical data of this study

No. Sample Location Altitude _(ft.) Rock _type Mineral _dated N

ρ

_s Ns

ρ

_i Ni

U (ppm) (± 2σ) χ2 age (Ma ± 1σ) Central age (Ma ± 1σ) P (χ2) %

1 BM 1 N31°40¢54² _{E76°56¢51²} 2614 Quartzite Zircon 27 2.54 1906 1.01 754 540.4 6.4±0.6 9.2±1.7 <1 2 BM 2 N31°41¢54²

E76°56¢19² 2572 Quartzite Zircon 21 1.25 359 1.17 335 626.1 4.1±0.4 4.1±1.0 1.5 3 MG 1 N31°42¢08² _{E76°58¢35²} 2560 Gneissic _granite Zircon 28 1.31 622 1.19 566 639.1 3.9±0.3 3.9±0.9 <1 4 MG 2 N31°42¢10²

E76°58¢37² 2541 Granite Apatite 19 0.045 102 0.429 978 36.5 1.8±0.3 2.8±0.5 <1 5 MG 3 N31°42¢38² _{E76°59¢32²} 2571 Granite Zircon 14 1.70 529 1.43 443 767.0 3.0±0.4 4.9±1.3 <1

6 MG 5 N31°39¢45²

E77°02¢47² 2965 Granite Zircon 13 4.06 652 3.15 506 1691.6 5.0±0.4 4.9±1.1 2.2 7 MNL 1 N32°18¢23²

E77°10¢58² 7060

Granitic

gneiss Zircon 33 2.04 2034 2.00 2000 1063.6 3.8±0.2 3.9±0.8 <1 7 MNL 1 N32°18¢23²

E77°10¢58² 7060 Granitic gneiss Apatite 21 0.039 38 0.52 496 44.1 1.9±0.4 1.9±0.4 22.2 8 MNL 3 N32°10¢43² _{E77°10¢51²} 4960 Granitic _gneiss Zircon 24 2.58 4116 2.28 3637 1223.7 3.7±0.2 4.3±0.9 <1

N = number of grains counted, ρs = spontaneous fission track density (106 cm-2); Ns = number of spontaneous tracks counted; ρ i = induced track density (106 cm-2); Ni = number of induced tracks counted; χ2 = probability of obtaining the observed value for degree of freedom; χ2 _{age correspond to the youngest group of ages from heterogeneous data to represent the last thermal event of the sample.} Central age, t (yr) = 6.12 x 10-8 x

i s r r

x f , where f is the neutron dose (neutrons/track),f zircon = 1.25 x1014_{,f apatite = 0.65 x} 1014

12

Figure Captions

Fig. 1. (a) Simplified tectonic map of the Himalayan orogen (Neuyamer et al., 2004). The rectangle indicates the position of the studied location in Himachal Pradesh enlarged in Fig. 1b. (b) Regional map of the Kulu-Beas valley (reproduced from Pandey et al., 2004) with sample localities marked as numbers 1 to 8.

Fig. 2. Probability-density plots of some of the samples. The mentioned ages are the Central ages. All samples show similar central and chi-squared ages with the exception of one sample from the MBT zone. Suffix Zr has been used for zircon FT age.

Fig. 3. Temperature-time plot for the rocks of Higher Himalayan Crystallines (HHC) and Mandi granite. There is a significant increase in cooling rates after the zircon closure temperature. The numbers in the graph denote the cooling rates.

Fig. 1a Jutogh Group HHCS Chail Metamorphics LH LKRWindow LH Mandi Granite Shali Belt LH Tertiary sediments SH Sample Location x x Fig. 1b

14 Fig. 2

出席國際會議心得報告

Ø 會議名稱：

23

rd

International Conference on Nuclear Tracks in Solids

Ø 會議時間：2006 年 9 月 11 日～ 2006 年 9 月 15 日，共五日

Ø 會議地點：Beijing, China

一、參加會議經過

本次23 屆的國際固體核徑跡會議（International Conference on Nuclear Tracks in Solids, ICNTS）是由國際核徑跡協會（International Nuclear Track Society, INTS） 與中國大陸的山西師範大學共同主辦。ICNTS 是個對於地球物理與核徑跡研究 相當重要的雙年會，會議主要的議程除了探討固體核徑跡的基本形成機制與未來 發展之外，也著重在固體核徑跡探測器的工作機理及在核子物理與核化學、天體 物理學與宇宙線物理、環境科學與生物醫學、輻射防護與劑量學、奈米材料與微 觀結構及核技術等方面的應用。 這次主辦國大陸方面的承辦單位雖然是山西師範大學，但會議舉行的地點選 擇在首都北京，除了國際交通較為方便之外，與會者也能利用會議的空閒時間飽 覽這個融合了古典與現代化的中國代表都市。會議第一天的議程由六零年代固體 核徑跡的發現者P. Buford Price 來揭開序幕。他介紹了五項 1980s 至 1990s 的工 作，帶領與會者回顧固體核徑跡在科學上的應用與發展。第二天的討論主題共分 為三個部分，分別為地球科學與地質定年、核物理與核化學以及放射線與輻射之 防護。晚上並安排了一場晚宴，讓大家有機會認識彼此並瞭解對方的研究領域。 第三天大會安排了一整天的旅遊行程，帶領與會人士參觀北京西北郊的八達嶺長 城與明十三陵，之後並舉行了一場Prof. Dr. Radomir Ilic 的追思晚會。Prof. Ilic 於今年年初逝世，在他的有生之年對於核徑跡的應用有許多傑出的貢獻，並毫不 保留地指導後進科學家，帶領他們踏入科學研究的領域。因此許多與 Prof. Ilic 相識的與會者皆出席了這場追思會。第四天的主題是核子物理與核化學以及奈米 材料與微觀結構及核技術。第五天，也就是會議的最後一天則討論了天體物理學 與宇宙線物理和研究方法、技術與軟體的應用。會期最後的閉幕典禮中，由本次 大會主席郭士倫教授宣布了下一屆大會的主辦國為義大利。

二、與會心得

能有機會參加國際會議我覺得對於各領域的研究者來說是相當不錯的機會， 對於有心從事科學研究的學生來講，更是難能可貴的經驗。除了能與各國相同領 域的研究人才互相切磋，直接面對面進行討論外，國際會議也提供了很好的管道， 讓我們能選擇自己感興趣的不同議題去進行吸收與瞭解。然而，這次二十三屆的 國際核徑跡會議卻有幾個讓與會者有個相當詬病的地方。首先，會議的議程與場 地安排不佳，主辦單位經常未盡告知之責，將議程與場地的異動適時與適當地傳 達給參與學者，使得與會者經常參與不到預期的演講與討論。再者，會議的場地 並不適當，尤其是進行海報展出的場地，由於非常的狹小、照明也不足，因此能 駐足在每張海報前的人數相當有限，討論的氣氛也不熱絡。最後，由於舉行會議 的場地與大會提供住宿的飯店並非在同一地點，因此交通與接送的問題相當重要， 但時常大會並沒有提供適當的交通工具或交通資訊，使得與會人士在會場空等。 這次我想要聽的兩個有關於核飛跡定年研究的演講因為大會的疏失而未能 參與，實在是相當的遺憾。此次一同前往的還有本研究室的三位研究生及國家地 震工程中心Vivek Walia 博士共計發表 5 篇研究成果(如後附摘要)，本人與 Walia 博士所進行的口頭報告竟被臨時更改時間安排在不同的議程內，且本人負責主持 的議程也臨時更改，令大家有些錯愕。不過，雖然主辦單位的作業協調不良，但 在會場內進行的討論還是很熱烈。尤其是在聽完了所有的議程後，對於國內的科 學研究更具信心了，不論是在報告的內容方面、投影片製作的流暢與美觀方面或 是演講者的口語表達清晰方面，台灣學者的表現令人難忘。

三、建議

我覺得台灣的學生有機會參加國際會議應該多爭取口頭報告的機會，因為這 種方式能更直接地讓各國學者或是學生認識我們的研究，對於學生本身，也是相 當好的英文口語訓練。再者，台灣應該多爭取舉辦國際會議的機會，讓學界有機 會認識台灣這個地方，進而增進學術交流合作的機會。尤其以地球科學這個領域 來說，台灣先天上就具有了得天獨厚的地理環境，國內外的合作交流近年來在國 內學者的奔走努力下也越來越蓬勃發展。舉辦國際會議除了有科學上的實質意義

四、攜回資料名稱及內容

會議手冊一份

Radioactive gas emission in hydrothermal area of the Tatun Volcano

Group, Northern Taiwan

Tsanyao Frank Yang1, Tefang F. Lan1, Hsiao-Fen Lee1 and Vivek Walia2 1. Institute of Geosciences, National Taiwan University, Taipei 106, Taiwan

2. National Center for Research on Earthquake Engineering, NARL, Taipei 106, Taiwan

Key words: radon, fumarolic gas, hot springs, Tatun Volcano Group

222_{Rn (radon) is the major radioactive gas in volcanic area. The study of radon} concentration variations in volcanic areas has been considered as a useful tool to investigate the volcanic activity in one area. Tatun Volcano Group, where close to Taipei basin and exhibits active fumaroles and hot springs, was chosen for first time systematical study of radon gas in hydrothermal area of Taiwan. Radon concentrations in soil and fumarolic gases were measured in situ by an analogous radon sensor with an external pump. The sensor is developed to separate the electronics from the measurement chamber, which contains the detector and the preamplifier which is completely covered by epoxy, and can be used in the aggressive environment. The total emission radon flux from the study area would be estimated for comparison with those from other active volcanoes in the world. Combined with gas composition and helium isotopic data, the emission of radioactive gas is mainly carried by the volcanic gases. Hence, it could be a good proxy for future monitoring the magma activity in this area.

Preferred mode of presentation: Oral or Poster

Preferred topic classification: Earth and Planetary Sciences and Dating

================================ Full name: Tsanyao Frank Yang

Postal address: Department of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan

Telephone: 886-2-33665876 Fax number: 886-2-23636095 Email address: [email protected]

Geochemical variation of soil-gas composition for fault and earthquake

precursory studies along Hsincheng fault in NW

Taiwan

Vivek Walia1_{, T. F. Yang}1,2_{, S. J. Lin}1_{, Wei-li Hong}2_{, C.C. Fu}2_{, , K. L. Wen}1,3 _{and C-H} Chen2,4

1_{National Center for Research on Earthquake Engineering, Taipei-106, Taiwan} 2_{Department of Geosciences, National Taiwan University, Taipei-106, Taiwan} 3_{Department of Earth Sciences and Institute of Geophysics, National Central University,}

Jhongli-32054, Taiwan

4_{National Applied Research Laboratories, Taipei 106, Taiwan}

The present study is proposed to investigate geochemical variations of soil-gas composition in the vicinity of geologic fault zone of Hsincheng fault in the Hsinchu area of Taiwan. Soil-gas surveys have been conducted across the Hsincheng fault, to find out the regional activity of this fault system. During the surveys soil-gas samples were collected along the traverses crossing the observed structures. The collected soil-gas sample bags are analyzed for He, Rn, CO2, CH4, Ar, O2 and N2. The data analysis clearly reveals anomalous values along the fault. The consistency of this pattern confirms that soil-gas can act as a powerful tool for the detection and mapping of active fault zones. A continues monitoring station has been established inside the Hsinchu National Science Industrial Park (HNISP) at the end of September, 2005. Preliminary results of the monitoring station shows that the site is good the earthquake monitoring and soil-gas variations have shown good correlation with impending earthquakes. Preferred presentation: Oral

Topic classification: E. Earth and Planetary Sciences and Dating

Dr.Vivek Walia, National Center for Research on Earthquake Engineering,200, Sec.3, Xinhai Rd., Taipei-106, Taiwan.

Telephone: +886-2-6630-0575, Fax: +886-2-6630-0858 e-mails: [email protected], [email protected]

Exhumation history of the Mandi Granite and adjoining tectonic units

across the MBT and MCT faults in NW Himalaya, India

Monika Walia1, Tsanyao Frank Yang1, Ravindra Kumar2, Tsung-Kwei Liu1 and Ling Chung1

1 _{Department of Geosciences, National Taiwan University, Taipei, Taiwan} 2 _{Center of Advanced Study in Geology, Panjab University, Chandigarh, India}

Continent-continent collision substantially affects the earth’s structure in the process of continental evolution. As a typical example, the Himalaya was brought about by collision of the Indian plate with the Eurasian plate, as well as continuous northward movement of the former. Continent-continent collision is still going on.

The Mandi-Manali transect in NW Himalaya, Himachal Pradesh, India, cuts across two major thrusts, namely, Main Boundary Thrust (MBT) and Main Central Thrust (MCT). Representative samples were collected from Mandi Granite intrusive body, while gneisses and schists from the Lesser and Higher Himalayan units.

There is no clear demarcation of the MCT in the study area. All rocks sampled contain quartz + muscovite ± biotite ± plagioclase ± epidote ± monazite ± zircon ± apatite ± tourmaline. Quartzite schists contain biotite and chloritoid where porphyroblasts of biotite are broken and rotated along the shearing planes and contain chloritoids along these planes. From lower to higher structural levels, the rocks follow a systematic progressive metamorphism and T and P increases indicative of inverted metamorphism in the region. Previous radiometric data from various segments in NW Himalaya reveal accelerated exhumation pulses after the initial India-Asia collision at ~55 Ma. Thermochronological data is expected to provide information of the geothermal past of the studied area. Therefore, we have performed the analysis of the fission track dating on the apatite and zircon separates from these various rock units to better define the cooling history of the host rock in the area.

Preferred presentation: Poster

Fission Track Ages as Evidence for the Thermal History of Eastern

Tibet and its Tectonic Implication

Ling Chung, Tsanyao Frank Yang, Sun-Lin Chung, Tsung-Kwei Liu, Monika Walia and Ching-Hua Lo (Department of Geosciences, National Taiwan University, Taipei, Taiwan)

Key words: Fission track dating, Jiali fault zone, eastern Himalayan syntaxis, tectonic evolution

Jiali Fault in eastern Tibet bounds the Karakoram-Jiali Fault Zone (KJFZ) in the east and cuts the Namche Barwa Syntaxis northerly. It also accommodates northward compression and shortening of India-Eurasia collision. As a result, Jiali Fault Zone plays a critical role to the tectonic evolution in eastern Tibet region. Representative samples have been collected for fission-track (FT) dating from Jiali fault zone (Tungmai, north of Namche Barwa Syntaxis) and its two easternmost branches, the Puqu and Parlung Faults (near Chayu). We have combined the FT dating results with previous thermochronological data to interpret the thermal history of eastern Tibet in this study.

FT ages are measured using external detector method. Zircon FT ages range from 11.8 to 9.4 Ma in the Chayu region, whereas age obtained from Tungmai is ca. 1.1 Ma. Apatite FT ages are ~6 Ma and ~1.2 Ma, respectively. Previous 40Ar/39Ar ages from Chayu area suggest a main phase of shearing KJFZ from ~18 to 12 Ma. Based on present FT ages, the shearing time of KJFZ can be extended from ca. 18-12 Ma to ca.18-6 Ma. The young apatite and zircon ages from Tungmai is the evidence of rapid cooling and exhumation of Namche Barwa Syntaxis at ~1 Ma while the Chayu area remain tectonically stable during this time.

Preferred mode of presentation: Poster

Preferred topic classification: Earth and Planetary Sciences and Dating

================================ Full name: Chung, Ling

Postal address: Gas Geochemistry Lab. R109, Department of Geosciences, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei, Taiwan 106 Telephone: 886-2-33665876

Fax number: 886-2-23636095

Variations of Helium and Radon Concentrations in Soil Gases from an

Active Fault Zone of NPUST Campus in Southern Taiwan

Ching-Chou Fu1, Tsanyao Frank Yang1,2, Jane Du3, Vivek Walia2, Tsung-Kwei Liu1 and Cheng-Hong Chen1,4

1. Institute of Geosciences, National Taiwan University, Taipei 106, Taiwan

2. National Center for Research on Earthquake Engineering, NARL, Taipei 106, Taiwan 3. National Pingtung University of Science and Technology, Neipu, Pingtung 91201, Taiwan 4. National Applied Research Laboratories, Taipei 106, Taiwan

Key words: Soil-gas, active fault, earthquake monitoring, NPUST campus, Taiwan

National Pingtung University of Science and Technology (NPUST) is one of the universities with biggest campus in southern Taiwan. It is believed that the active Chaochou Fault may cut through the campus based on geomorphological, geophysical and geological studies. Soil-gas survey was performed along several profiles in the campus for systematical analysis of gas compositions, including He, Rn, CO2, Ar, O2 and N2. The results show that helium, radon and carbon dioxide concentrations in the soil gas reveal anomalies for some specific positions. Trace of these positions coincides with the surface trace of geological and geomorphological characteristics of the Chaochou Fault in southern Taiwan.

In this study, some sensitive sites were chosen for soil helium and radon gas continuous monitoring at the suspected fault scarp in the NPUST campus. After continuous measurement for several months, some anomalous high concentrations can be observed. These anomalies usually appeared few days before the earthquakes, which mainly occurred in southern Taiwan with magnitude larger than 4.0. It suggests that the variations of soil gas compositions may reflect regional crustal stress/strain changes prior to earthquakes and hence, it can be used for future earthquake monitoring at fault zone in the area.

Preferred mode of presentation: Poster

Preferred topic classification: Earth and Planetary Sciences and Dating

================================ Full name: Ching-Chou Fu

Postal address: Department of Geosciences, National Taiwan University No.1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan Telephone: +886-2-33665876