Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies
Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies
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全球地质图是构造速度计——根据面积年龄频率计算的岩石循环速率
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发表时间：
2009
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影响因子：
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通讯作者：
E. Rothman
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作者：
B. Wilkinson;B. McElroy;S. Kesler;S. Peters;E. Rothman
火山、沉积、深成和变质岩单元（岩石体）的年龄和当前暴露区域之间的关系记录了深度和形成速率、随后的构造沉降和埋藏速率、和/或隆起和侵蚀速率之间复杂的相互作用。因此，它们有可能成为有效的深时地质速度计，提供对主要岩石储层（岩石循环的核心过程）之间物质转移速率的定量洞察。来自两个地图来源（加拿大地质调查局 [GSC] 和联合国教育、科学及文化组织 [UNESCO] 的粮食及农业组织 [FAO]）的所有大陆暴露的岩石面积范围表明，火山岩、沉积岩、深成岩和变质岩分别占全球暴露量的约 8%、73%、7% 和 12%。所有绘制的岩石类型的面积与年龄图显示出幂律关系，每百万年约 6.5% 的大陆面积会被较年轻的（约 10% 火山；90% 沉积）单位重新覆盖，并且露头年龄每增加 1%，岩石暴露面积就会减少约 0.86%（r 2 = 0.90）。火山岩和沉积岩体的面积-年龄关系类似于所有岩石单元定义的幂律分布（因为大约 81% 的测绘面积由这两种岩性组成），反映了暴露量随着年龄的增加而逐渐减少。从长远来看，大陆表面被新的火山岩和沉积物覆盖，覆盖率分别为~1.5 和 12.1 × 10 6 km 2 /Ma。与地球表面形成的幂律分布的火山岩和沉积岩相反，深成岩和变质岩的年龄频率分布表现出对数正态关系，模式约为 1。分别为 154 和 697 Ma。深成岩和变质岩缺乏较年轻的暴露反映了这样一个事实，即这些岩石类型是在深处形成的，因此它们的暴露需要一定的构造作用持续时间。不断增加的模态年龄，从火山和沉积层序的第四纪，到侵入岩的早中生代，到变质岩的新元古代，表明抬升需要更长时间的地质时间才能将更深层形成的岩石带到地球表面。对这些主要岩石类型观察到的两种不同的年龄频率分布——火山岩和沉积岩的一般幂律年龄分布以及深成岩和变质岩年龄的对数正态分布——反映了地层深度和垂直构造位移平均速率之间的相互作用。每种主要岩石类型的年龄频率分布都由一个模型密切复制，该模型假设各个地壳元素在地质时间和地壳深度上表现为大量的随机游走，并且与构造隆起相关的地表侵蚀过程在该随机游走空间上施加了吸收边界。模型预测的年龄频率与全球地图数据中明显的年龄频率之间的比较表明，地壳沉降和隆起的平均速率在大小上大致相等，地壳深度的岩石体垂直构造扩散的平均速率约为每百万年半公里。隆起和沉降的速率很大程度上取决于构造分散的持续时间（岩石年龄）；然而，地图年龄频率所建议的每百万年数百米的平均速率与根据更传统的地质计时仪确定的数百个已公布的侵蚀隆起和折返速率的预期相同。该协议表明地质图可以作为地质岩石循环的有效深时速度计。
Relations among ages and present areas of exposure of volcanic, sedimentary, plutonic, and metamorphic rock units (lithosomes) record a complex interplay between depths and rates of formation, rates of subsequent tectonic subsidence and burial, and/or rates of uplift and erosion. Thus, they potentially serve as efficient deep-time geologic speedometers, providing quantitative insight into rates of material transfer among the principal rock reservoirs—processes central to the rock cycle. Areal extents of lithosomes exposed on all continents from two map sources (Geological Survey of Canada [GSC] and the Food and Agricultural Organization [FAO] of the United Nations Educational, Scientific, and Cultural Organization [UNESCO]) indicate that volcanic, sedimentary, plutonic, and metamorphic rocks occupy ~8%, 73%, 7%, and 12% of global exposures, respectively. Plots of area versus age of all mapped rock types display a power-law relation where ~6.5% of continental area is resurfaced with younger (~10% volcanic; 90% sedimentary) units every million years, and where areas of rock exposure decrease by ~0.86% for each 1% increase in outcrop age (r 2 = 0.90). Area-age relations for volcanic and sedimentary lithosomes are similar to the power-law distribution defined by all rock units (because ~81% of mapped area consists of these two lithologies) and reflect progressive decrease in amount of exposure with increasing age. Over the long term, continental surfaces are blanketed by new volcanic rocks and sediments at rates of ~1.5 and 12.1 × 10 6 km 2 /Ma, respectively. In contrast to power-law–distributed volcanic and sedimentary rocks that form at the Earth9s surface, age-frequency distributions for plutonic and metamorphic rocks exhibit lognormal relations, with modes at ca. 154 and 697 Ma, respectively. A dearth of younger exposures of plutonic and metamorphic rocks reflects the fact that these rock types form at depth, and some duration of tectonism is therefore required for their exposure. Increasing modal ages, from Quaternary for volcanic and sedimentary successions, to early Mesozoic for intrusive rocks, to Neoproterozoic for metamorphic rocks, demonstrate that greater amounts of geologic time are required for uplift to bring more deeply formed rocks to the Earth9s surface. The two different age-frequency distributions observed for these major rock types—a general power-law age distribution for volcanic and sedimentary rocks and a lognormal distribution for plutonic and metamorphic rock ages—reflect the interplay between depths of formation and mean rates of vertical tectonic displacement. Age-frequency distributions for each of the major rock types are closely replicated by a model that presumes that individual crustal elements behave as a large population of random walks in geologic time and crustal depth, and where the processes of surficial erosion associated with tectonic uplift serve to impose an absorbing boundary on this random-walk space. Comparisons between model-predicted age-frequencies and those apparent in global map data suggest that mean rates of crustal subsidence and uplift are approximately equal in magnitude, with mean rates of vertical tectonic diffusion of lithosomes from crustal depths of formation of about half a kilometer per million years. Rates of uplift and subsidence are strongly dependent on durations of tectonic dispersion (lithosome ages); however, mean rates on the order of hundreds of meters per million years suggested by map age-frequencies are the same as would be anticipated on the basis of hundreds of published rates of erosional uplift and exhumation determined by more conventional geochronometers. This agreement suggests that geologic maps serve as effective deep-time speedometers for the geologic rock cycle.