Jonathan R. Major. BYU Department of Geological Sciences

Transcription

1 The tectonic evolution and regional significance of Kisar island, Indonesia: OR Early Orogeny Exposed Jonathan R. Major BYU Department of Geological Sciences

2 Kisar is an anomaly Why Kisar? Located between the Outer (nonvolcanic) Banda Arc and the Inner (volcanic) Banda Arc Emerges from 3km+ of ocean! The 400 km of young arc-continent collision between Timor and Tanimbar islands is practically unstudied The potential for geologic hazards for many parts of Indonesia is still poorly understood.

3 Kisar Island Kisar Island is strangely located between the non-volcanic outer Banda arc and the volcanic inner arc. QUESTIONS: WHY IS IT THERE?? WHERE DID IT COME FROM?? IS IT STILL TECTONICALLY ACTIVE?? From Standley & Harris, 2009 Map from Kaneko et al., 2007

4 Hamilton, 1978

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6 Why Kisar? Coral terraces and exposed metamorphic rocks make Kisar an ideal location to quantify uplift (a key parameter of deformation) of the region Kisar is located in a forearc suture zone between the Australian continent and the Banda Arc It has many characteristics of a gneiss dome

7 Hinschberger 2003

8 Motivations

9 Previous research The Dutch: Geologists have studied the region since the 1800s Dutch Snellius expedition in 1940 s and joint Dutch/Indonesian Snellius II in 1980 s Geophysical work (gravity, seismic, GPS) 80 s and 90 s Japanese team (metamorphic petrology) visited in 90 s

10 Geophysical data: GOAL: Investigate arc-continent collision Includes: deep reflection seismic reflection, gravity, and GPS data Found that part of Australian continental margin has subducted below the Banda Arc (gravity) Backthrust imaged near Kisar The Timor trough was assumed dead but GPS measurements show 20 mm/yr of convergence between Australia and Timor Little field work has followed

11 B.I.R.P.S. Snyder, 1996

12 So why Kisar? Kisar forms an important clue to the relations between the two [arcs]. -Kuenen 1942 Conclusions from geophysical data have not been confirmed in the field Kisar s metamorphic rocks may be the youngest exposed on earth! Region has up to 7 cm/yr of motion! Still, nobody knows exactly why Kisar is there!

13 Coral Terraces: Key to Quaternary Deformation

14 Working reference map

15 Coral terraces, U-Th ages Terrace II (120Ka) is warped = ACTIVE DEFORMATION! Coral sample ages grouped at Ka

16 Coral cont. Curves and data don t fit exactly, but calculated surface uplift rate is within expected range More age data will give greater confidence in estimations

17 Coral cont. Surface uplift rate 0.1 to 1.0 m/ka, depending on how to extrapolate age data 2 loose samples collected yielded 100 year ages evidence of tsunami? U-Th alpha counting methods on giant clam shell sample may give better control on terrace ages Several samples not yet analyzed

18 Possible tsunami deposits

19 Kisar s core Metasedimentary rocks ranging from phyllites, schists, to amphibolites Scattered mafic intrusions and ultramafic bodies Foliation s1 varies, but structures are dominantly east-verging Perpendicular fracture sets roughly oriented N-S and E-W 3 normal faults and one reverse fault were found, but not active.

20 Outcrops and samples

21 Metamorphics Identical to Aileu Formation found on N coast of neighboring Timor Original bedding is not preserved Similar to described metamorphics surrounding mantled gneiss domes P-T data, age data + known geothermal gradient will yield exhumation rate GIS maps of structural data needed

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23 Questions Is it an active gneiss dome? Is it part of a backthrust? Is it both? What is relationship between Kisar s metamorphics and those on Timor? How fast is (short term) surface uplift, how does it compare to (longterm) exhumation rate? What is the tectonomagmatic setting of mafics/ultramafics?

24 LANDSAT false-color image

25 What we can say now Structures are dominantly eastverging Antiforms/synforms found, uplifted coral = Kisar IS actively deforming!! The region IS capable of generating large earthquakes and tsunamis We can learn much about orogeny from one small island

26 Backup slides

27 Hamilton, 1978

28 Hinschberger 2003

29 Uranium and Thorium isotopic compositions and 230 Th ages for Indonesian coral samples MC-ICPMS, Thermo Electron Neptune, at NTU. Weight 238 U 232 Th d 234 U [ 230 Th/ 238 U] [ 230 Th/ 232 Th] Age Age d 234 U initial ID g ppb ppt measured a activity c ppm d uncorrected corrected c,e corrected b OLT ± 5 98± ± ± ± ,602± ,601± ± 2.4 NAB ± 3 43± ± ± ± ± ± ± 2.3 NAB ± 4 45± ± ± ± ± ± ± 2.7 JAB ± 4 299± ± ± ± ,685± ,680± ± 3.9 JAB ± 4 545± ± ± ± ,072± ,064± ± 3.2 PUL ± 4 29± ± ± ± ,829± ,829± ± 3.7 PUL ± 6 336± ± ± ± ,211± ,206± ± 3.9 UHU ± 7 93± ± ± ± ,374± ,373± ± 5.0 POL ± 7 275± ± ± ± ,650± ,647± ± 3.9 YEN ± 5 567± ± ± ± ,665± ,658± ± 3.5 Analytical errors are 2s of the mean. a d 234 U = ([ 234 U/ 238 U] activity - 1) x b d 234 U initial corrected was calculated based on 230 Th age (T), i.e., d 234 U initial = d 234 U measured X e l234*t, and T is corrected age. c [ Th/ U]activity = 1 - e - l 230T + (d 234 Umeasured/1000)[l230/(l230 - l234)](1 - e -( l l 234) T ), where T is the age. Decay constants are x 10-6 yr -1 for 230 Th, x 10-6 yr -1 for 234 U, and x yr -1 for 238 U (Cheng et al., 2000). d The degree of detrital 230 Th contamination is indicated by the [ 230 Th/ 232 Th] atomic ratio instead of the activity ratio. e Age corrections were calculated using an estimated atomic 230 Th/ 232 Th ratio of 6.5 ± 6.5 ppm.

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