PERTEMUAN PERSATUAN (Meetings of the Society)CERAMAH TEKNIK TECHNICAL TALKON THE LINK BETWEEN OROGENIC SHORTENING AND “BACK-ARC” EXTENSIONAL COLLAPSEIN LOW TOPOGRAPHY OROGENSL. Matenco 1 and M. Ducea 21Netherlands Research Centre for Integrated Solid Earth Science, VU University Amsterdam2<strong>Department</strong> of Geosciences, University of Arizona, Tucson, AZ, USA19 October 2011<strong>Department</strong> of <strong>Geology</strong>, University of MalayaAbstract: Classical models of orogenic evolution assume that back arc basins form in the hinterland of orogens,collapsing the upper plate above oceanic subduction zones. This is a common characteristic of all low-topographyorogens of Mediterranean type or SE Asia subduction zones, driven by the fast roll-back of subducted slabs. Thisextension may take place far at the interior of the upper plate, as is the case in various segments of the Carpathiansor in the core of the SE Asian domain, but in most cases of the Dinarides, Apennines or Hellenides it take placesuperposed or far into the foreland of oceanic suture zones. Therefore, the term back-arc extension in many cases ismisleading, as exhumation along major detachment zones takes place in the core of the orogen (Rif, Betics), in theaccreted crustal material of the lower plate (Apennines, Dinarides) or even in the fore-arc (Aegean, Sunda-Banda arc).In all these subduction zones, collision has largely duplicated crustal blocks from the lower plate and has graduallyshifted subduction zone far towards the lower plate. As a result, crustal thickening takes place in the foreland of theorogen, in contrast with the typical crustal roots of the high convergence orogens, such as the Alps or Himalaya.This demonstrate an active shift of the main subduction zone, the position of slabs detected by teleseismic mantletomography is displaced to the foreland and cannot be connected with the position of the lower plate crust beneaththe orogen. This shift is associated with large scale magmatism with unusual large crustal signatures, atypical forsubduction related magmas. These observations demonstrate the need for an active reconsideration of existing orogenicmodels which should include displacements of subduction zones during orogenic shortening and an active investigationof the role of continental subduction and associated magmatism during various phases of mountain build-up.238Warta Geologi, Vol. 36, No. 3&4, Jul–Dec 2011
PERTEMUAN PERSATUAN (Meetings of the Society)MALAM JURUTERA 2011MALAM JURUTERA 2011 was held on 21 st October 2011 at the <strong>Department</strong> of <strong>Geology</strong>, University of Malaya.It featured 2 speakers, namely: Ir. Yee Thien Seng (Consultant) and Ir. Pan Kok Loong (Consultant).Ir Yee discussed various issues in slope engineering in Malaysia, such as problems with frequent slope failures,Terzaghi’s soil mechanics, checkers versus submitting engineers, reports on post-failure investigations such as for thecollapse of the Highland Tower and the Bt. Antarabangsa landslide. He also compared some Hong Kong practiceversus the Malaysian practice.Ir. Pan discussed the practice of geotechnical engineering in Hong Kong and shared his some 10-year workexperiences there. The 3 main topics discussed were: foundations, slopes and deep excavations. These were illustratedwith numerous examples or case studies in Hong Kong.As usual, there were very lively discussions on the topics presented for the Malam.My sincere thanks again to the speakers for their support and contributions to the Society’s activities.Tan Boon KongChairman,W/G on Engineering <strong>Geology</strong>, Hydrogeology & Environmental <strong>Geology</strong>Warta Geologi, Vol. 36, No. 3&4, Jul–Dec 2011239