Brancolini, G.

Davey, F.

de Santis, Laura

Prato, S.

English

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Subsidence at the Cape Roberts Project (CRP) Drillsites from Backstripping Techniques, Victoria Land Basin, Antarctica 'Istituto Nazionale di Oceanogi'.ifia c ill (Ieofisica Speriinentale Triesic Italy qnstiiuie of Geological a n d  Nuclear Sciences - New Zcalami a p p l i e d  Physics and Hnginecring - A.l',H. Research Triesle. Italy Abstract - The tectonic subsidence of the western niargiin of the Victoria Idand basin has been estimated from the physical properties and ages of the sediments in the Cape Roberts Project drillcores 212A and 3, using hackstripping techniques. The analysis indicates a total tectonic subsidence of about 660 in at this location between 34 Ma and the present. Two main trends are defincd, i) about 230 m11ii.y. from 34 Ma to 32.5 Ma, and ii) about 23 m1m.y. from 32.5 Ma to 21 Ma. Since 21 Ma, the subsidence is not well constrained. Extrapolation indicates a very low subsidence rate, but uplift within this period may have greatly affected the estimate. Seismic reflection data further east, towards the centre of the basin, indicates several episodes of extension and associated high subsidence rates since 21 Ma. INTRODUCTION Ail initial assessment of the geohistory of the Victoria Land basin (VLB) at the CRP drillsites was presented in the Initial Report of the CRP-3 drill core (Cape Roberts Science Team (CRST) 2000, Fig. 7.6, p. 197). It was based on ages and thickness from all the three drillholes, with the core added to each other and assumed to be under CRP1 site with allowance made for inferred overlap or gaps. The diagram shows t h e  total subsidence of the basement of the VLB through time. The  total  subsidence is caused by tectonic driving forces and sediment load. We apply t h e  backstripping technique of Steckler & Watts (1 978) to the CRP2 and CRP3 drillcores separately in order to remove the sediment load effects from the to ta l  subsidence and then obtain the  tectonic contribution through t ime at each location. Our  models for CRP2 and CRP3, when put together, show h o w  the tectonic subsidence rate has changed throughout time at these sites. Simple subsidence is assumed, and no allowances made for  any uplift events. The results will, therefore. give minimum subsidence and subsidence rates. METHODS The backstripping technique comprises: removing e a c h  of the sedimentary units,  start ing with the youngest one and progressively decompacting the lower units (decompaction); correcting the depth of each unit for the isostatic uplift after having removed the upper sediment and water load ( isosta t ic  compensation); correcting the water depth taking in consideration the depth at the time of deposition, that determines its position relative to a datum (such a s  present-day sea level) correcting for long term, global eustatic sea level changes. For  the modelling a t  the CRP2 and C R P 3  drillsites, the present porosity data from the drillcore has been used (CRST, 1999 and CRST, 2000), and local isostatic compensation assumed. T h e  CRP-2  and CRP-3 cores have been subdivided into 4 and 5 units respectively, bounded by those unconformities/horizons recognised to be  most significant by other studies on the CRP cores. Tables 1 and 2 list the unconformities used in this study for CRP-2 and CRP-3 respectively. T h e  thin breccia and conglomerate deposited above the basement (Beacon Supergroup quartzose sandstone) (between 823.1  1 m and 790  mbsf) a t  CRP-3 is assumed to represent the lowermost rigid base in our modelling for the CRP-3 drillsite. The curve on the right of the diagram in figure 1 shows the tectonic subsidence of this horizon throughout time and it is reasonably representative of the tectonic subsidence of the geologic basement. CRP-2  did not reach the same breccia and conglomerate layer or the Beacon basement, and the bottom of the drillcore sampled glacimarine mudstone and sandstone. The seismic section across the two drillsites (Fig. 2) does not allow extrapolation of the depth of the breccia layer from CRP-3 to the CRP-2. *Corresponding author (ldesantis@ogs.trieste.it) 138 I , .  De Siinlis et  al. '7th. I - CRP-2 horizons used for this work. (meters bsf) (meters hsl) 130 l[) I 24 1 1 307 1 10 1 7 ; 0 5  1 443 1 0 bottom 1 0 Notes: *From the CRP-2 initial report; bsf bencath seafloor: is1 beneath sealevel. Therefore, the absolute values of the two curves in f i p ~ r e  1 cannot be used to construct one combined curve, because they represent the theoretical p-' '1 1aeo- depths  throughout  t ime,  resul t ing  from tectonic vertical movement, of two different horizons within the  sedimentary co lumn.  These  horizons are the  breccia and conglomerate layer at the bottom of the CRP-3 core  and the  g lac imar ine  mudstone a n d  sandstone at the bottom core at CRP-2. However, these two curves may be used to see the trend of the tectonic subsidence that has occurred: - fsrom the time of deposition of marine sediments above the  breccia layer (34 Ma)  to the  t ime of deposition of sediments at the top of the CRP-3 site (3 1 Ma), and - from the t ime of deposit ion of sediments at the bottom of CRP-2 (30 Ma) to the time of deposition of sediments at the top of CRP-2 (ca. 21 Ma). The dashed line in figure 1 represents the tectonic subsidence of the basement at CRP-3 site between 3 1 Ma and present, inferred from the trend obtained at CRP-2 (from 30 to  21 M a j  and the  calculated depth of basement (660 m) caused by the tectonic subsidence at the present time at the CRP-3 site. The palaeo-depth  informat ion obta ined f rom bio-  stratigraphic studies on the CRP-3 core have a wide range of uncertainty, about 50-120 m.  This would affect the computation of the absolute values of the curve in figure 1, but not significantly its general t rend.  I t  must  b e  a l so  noted that  the  age  of the  unconformity at 443 mbsf in the CRP-2 core is not well constrained and probably lies within the range from 27 to 29.5 M a .  The  age  selected within this range would affect the shape and the gradient of the curve  in  f igure  1 tha t  i l lus t ra tes  the  tec tonic  subsidence at CRP-2. RESULTS AND DISCUSSION The results of backstripping analysis of the core at the drill sites indicate two main trends in tectonic subsidence. A fast subsidence occurred from about 34 Ma to 32.5 Ma,  the younger time corresponding to.  95.48 mbsf at  CRP-3 site. A s lower  period of subsidence  is  observed f rom 32.5 M a  to 21 M a  (Fig .  1) .  T h e  dep th  of 95.48 mbsf  a t  CRP-3 corresponds  to t h e  base  of sequence  2. T h e  fill>. 2 - CRP-3 liorimns used for  this work, 1 Age- (Ma) 1 Present depth T- I'alco-water depth  " 1 Notes: *From the CRP-3 initial report and from Florindo i.-l a l .  (this volume) and Hannali et al.  (this volume): bsf lu'm*:illi scafloor: bsl bencath sea level. sedimentary log in the CRP3 initial result voliimr shows that the base of sequence 2 is represented by ;I l i thologic  change from :I coarse unit to I'iiu* sediments. Moreover there is a significant increase in tin' number of the stratigraphic sequences, erosion e \ u \ t s ,  and diamictite and conglomerate layers be low 11ir depth  of 115-120 m (CRST 2000, Fig .  3..3). I n  addition, metamorphic rocks are absent above  this depth while they are common below it (CRST 2000, F ig .  4.21, and three  large-scale fault  z o n e s  an.' observed below this depth (CRST 2000, Fig. 2.14). A detailed correlation between seismic and lithologic boundaries is limited by the resolution of the seismic data  ( c a .  20 m, see  Henrys et  al .  this  v o l u m e ) ,  However the synthetic log constructed by Henrys et al. (this volume) suggests that the depth of 95.48 111 bsf at  CRP-3 corre la tes  with the  r e f l ec to r  " p " .  or ig inated  by a s t rong acoustic ref lec t ion at tin: boundary between a coarse layers package (at least 20 m thick) overlying by fine sediments. A change i n  the subsidence rate from fast to slow, that occurred at about 95.48 m bsf, is consistent with the geometric TECTONIC SUBSIDENCE 7000 5 10 15 20 25 30 35 AGE before present (Ma) Fig. I - Tectonic subsidence curves obtained using backstripping technique at the CRP-2 (squares) and the CRP-3 (circles) drillsites. The dashed curve represents the extrapolated tectonic subsidence of the basement at CRP-3 site between 31 Ma and present. This curve is inferred from the trend at CRP-2 (from 30 to 21 Ma) and the depth of the basement caused by the tectonics at present time at the  C R P - 3  s i t e .  T h e  e r ro r  bars  indicate  the range  of the  uncertainties in the palaeo-depth information and age. Subsidence at the Cape Roberts Pro,ject (('RP) D r i l l s i k s  from Hiicksirippin<Techniques 139 Line NBP 89 cRp-3 I km 7EST CRP-2 CRP-1 USGS 401 1 Fig. 2a - Seismic section of multichannel l ine NBPX9 across the drillsites 1 km Line NBP 89 ___ WEST CRP-2 CRP-1 USGS 401 EAST Fig. 2b - Line drawing of multichannel seismic line NBP89 across the drillsites. pattern of the seismic reflectors observed on the profile across the drillsite (Fig. 2). On this seismic l ine the sedimentary strata lying below the horizon "p" that correlates with the 95.48 m bsf at CRP-3 drill site dip toward east with high angle. This same horizon is onlapped by reflectors that dip toward east with a gentler angle. No significant change in the tectonic subsidence r a t e  is observed a t  the t ime ( c a .  3 4  My) of the change  in deposit ional s tyle  in  the  CRP-3  core  recorded by the sedimentary facies at the depth of 5 8 0  mbsf. In the Initial Report (CRST, 2000) the depth of basin deposition was interpreted to become shallower for the core sediments above about 580 m bsf, suggesting a decline in the rate of subsidence that a l lowed the basin to fi l l  to  a higher  level.  T h e  penetration in depth of the seismic profile across the drillsite is not sufficient to distinguish any relevant acoustic and strata geometry change at this depth. A major erosion event was recorded at depth of 443  m bsf at the CRP-2 site although the age of the hiatus is not well constrained (27-29.5 Ma; CRST, 1999). Across the 443 mbsf unconformity, a change from thick to thinner sedimentary cycles was also observed and it was previously inferred to represent a decreasing in the subsidence rate (CRST 1999). We do not observe any significant change in the tectonic subsidence rate during the time of formation of the unconformity across 443 mbsf. The subsidence trend during this time is similar to that between 32 and 31 Ma at the CRP-3 site (Fig. l) ,  even considering the uncertain chronology for the hiatus at 443 mbsf. T h e  seismic l ine NBP 89  (Fig. 2) shows that the strata above and below the unconformity at 443 mbsf dip toward east with similar angle, suggesting to us that no large variation in the tectonic subsidence rate occurred during the deposition of these strata. The reflector that correlates with the unconformity at 443 mbsf truncates the underlying strata and is a downlap surface and the sediments below the unconformity are generally finer than those above the unconformity (CRST, 1999). This suggests that the EAST WEST 5 km north of the CRP sites 10 KM - --! n IT90A-69 USGS403 USGS404 2000 Shot@"* 2800 2000 200 1000 1600 10 l"i,i>. 3 - Line drawing of a composite multichannel seismic section made by IT69. US 403 and U S  404 across the VLB  about 5 km norlli o f  [he CRP drillsites. change  f rom thick to thinner cycles ,  previously inferred to represent a decreasing in the subsidence rate (CRST, 19991, could be explained with sea level change  andlor sediment supply  ra ther  than with tectonics rate change. Another major erosion event was recorded a t  depth of 307 mbsf at the CRP-2 site. The age of the end of this erosion event is given by the age of the sediments lying above it, that is 24.1 Ma, although the hiatus span is not well constrained because the age  of sediment  below the unconformity  is s t i l l  uncertain (24.1-29 Maj. The seismic line across the drillsite (Fig. 2) shows that the unconformity at 307 m bsf at the CRP-2 site correlates with a reflector that  sharply truncates eastward inclined strata with high angle. East of the CRP-2 drill site, those inclined strata are onlapped by younger strata that dip toward east at a smaller angle. This geometric configuration suggests that the time span of the unconformity recorded at 307 m bsf could be relatively long, considering that erosion was taking place at the drillsite while deposition was taking place to the  eas t  of the  dri l lsi te.  Unt i l  the  age  of the  sediments below this erosion unconformity is known, we  cannot  es t imate  the  amount  of eros ion that  occurred and the  actual  t i m e  span  of such an  unconformity. The backstripping of the CRP-2 core shows no significant deviation in the overall tectonic subsidence rate from the general trend observed since 32.5 Ma. Extrapolation to  the present indicates a much slower subsidence rate at the CRP drillsites since the formation of the unconformity at 21  Ma. This may reflect the uplift and erosion after subsidence of this region since that  t ime (Davey & Henrys  1999).  Seismic sections across the Victoria Land Basin (Cooper et. al. 1987, De Santis et. al. 1994) show that a significant amount of the sediments deposited into the basin is younger than 21 Ma (Fig. 3). These sediments are contained within several packages ol' strata that dip toward east with relatively high angle, that are bound by truncation unconformitics, and  onlapped by more recent strata. The eastern margin of the basin is intruded by magmatic bodies of relatively young age, clearly folding up the sedimentary strata and the present sea floor (Fig. 3). This would suggest that several episodes of fast extensional tectonics and associated subsidence a lso  occurred in and  after Miocene time, and gradually migrated toward the eastern sector of the VLB (De Santis et  al., 1994). The  result of such a tectonic model would be the overall subsidence slow down in the westernmost sector of the VLB, around the CRP site area. Further work still needs to be done to estimate the amount of stretching and the tectonic subsidence that occurred during this phase. CONCLUSION Tectonic subsidence, at rates of about 230 m/m.y, affected the area around the CRP drillsites from about 34 Ma to about 32.5 Ma. After that time, subsidence rates dropped to about 23 m1m.y. until about 20 Ma. The age and the calculated fast rates of the tectonic subsidence affect ing the  western Ross  sea  is consis tent  wi th  s tudies  made  in  the  Centra l  and Eastern Ross Sea (De Santis et al., 2000) that suggest an Olip,ocene age of t h e  basin o p e n i n g  p h a s e  in t h o s e  r e g i o n s .  Since 20 Ma, e x t r a p o l a t i o n  of t h e  t e c t o n i c  s u b s i d e n c e  c u r v e s  i n d i c a t e s  a p e r i o d  o f  v e r y  low s u b s i d e n c e .  S e i s m i c  r e f l e c t i o n  data across the V L B  i n d i c a t e s  t h a t  e x t e n s i o n a l  t e c t o n i c s  was d i a c h r o n o u s  w i t h i n  t h e  VLB and i t  was p r o g r e s s i v e l y  y o u n g e r  t o w a r d  c ; i s t ,  as p r e v i o u s l y  o b s e r v e d  by DC S a n t i s  et al. (1994), and possibly t o w a r d  s o u t h .  We b e l i e v e  tha t  t h e  a p p a r e n t  m a j o r  s low c l o w n  o f  t h e  overall s u b s i d e n c e  r a t e  a f t e r  a b o u t  20 M;) m a y  h e  t h e  r e s u l t  o f  s i i h s c c l u e n t  u p l i f t  of t h e  r e g i o n  and m i g r a t i o n  o f  t h e  e x i c n s i o n a l  t ec ton ics  t o w a r d  cast ant! s o u t h .  A C K N O W L E D G M E N T S  - T h e  work was  supporled by the NZ F o u n d a t i o n  for R e s e a r c h  S c i e n c e  a n d  T e c h n o l o g y  G r a n t  (..'OS815 ( F D )  and  b y  P r o c y r a ~ i ~ ~ ~ i ~ i  Nuzionc i le  (1; RiMirlio in  Antar t ide  ( P N R A )  un t le r  t h e  CRI' p r o j e c t  (LDS).  C a r l a  De C i l l i a  ( O G S )  h a s  p r o v i d e d  c r u c i a l  assistencc for  depth converting of the seismic data. We t h a n k  B .  L u y e n d y  a n d  L .  B a r t e k  and t h e  O f f i c e  o f  Polar  Programs (USNSF) for  kindly providing access to the seismic NI3P data. W e  appreciate  the  c o m m e n t s  and sugges t ions  o f  our  three reviewers. REFERENCES Cape Roberts Science Team, 1999. Studies from the Cape Roberts Project. Ross Sea. Antarctica. Initial Report on CRP-212A. T e r n  Antartica. 6, 1 - 173. Cape Kohcris Scieiicr 'I'riim. 3000. Stiulies froin the C'iipc Roberts lrii,jci'i, Ross Si'ii, Aiitiirrti~~ii.l i i i t i i i l  R c ~ ~ o r t  on CRP-3. Terra /Itt/arii,;i. 7. 1 209. Cooper A.K., Diivry l"..l. Iv Hrlirrmlt . l . ,  1987. Scisinii; straiigi-aphy and structure of ilic Virioriii l.;nul biisin. wcsterii Ross Sea. Antiiiviicii. In: A.K. Cooprr i l  I-'..), Diivi'y (cds.). The A/it(ii.ctic Coiiisficnia! Marfin: fA,olo,qx 4 (i(~opliy.\ic.s of t h e   waster)^ f<i~,s.s St'a. (';rcu;n-/'tictfic C o u n c i l  f o r  I ^ i ~ e r ^ \  & Miwrcil  /?(~,soiim~.'i. Hiirth Sciences Sci ics, 51%. l Ioiiston, Tcx., 27-65. Davey l-'..l. & llenrys ,?.A., I'W. Vcrticiil tecionics in McMurdo Soinui, Antiii-clicii. Alistnict. 8"' International Symposium on  Aiitarctic liarth Science. Wcllingion. p 81. Ile Santis  I... Hriincolini ( i .  "k Husctti M, .  1994. Structural  evolution of tlic Victoriii l.iiiul Hasin Soiitli of the Drygalsky c c  Toinigue (Western RONS Srii). Pi'rci Aiittirlic(i. 1, 107-1 10. 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Subsidence at the Cape Roberts Project (CRP) Drillsites from Backstripping Techniques, Victoria Land Basin, Antarctica

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